PCC

Send Orders for Reprints to [email protected] Current Organic Synthesis, 2016, 13, 000-000

1

Recent Advances in Application of Pyridinium Chlorochromate (PCC) in Organic Synthesis Majid M. Heravia,*, Azadeh Fazelia and Zeinab Faghihia a

Department of Chemistry, School of Science, Alzahra University, Vanak, 1993891176 Tehran, Iran Abstract: Pyridinium chlorochromate (PCC) is an important reagent in organic synthesis used primarily for the selective oxidation of alcohols to give carbonyl compounds. Although a variety of related compounds are known with similar reactivity, PCC offers exclusively the advantage of the selective oxidation of alcohols to aldehydes, whereas many other reagents were less selective. Disadvantages of using PCC are the tedious reaction workup and its toxicity, very well compensated by selective oxidation, observed using this reagent as an oxidant. This useful oxidant was first synthesized and used by E. J. Corey and J. William Suggs in 1972.

Keywords: Pyridinium chlorochromate (PCC), organic synthesis, alcohol, oxidation, carbonyl compounds, selective oxidation.

Majid M. Heravi

1. INTRODUCTION The Oxochromium (VI) reagents are the most important oxidants in organic synthesis. These reagents are most commonly used for the conversion of alcohols to carbonyl compounds [1]. Among the oxochromium (VI) reagents, Jones reagent (chromic acid) was introduced by E. R. H. Jones in 1949. Jones reagent is a solution of chromium trioxide in dilute sulfuric acid and acetone. This reagent directs the conversion of primary 1 and secondary 3 alcohols to carboxylic acids 2 and ketones 4 respectively (Scheme 1) [2]. CrO3 OH

acetone

R

1

OH 2

R= alkyl, aryl CrO3

OH

N R

CrO3, CH2Cl2 1

5 R= alkyl, aryl

In 1975, pyridinium chlorochromate was introduced by E. J. Corey and J. W. Suggs. This reagent is prepared by the addition of pyridine at 0 oC to a mixture of hydrochloric acid and chromium trioxide (Scheme 3). The produced yellow-orange solid is pyridinium chlorochromate (PCC) which is not appreciably hygroscopic [4].

O

aq. H2SO4 R1

R2

acetone

3

H

R

OH

Scheme 2.

O

aq. H2SO4 R

O

N

R2

R1

HCl + CrO3

[CrO3Cl]-

HCrO3Cl

4

0 oC

N H Pyridinium chlorochromate (PCC)

R1= alkyl, aryl R2= alkyl, aryl Scheme 3. Scheme 1.

Collins reagent (Chromium trioxide/ pyridine complex) was introduced by J. C. Collins in 1968. Collins reagent is the complex of chromium (VI) oxide with pyridine in dichloromethane. This reagent is employed to convert primary alcohols 1 to aldehydes 5, with tolerating many other functional groups within the molecule (Scheme 2). Collins reagent is especially useful for oxidation of acid sensitive compounds [3].

*Address correspondence to this author at the Department of Chemistry, School of Science, Alzahra University, Vanak, 1993891176 Tehran, Iran; Tel: +98 912 1329147; Fax: +98 21 88041344; E-mail: [email protected] 1570-1794/16 $58.00+.00

PCC can be used for the efficient and selective oxidation of various primary 1 and secondary alcohols 3 to the corresponding aldehydes 5 and ketones 4 respectively (Scheme 4). Due to the milder and less acidic nature of PCC compared with the Jones reagent, PCC promotes fewer side reactions. The selective conversion of primary alcohols to aldehydes instead of carboxylic acids is one of the advantages and merits of this reagent. In contrast to the Collins reagent, PCC is easier and safer to prepare. The Collins reagent is both difficult and dangerous to prepare. Besides, the latter is very hygroscopic and can inflame during preparation. In order to complete the oxidation reaction, Collins reagent is typically used in a six fold excess and it is only used to convert the primary alcohols to aldehyde. Furthermore, the PCC is not hygroscopic, it is © 2016 Bentham Science Publishers

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Current Organic Synthesis, 2016, Vol. 13, No. ?

Heravi al.

O PCC R

H

R

OH 1

5

R= alkyl, aryl

OH

O PCC

R1

R2 3

R1 R1= alkyl, aryl R2= alkyl, aryl

4

R2

Scheme 4.

stable, commercially available and can be also used for the oxidation of secondary alcohols to the corresponding ketones. Due to the advantages mentioned, now a day, PCC is largely employed instead of Collin reagent for many purposes. It is worthwhile to mention that PCC is soluble in many organic solvents. However, many chemists commonly prefer to perform the oxidation in dichloromethane at room temperature [5].

is of great synthetic value if a protected functional group could be transformed into other functionalities without passing through intermediary unmasked stages. T. Sato and his coworkers in 1989 have reported a versatile method for one-pot transformation of tetrahydropyranyl (THP) ethers 6, 8, 10, the most important protective form of hydroxy groups, into various functionalities (Scheme 5). They have been converted via the one-pot reactions into aldehydes or ketones by treatment with sustenance in the presence of BF3.OEt2. This reaction has been followed by addition of PCC as an oxidant. Scheme 5 shows this transformation on allylic, benzylic, and secondary alkyl THP ethers 6, 8, 10 [61]. Luzzio et al., found that the silica gel modification facilitated the process of standard PCC protocol and these reactions could complete the entire procedure in 3 hours. The model procedures were the oxidation of cis,trans-4-tert-butylcyclohexanol 12 to 4tert-butylcyclohexanone 13 and d,l-menthol 14 to d,l-menthone 15 During the PCC/ silica gel-promoted oxidation the reduced chromium by-products have been adsorbed on silica gel. PCC/silica gel oxidation reactions, in the anhydrous conditions have served to minimize the formation of the side reactions products (Scheme 6) [5].

We are interested in oxidation [25-28] and electrochemical oxidation [29, 30]. We also had some successful endeavors, in applications of chromium-based oxidative agents in various oxidation reactions [31-37]. Armed with these experiences, our interest lies in the applications of name reactions [38-52] and certain reagents in organic synthesis [53-60]. In the present review, we try to highlight the applications of pyridinium chlorochromate as one of the most important and selective oxidant in organic synthesis. 2. OXIDATION OF ALCOHOLS TO CARBONYL COMPOUNDS Protection of functional groups is an unavoidable process in organic synthesis. A protecting group is removed after completion of required manipulations on a molecule, and the regenerated functional group undergoes further desired modifications. Therefore, it

PCC/SiO2 CH2Cl2

OTHP

13

12

PCC/SiO2 CH2Cl2

OH

Scheme 6.

In continuation of V. Chakraborty’s work on the development of highly efficient oxidation protocols, it was observed that the oxidation of neat alcohols with pyridinium chlorochromate (PCC) under microwave irradiation can be carried out much more quickly than using conventional techniques. In this reaction the alcohols 3 reacted with PCC in dry CH2Cl2 under microwave irradiation (2.45 GHz) for 2 min at ambient pressure, to yield the corresponding carbonyl compounds 4 in excellent yields. Any evidence for either the formation of side products or over oxidation have not been observed (Scheme 7) [62].

Ph

O

2) PCC, r.t, 18 h 7

6 Ph

OTHP

1) Bu3SnSMe, BF3.OEt2, 0 oC, 7 h

PhCHO

2) PCC, r.t, 15 h 9

8 1) Bu3SnSMe, BF3.OEt2, 0 oC, 6 h

O

OTHP 2) PCC, r.t, 12 h 10 Scheme 5.

O

15

14

1) Bu3SnSMe, BF3.OEt2, 0 oC, 10 h Ph

O

OH

In the vast majority of oxidation reactions, the numerous versions of oxochromium (VI) are considered as the first choice of reagent and PCC stands above all. The applicability of PCC in the multi-step synthesis of natural products as a key step has been previously reported [6-9]. Among which, is a review article by Luzzio, et al, regarding 1,3-oxidative transformation of allylic alcohols, a review concerning the synthesis of pleuromutilin [10].. They show vividly the importance of PCC as a selective oxidative reagent in oxidation steps. It has also been applied in the synthesis of steroid, as an important and a vital biological class of compounds [11-14]. Interestingly PCC can be used for identification of isomers [15, 16] as well as several kinetic studies [17-24].

11

Recent Advances in Application of Pyridinium Chlorochromate (PCC)

R1

PCC, CH2Cl2

OH

Microwave irridiation

R2

3

solvent-free conditions. The results of these reactions showed that reactions in solution proceeded with higher yields and required longer reaction times in comparison with those under solvent–free conditions. The reaction workup was easy, and the pure products were obtained by a simple vacuum distillation in good to excellent yields. Lack of cleavage of C(O)P bond in both solvent free conditions and in solution was a strong practical advantage of the method (Scheme 11) [66].

R1 O R2

R1= alkyl R2= H, alkyl

3


4

Scheme 7.

Hg[C6H4(CH2OH)-2]Cl 17 has been prepared from the reaction of dilithiated benzyl alcohol 16 with HgCl2. The produced Hg[C6H4(CH2OH)-2]Cl 17 has been smoothly oxidized to Hg[C6H4(CHO)-2]Cl 18 by reaction with pyridinium chlorochromate (PCC) (Scheme 8) [63].

The epimeric mixtures of alcohols 23, 24 at the hydroxylic carbon, both with the same S configuration at the benzylic carbon, have been obtained from the reaction of the lithium carbanion with aldehydes 5. The oxidation of their hydroxylic groups have produced enantiomerically pure ketones 25 of known configurations. As expected, the use of PCC at room temperature in dichloroH O

HO

HO 1) n-BuLi/TMEDA

PCC Hg

Cl

Hg

Cl

2) HgCl2 17

16

18

Scheme 8.

4-Methylpyridinium chlorochromate (-picolinium chlorochromate) is a derivative of PCC. It is commercially available and known to be less toxic than pyridine in some aspects. This oxidant is stable and can be stored in air for several months without any loss of its reactivity. Oxidation of benzyl alcohols and cyclic alcohols to the corresponding aldehydes and ketones has been easily achieved by the using of this oxidant and alcohols in dichloromethane at room temperature. Benzyl alcohol and its derivatives with electronreleasing and electron-withdrawing groups have been oxidized to the corresponding aldehydes and ketones in good to excellent yields (Scheme 9) [64]. +

NH

H3C R1R2CH-OH

3

1

+

No Solvent O

O R

O

H

R

R

19

5

R= aliphatic

O

CH2Cl2, r.t aryl, alkyl R2= H, alkyl

OH

PCC-Al2O3

ClCrO3R1R2C

R1=

R

R= aromatic and vinylic

Scheme 10.

4

O R

P(OEt)2

Scheme 9.

S. Bhar and S. K. Chaudhuri in 2003 found that using the pyridinium chlorochromate adsorbed on neutral alumina (PCCAl2O3) under solvent-free conditions could oxidize selectively primary aliphatic alcohols to alkyl alkanoates 19 whereas primary benzylic and primary allylic alcohols produced the corresponding aldehydes 5. Secondary aliphatic and aromatic alcohols 3 could oxidizes to ketones 4 without isomerization and polymerization of double bonds, overoxidation and other side reactions (Scheme 10). The important advantages of this procedure included (a) operational simplicity, (b) eliminating toxic organic solvents as reaction medium, (c) good yield of the oxidized products (68–89%) with high purity (by immobilization of the chromium by-products on the surface of alumina), (d) mild reaction conditions, (e) good selectivity and (f) general applicability accommodating a variety of substitution patterns [65]. -Hydroxyphosphonates 20 have been converted to their ketophosphonates 21 using PCC as an oxidant in solution or under

O PCC

R

P(OEt)2

Solvent-free, r.t OH

60-88% 20

R= alkyl, aryl

O 21

Scheme 11.

methane has afforded the desired diastereomerically pure ketones in excellent yields (Scheme 12) [67]. A convenient method for oxidation of alcoholic groups to corresponding carbonyl compounds with PCC supported on Al2O3 as an oxidant under solvent free conditions was reported. PCC supported on alumina has been prepared by simply grinding Al2O3 with PCC in the ratio 5:1 (w/w). In this method the starting alcohols in a mortar with grinding by a pestle in the presence of supported PCC on Al2O3 and little drops of t-BuOH have been oxidized to the corresponding carbonyl compounds (Scheme 13) [68].

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Heravi al.

R1 O

O

S

O

S

S

1. LDA, -78 oC

Tol

Tol

Tol +

O

2.

5

H

R

S

R= aryl, alkyl 22

S

R

OH

R

23

24

anti

syn

OH

O S PCC

Tol

CH2Cl2, r.t S R

O

25 Scheme 12.

OH

O

PCC-Al2O3 No Solvent

R2 R1,

3

R2=

alkyl, aryl

R1

MnO2/SiO2 or PCC/Al2O3 R3M

R2 4

H2NR1

N

27

R3M

R1

28

OH

Scheme 13.

In another method for oxidation of alcoholic groups to corresponding carbonyl compounds, CuSO4 and PCC were crushed together in a mortar to form an intimate mixture, and then poured into a flask. Neat alcohol and some drops of t-BuOH were added and irradiated in a domestic microwave oven at 90% power for the short reaction times. Primary and secondary saturated aliphatic, benzylic and heterocyclic alcohols were converted to the corresponding carbonyl compounds in very good to excellent yields with very short reaction times (10-30 S) (Scheme 14) [69]. R1

PCC/CuSO4

29 R3M= Me3Si, R1= CH2Ph

R3 30

R2

O Ph R3M= Me3Si,

N

R1=

R3M= Me3Si, R2= MeCO, R3= COOMe

N Me

O Microwave

R 3M

H2CR2R3

R1

OH R2

PCC/Al2O3

26

Me

R3M= Me3Si, R2= R3= CN

R2 R3M= Et3Ge, R2= MeCO, R3= COOMe

R3M= Et3Ge, R1= CH2Ph

irradiation 4

3 R1= alkyl, aryl R2= H, alkyl, aryl

R3M= Et3Ge, R2= R3= CN

O Ph N

Scheme 14.

The one-pot, solvent free microwave-assisted syntheses of ynimines 30 (using MnO2/ SiO2) and 1,3-enynes 32 from 3organosilyl-(germyl)prop-2-yn-1-ols 28 were developed by A. V. Mareev and his co-workers. The efficiency of an oxidant in these tandem processes depended on the nature of N- or C- nucleophiles. The oxidation of alcohols with PCC/Al2O3 at 25 °C proceeded more rapidly than that with MnO2/SiO2 (Scheme 15) [70]. A method for preparation of chiral building block (ketoester) was developed. The reaction of ketal 31 with phenylboronic acid 32 in toluene produced phenylboronate 33. The crude ester, homoge-

R3M= Et3Ge, R1=

N Me Me

Scheme 15.

nous on TLC, was oxidized with PCC affording ketoester after purification by flash chromatography in good yield. The structure of ketoester compound has been confirmed by its spectroscopic data. Moreover the 1HNMR and MS spectra of ketoester were in agreement with the data reported for the racemate (Scheme 16) [71].



OMe

OMe

5

OH

PhB(OH)2, p-TsOH 32

O O

O toluene, 40-50 oC, 2 h B

OMe

OH

OMe

Ph

O 33

31 OMe

O

PCC, CH2Cl2, 3 A MS O

r.t, 1 h, 70 %

34

B OMe

Ph

O

Scheme 16.

was 36 wherein only the secondary hydroxyl group has been oxidized while the primary hydroxyl group has remained intact (Scheme 18) [73].

M. Hunsen reported a facile and efficient oxidation of primary and secondary alcohols to aldehydes and ketones. They were oxidized to the corresponding carbonyl compounds using only 2 mol% of PCC and 1.05 equiv of the co-oxidant, H5IO6, in acetonitrile. This procedure afforded the carbonyl compounds in high yields in short reaction times (Scheme 17) [72]. OH R1

O

PCC, H5IO6 R2

3

D. J. Phillips and his co-workers could control the acidity of reaction conditions by the addition of a buffering agent, such as imidazole, which would suppress the re-protonation of the Wittig reagent. The studies showed that oxidation over a 2 h period followed by the addition of 2 equiv of imidazole and Wittig reagent have produced the reasonable yields of duster 40. Further improvements in yields were obtained by increasing the reaction time in the oxidation step to 4 h and the addition of the imidazole 1 h before Wittig reagent in the reaction mixture followed by an additional 19 h stirring. Under these conditions, the desired diesters were produced in good yields (Scheme 19) [74].

R1

CH3CN

R2 4

R1= aryl, alkyl R2= aryl, alkyl

Scheme 17.

Oxidation of compound 41 with PCC has also resulted in the formation of the compounds endo and exo bicyclo[6.1.0]nona-3,5diene-9-carboxaldehydes 42 (Scheme 20) [75].

OH (R2)R1

CH

CH

PCC

R2(R1)

OCH2(CH2OCH2)nCH2OH 35 (R2)R1

Oxidation of benzoin 43 to benzyl 44 under microwave irradiation via chromium oxidants such as pyridinium chlorochromate, benzimidazolium fluorochromate, pyridinium fluorochromate, piperdinium chlorochromate and ammonium chlorochromate supported on alumina as a catalyst and solid support was reported and found promising. Alumina led to increase the yield of the product in shorter reaction times. The durability, easy work up and efficiency of chromium oxidants were considerably increased by using alumina as a solid support (Scheme 21) [76].

R1=

CH3(CH2)4 R2= CH2CH=CH(CH2)7CO2CH3

CH

R2(R1)

CO

OCH2(CH2OCH2)nCH2OH 36

Scheme 18.

M. Koprowski and his co-workers reported their results in the synthesis of the polyalkoxy substituted -aryl -ketophosphonates, which may be utilized in the syntheses of lignans as organophos-

In 2006, selective oxidation of 35 to an unexpected product has been studied using PCC. The unexpected product of the reaction

PCC, CH2Cl2, r.t

R1

Imidazole HO n

CO2Et

Ph3P=CHCO2Et

Ph3P=CHCO2Me

38

37

39 R1

R1= H or Me n= 2 or 3 R2O2C

n 40 R2=

Scheme 19.

or

Me or Et

CO2Et

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Heravi al.

H

H CH2OH

PCC, CH2Cl2

CHO

H

H

41

42

Scheme 20.

O

O Acidic Alumina, various chromium based oxidant Microwave irridiation

OH

O

1 min

44

43 Chromium based oxidant Pyridinium chlorochromate Benzimidazolium fluorochromate Pyridinium fluorochromate Piperdinium chlorochromate Ammonium chlorochromate

Yield % 98 70 100 80 60

Scheme 21.

OH

O P(O)(OR)2

O

PCC

O

P(O)(OR)2

O

OBn

O

OBn

CH2Cl2, 2 h, 20 oC

46

45 MeO

OMe OMe

MeO

OMe OMe

R= Me, Et

OH

O

MeO

O

P(OMe)2

O

MeO

P(OMe)2

PCC CH2Cl2, 1 h, 40 oC

MeO OMe

MeO OMe

47 OH

O

O P(OMe)2

O

48 O P(OMe)2

O PCC

O

CH2Cl2, 2 h, 40 oC

X X= Br, H

48

O

X 49

Scheme 22.

phorus reagents transferring the aryloylmethyl functionality. They presented general investigations on the effective oxidation of aryl--hydroxyphosphonates 45, 47, 49 substituted in the aryl moiety by electron rich alkoxy substituents, for the first time addressing this problem. The oxidation of this synthetically useful and sensitive subgroup of -hydroxyphosphonates is usually troublesome unlike oxidation of their -alkyl substituted analogs and could be

achieved in every particular case by two steps: (1) a choice of appropriate oxidizing agent for a selected substrate and (2) a careful tuning of reaction conditions. They found that oxidation under standard conditions by PCC (CH2Cl2 at room temperature) and at elevated temperature (reflux in CH2Cl2) gave good results (Scheme 22) [77].



OH AcOH/conc. H2SO4 HO

OAc PCC, CH2Cl2 HO

C6H5CH3/reflux

OAc OHC

51

50

7

52

O O

1) t-BuOK/THF, 0 oC

CH3CH2CH2CH2P+Ph3Br-

2) 3/THF, –15 oC

54

NaNO2, HNO3

54 O

63/64 = 24:76

55

53 O 55 63/64 = 87:13

Scheme 23. R2

R1

56

HO R1= alkyl R2= alkyl

PCC R2

R1

O

57

PCC

PCC

PCC

R2

R2

R2

R1

R1

R1

PCC

O

60

O

59 O

O

58 OH

Scheme 24.

Grapholitha molesta is a pest that damage peaches, apples, pears, apricots, etc. Its pheromone was isolated and identified as (Z/E)-8-dodecenyl acetate (54 and 55) by W. Roelofs in 1969 [7880]. The sex pheromone of Grapholitha molesta, (Z/E)-8dodecylen-1-ol acetate 54, 55, were synthesized from the reaction of 1,8-octanediol 50 as starting material via mono-esterification, PCC oxidation, and Witting reactions. Transformation of Z/E 83:17 compound into compound with Z/E 24:76 was resulted by sodium nitrite and nitric acid (Scheme 23) [81]. S. S. Korde and his co-workers reported that the oxidation of homoallylic steroidal alcohols 56 with PCC led to four products 57, 58, 59 and 60. Thus, the reaction followed a path where pyridinium chlorochromate first oxidized the homoallylic alcohol to a homoallylic ketone 57. Due to a mild acidic nature of PCC, some of the

compound 66 isomerised to the conjugated ketone 58. Excess of PCC then has oxidized the 5-en-3-one 57 to 6P-hydroxy 4-en-3one 60 and finally to 4-en-3,6-dione 59 (Scheme 24) [82]. The synthesis of new amino alcohols (+)- and ()-3-exo-amino7,7 dimethoxynorbonan-2-exo-ols 65 were described. The (+)- or ()-7,7-dimethoxy-1,4,5,6-tetrachlorobicyclo[2.2.1]hept-5-en-2endo-ol 61, obtained from the enzyme catalyzed transesterification of the racemate, has been reduced and dechlorinated (Na:NH3; ethanol), followed by pyridinium chlorochromate oxidation of the resultant alcohols 62 to the corresponding ketones 63. The enantiopure amino alcohols 64 was then produced via some steps in good yields (Scheme 25). This constitutes an effective and convenient synthetic approach for the preparation of new chiral auxiliaries, synthons and building blocks [83].

8


OMe

MeO

OMe

MeO

Cl

Cl

Heravi al.

Na, NH3 liq

MeO

OMe

PCC

NH2

H

ethanol

H

OMe

MeO

H Cl

Cl

O

OH

OH 62

61

OH

H

63

64

Scheme 25.

HO

CO2R3

O

O

O

CO2R3

PCC R1

CO2R3

O O

R1

chiral pyrrolidine 67

R2

R2 68

+

70

50 mg R2

R1 65

silica 66 R2

O R1= Bn, Et, iPr R2= Me, Ph, 4-ClC6H4 R3= Et, Me

O CO2R3

R1 69 Me

chiral pyrrolidine: Me

N H

75

Me Me 67 Scheme 26.

OH

O PCC

OH

CH2Cl2, r.t

71

O

72

Scheme 27.

K. Juhl and K. A. Jørgensen described that chiral enamines, generated from aldehydes and chiral pyrrolidines, could act as electron-rich alkenes and undergo an enantioselective hetero-Diels– Alder (HDA). The HDA reaction of aldehyde 65 and enones 66 in the presence of a chiral pyrrolidine catalyst 67 produced the two anomers of hemiacetal and aldehyde. Oxidation of this mixture with pyridine chlorochromate yielded lactone 70 as a single diastereomer (Scheme 26) [84].

The reaction of cis-71 with pyridinium chlorochromate in CH2Cl2, also gave the quinine 72 in quantitative yield. (Scheme 27) [85]. Treatment of 73 with an equivalent amount of OsO4 in pyridine resulted in a mixture of cis-74 and cis -75. The diol mixture reacted with PCC to result in a mixture of diones 76, 77. This mixture, on reaction with NaBH4 in EtOH, afforded the mixture of trans-78 and trans-79 (Scheme 28) [85].



9

OsO4 + Pyridine, r.t OH

HO

OH

OH

74

73

75

NaBH4

PCC +

EtOH, r.t

CH2Cl2 O

O

O

O

76

77

+ OH

HO

OH

OH 79

78 Scheme 28.

HO

HO

CO2H

HO

OH

CO2Me

HO

O

O OMe

O

OH

CO2Me

CO2H

81%

O

HO

HO

PCC CH2Cl2

Me

OH OH

Me

MeO Me 80

O OMe

O MeO Me

81

82

83

Scheme 29. OH H

Me

O

Me

H

Me PCC, CH2Cl2

Me

molecular sieve H

H

H

Me

Me

Me

Ph3P=CH2, toluene Me

Me H

84

H 85 (71%)

H

H 86 (40%)

Scheme 30.

A convenient synthesis of dehydroquinic acid was described. After protection of hydroxyl groups of 80, PCC oxidation of protected trans diol of quinic acid 81, produced fully protected dehydroquinic acid 82. The dehydroquinic acid 83 was then afforded via some steps (Scheme 29) [85]. Hirsutene 86 is a triquinane sesquiterpene, a popular target for synthesis because of its molecular structure and role in biogenesis, and a test case for new methods of cyclopentanoid synthesis from salicyl alcohol. In a step for the total synthesis, 1,4,4trimethyltricyclo[6.3.0.0]undecan- 11-one 85 was produced upon

reaction of 84 with PCC a known precursor. Wittig methylenation on this precursor finally gave hirsutene 86 (Scheme 30) [86]. Oxidation of 87 with PCC produced 88, which was then reduced with NaBH4 (5 equiv) and MeOH (50 equiv) in dichloromethane to give the 89 with a high stereoselectivity. Similar oxidation/reduction strategy of 90 to 91 was employed giving a high stereoselectivity. Benzoyl migration of 92 was effected in 60% aqueous pyridine for 4 days at 100 oC to give a mixture of 92 and 93 (30:70). By repeating the migration reaction of 92, the 93 was obtained in 83% overall yield. Further inversion of 3-OH of 93 by

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Heravi al.

OMOM TBDPSO BzO

OMOM

OMOM TBDPSO

PCC, molecular sieve 4 A CH2Cl2, reflux

OMOM

TBDPSO

NaBH4, MeOH

BzO

OMOM

CH2Cl2, 0 oC

BzO

OMOM

OH

O

OH

87

88

89

OMOM TBDPSO HO

OMOM

OMOM PCC, molecular sieve 4 A

OMOM

TBDPSO

TBDPSO

NaBH4, MeOH

CH2Cl2, reflux

O

OMOM

OBz

OBz

90

91

CH2Cl2, 0 oC

R2 O

R1= Bz, R2= H 92 60% aq pyridine, 100 oC, 4 days

PCC, molecular sieve 4 A

OMOM TBDPSO

CH2Cl2, reflux

BzO

OMOM OR1

OMOM

NaBH4, MeOH

R1= H, R2= Bz 93 OMOM

TBDPSO

CH2Cl2, 0 oC

BzO

OMOM

O

OH

94

95

Scheme 31.

OH OH

O

OH

O

PCC or Dess-Martin

+

OH periodinane CH2Cl2

96

97

98

Scheme 32.

O HO

Bn N R3

R1

O

K. Johannes and his co-workers reported the synthesis of oxabutyrolactams from 2,5-dihydrooxazoles. The 4-hydroxyoxazolidines 99 was oxidized in the presence of PCC to generate the desired lactams 100 (Scheme 33) [89].

O

R2

O

Bn

PCC, CH2Cl2, r.t

N

In a pathway for selective synthesis of c-allylbutenolide (cadduct) 103, as shown in Scheme 34, PCC oxidation of homoallylic alcohol 101 was an important part of the protocol. Oxidation of the alcohols 101 and then treatment with Et3N provided 102 in a moderate yield. Although consequently, E-isomer of 102 was formed in low yield of 8–13% during the isomerization, the major Z-isomer was separated and used purely in next stage [90].

R3

52-83% R1

99

O

R2

100

Scheme 33.

the similar oxidation/reduction strategy provided 95 with a high stereoselectivity (Scheme 31) [87]. Without further purification from previous step, the 96 was oxidized to target structure 97/98 using PCC on celite (Scheme 32) [88].

Ph

OH

1. PCC COOMe

101

O

O

Ph

2. Et3N

O

COOMe Ph

Ph

Scheme 34.

Ph

The oxidation of 2-chloro-3-(4-hydroxyphenylamino)-1,4naphthoquinones 104 with cerium ammonium nitrate directed to the formation of the consequent 2-chloro-3-(4 oxocyclohexa-2,5dienylideneamino)-1,4dihydronaphthalene-1,4-diones 105. Compound 104d was oxidized to the corresponding quinone imine 105d

102

Ph 103



O

O H N

Cl R1

11

R2 (NH4)Ce(NO3)6

N

R2

OH

Cl

O

O

R1

104a-d

O 105a-d

O N PCC

104d

105d O

Cl R1

O

104, R1=R2=H(a); R1=H,R2=Me(b); R1=OH, R2=Me(c), H(d); 105, R1=R2=H(a), R1=H,R2=Me(b), R1=OH,R2=Me(c) Scheme 35.

O

OH PCC

O

O

O

O

CH2Cl2 O

O

107

106 Scheme 36.

PCC, CH2Cl2 rt., 2h, 93% 108

OH

109

O

110

N(CH2)nR

NH(CH2)nR

111

Scheme 37.

(R) (S)

(S)

OH

O H

H

H

PCC, CH2Cl2

H HO

112

113

114

Scheme 38.

using pyridinium chlorochromate since the reaction of compound 104d with cerium ammonium nitrate had another path (Scheme 35) [91]. In stereoselective total synthesis of (S)- and (R)-3-allyloxypropane-1,2-diol, one of the key chemical was undoubtedly PCC oxidation of 1-benzoyloxy-3-allyloxypropan-2-ol 106 to 1benzoyloxy-3-allyloxypropan-2-one 107 which gave a fairly good yield. It is worthy to mention that attempts to perform the aforementioned oxidation by commercial enzymes was, at that time, unsuccessful (Scheme 36) [92]. Several libraries of similarly N-substituted 8aminopentacyclo[5.4.0.02,6.03,10.05,9]undecanes 110 N-methyl-8aminopentacyclo[5.4.0.02,6.03,10.05,9]undecanes 111, and its ana-

logues were synthesized and screened against a panel of CNS target. Compounds of these types were potent Receptor ligands with low levels of subtype selectivity. Several compounds of 110 were evaluated for the ability to inhibit nitric oxide release in vitro. Alcohol 108, followed by oxidation with PCC to yield preferred ketone 109 (Scheme 37) [93]. The synthesis and evaluation of a series of cholesterol sidechain analogs such as 114 was reported. It was considered as mechanistic probes of three important Mycobacterium tuberculosis cytochrome P450 enzymes selectively oxidize the methyl-branched cholesterol side-chain. Oxidation (PCC, CH2Cl2) of 112 provided aldehyde 113 as a key intermediate, which permitted simple construction of 114 (Scheme 38) [94].

12


Heravi al.

O

PCC, DCM

DIBAL-10, DCM O

O

0°C, 2h, 67%

O

115

OH

rt, 1h, 95%

116

HO

O

OH

O O

garsubellin A 118

117 O O

O

Scheme 39.

In an approach to the total synthesis of bioactive natural product, garsubellin A 118, enol-lactone 115 was initially subjected to reduction using, DIBAL-10 to provide bicyclo[3.3.1]nonanone derivative 116 as a mixture of C-1 hydroxy epimers. In the following, the latter in a key step, underwent PCC oxidation to give a single bicyclo[3.3.1]nonane based dione 117 incorporating, the bicyclic core of garsubellin A 118 (Scheme 39) [95]. Similar to (7E)-121, isomer (7Z) of cyclohexadec-7-enone 121 has unique and crumbly musk odors with tobacco-type ambary. However, it is more waxy and does not have a really pleasant odors, whereas (7E)-6 is more flowery, charming, and has hay like odors. An efficient approach to the total synthesis of 121 involves the application of standard PCC oxidation on compound 119 in CH2Cl2 to furnish the desired cycloalkynone 120 in high yield, albeit, it should be purified by solvent-consuming flash column chromatography. The target 121 was then produced via Lindlar catalyzed hydrogenation of 120 (Scheme 40) [96].

PCC CH2Cl2 OH

O

119

120

7Z (7Z)

121

O Scheme 40.

In a part of total synthesis of higher terpenoids, oxidiation of 122 (Scheme 41) to the corresponding carbonyl compound was necessary. For this purpose, various standard oxidative systems such as, SO3:Py, n-bromoacetamide in t-BuOH or t-BuOH in the presence of Py, n-bromosuccinimide in aqueous dioxane, or active MnO2 was performed and found, ineffective. Although, oxidation of 122 using PCC proceeded smoothly, but a mixture of compounds

was formed. Notably, the ratio of the components depended on the amount of used oxidant. Upon oxidation with 1 equivalent of PCC, compound 122 was converted to 7-oxo-14'-hydroxytetrahydroisopimarate 123 (23% yield), 7'-hydroxy-8,14- epoxydihydrosandaracopimarate 124 (21% yield). However, complete conversion was achieved when 1.5 eq. of oxidant was used. Increasing the reagent to 3 eq. generated, 7-oxo-derivatives 123 (39%) as a major product along with 152 (30%) and 7-hydroxydihydrosandaracopimarate 126 (6%). Using upper eq. of oxidant did not improve the yield of the desired compounds most probably due to isomerization of chromic ester into 7-oxychromate B followed by generation of 8',14'epoxychromate which upon, hydrolysis gave epoxy alcohol 124 and oxidation provided 7-keto-derivative 125. Notably a similar conversion along with generation of several epoxy derivatives was detected when 14-hydroxypimarate was oxidized using collins reagent [97]. (+)-Zoapatanol 130 (Scheme 42) is one of the several diterpenoid oxepanes extracted from the leaves of Mexican plant called Montanoa tomentosa. Interestingly, in the ancient era, a syrup obtained from this extract has been traditionally used as a contraceptive. Nowadays it has been found that zoapatanol metabolites are in charge for the contraception. Among these, the oxepane scaffold stirred up the interest of organic synthetic chemists in attempting its total synthesis. As a part of total synthesis, the crude alcohols 127 was oxidized to the ketones 128 using PCC in CH2Cl2 at room temperature for 10h. After total consumption of the starting alcohols 127, monitored by TLC, the reaction mixture was filtered through a celite pad to remove the chromium metal. After several steps, oxepanes library 129 were synthesized as (+)-Zoapatanol—Inspired compounds [98]. PCC was also used in a key step in total synthesis of natural compounds such as familiar virolin 133 (Scheme 43) along with norephedrine, and 3-hydroxy-2-phosphonylmethoxypropyl adenine. The crude mixture of alcohols 131a and 131b was subjected to standard PCC oxidation in CH2Cl2, in the presence of molecular sieves (4 Å) to afford ketone 132 in high yield. Notably, compound 132 was found to be a mixture of two rotamers [17]. 2.1. Oxidation Of Unsaturated Alcohols to , - Unsaturated Carbonyls Schneider and his co-workers described a mild method for the synthesis of acyclic -nitroenones. The reaction of nitro epoxydes


H


H

H OH

H

OH

O

PCC

O

+ H CO2CH3 122

+

+

O

H CO2CH3

OH

H CO2CH3

126

O

H CO2CH3

124

123

13

125

Scheme 41.

OR1

HO R O O (+)-Zoapatanol 130

Y

O H

R3-O HO

O

O

H

N

PCC, CH2Cl2 R2

O

H

rt, 10h

R2

O

R1 127

R2

O

R1

R1 oxepanes library

128

129

Scheme 42. OMe OMe OMe O O

+ OH

O

O O

O Mol seive 4A° 30 min,85% OH

131a

OMe

OMe

OMe PCC, DCM

O

OMe

OMe

OMe

O 131b

OH virolin 133 ( threo isomer)

132

Scheme 43.

CR3NO2

CHR3NO2 R1H2CC

CHR2

R1H2C

C

O

CHR2 OH

134

135

PCC, CH2Cl2 25

oC,

CR3NO2 R1H2C

)))), 5 h

C

CHR2 O

136

Scheme 44.

134 with silica gel or aluminium isopropoxide, nitro ethylenic alcohols gave 135. Their oxidation by PCC under sonication led to synthesize the -nitroenones 136 (Scheme 44) [99]. The simple oxidation of 137 with PCC in dichloromethane at room temperature led to produce the aldehyde 138. ,-Unsaturated aldehyde 138 has smoothly reacted with organozinc or organocuprates to give the corresponding allyl alcohols 139 in good yields and oxidation of allyl alcohols with PCC gave the ,-unsaturated ketones 140 (Scheme 45) [100].

Substituted homoallylic alcohols 137 were oxidized using four different oxidants. When oxidizing with PCC or zinc chlorochromate, the corresponding carbonyl compounds 138-141 were produced. When oxidation of the same substrates was performed with t-BuOOH, the corresponding allylic oxidation products together with epoxidation products were obtained (Scheme 46) [101]. Scheme 47 shows the synthesis of tetronolactonyl aldehydes 143 originally from L-ascorbic acid. The oxidation of intermediate allyl alcohols 142 with PCC in dichloromethane and in the presence

14


Heravi al.

O

O

OH

B

PCC

RZnCl

H

B

O

O

THF, -40 oC

O 137

Hb

O

R

B OH

R= Me, Et, n-Bu, Ph, allyl

138

Hb

PCC

O

R

O

B

CH2Cl2, r.t

Ha

O

139

O

Ha

140

Scheme 45. R3 R2

O

Method A

+

(PCC)

R1

R1

H

O

R3

139

138

R2

R1

OH

R3

R3

R2

137

R2

O

O

Method B +

R1= H, CH3, Ph R2= H R3= H, CH3

(ZCC)

R1

O

138

+ R1 140

R1

+

R1

H

H

O

O 139

141

Scheme 46.

of molecular sieves (4 Å) led to the formation of tetronolactonyl allylic aldehydes 143 in good yields. The structures and geometry (Z) of these compounds were determined on the basis of their spectroscopic data [73]. OH

O H O

PCC, CH2Cl2

O

O

O

4A MS, 0 oC RO

OR

30 min

RO

OR

142

143

66%, 65%

53%, 54%

Scheme 47.

The PCC oxidation of the homoallylic alcohol 144 to 145, double bond isomerization, and zinc-mediated Barbier type alkenylation protocol have led to regioselective synthesis of alkenylbutenolides 146 in moderate to good yields (Scheme 48) [90].

G.K. S. Prakash and his co-workers reported a simple route to synthesis 3- trifluoromethyl-2-cycloalkenones 149 by oxidative rearrangement of trifluoromethylated tertiary allylic alcohols 148, obtained from trifluoromethylation of several conjugated enones 147 with PCC in the presence of a small amount of concentrated H2SO4. This reaction was limited to cyclic enones 150. It was noteworthy to mention that the amount of H2SO4 was effective in progress of the reaction. Using PCC alone or adding a drop of H2SO4 did not give the rearranged products. On the other hand, one equivalent (or more) of H2SO4 led to decomposition of PCC without any improvement in the yields of the final rearranged products. As a cyclic enones, eucarvone 150 was chosen as a substrate with two conjugated double bonds. Reaction of trifluoromethylated alcohol with PCC under the described conditions produced a product with an exact mass which was 16 masses higher than the expected oxidation product. The spectra showed the product had the dihydrofuran structure. It was possible that PCC under acidic conditions hydroxylates the farthest double bond, creating a carbocationic center 152 that was subsequently attacked by the hydroxyl geminal to the trifluoromethyl group 153. The resulting intermediate alcohol bearing the dihydrofuran ring 154 was oxidized to the final product (Scheme 49) [102].


R2


R2

OH

O allyl bromide

1. PCC CO2Me

R1

2. Et3N

CO2Me

R1

O

R2

15

O

Zn/THF R1

144 (syn)

146 (51-81%)

145 (Z) R1= Ph, 2-BrPh, -CH=CHPh R2= Ph, 4-ClPh, Me

Scheme 48.

O

F3C 1) CF3SiMe3/TBAF (cat)

n

CF3

OH PCC

n

n

H2SO4

2) TBAF (1eq)

O R

R n= 1, 2 R= H R1= H, CH3, 2-propenyl

147

R1

R

148

149

1) CF3SiMe3/TBAF (cat) O

CF3

2) TBAF (1eq)

OH

150

R1

PCC H2SO4

151

+ CF3

HO

HO

O

CF3

O

O

CF3

OH

152

153

154

Scheme 49.

When alcohol 155 reacted with pyridinium chlorochromate instead of Jones reagent, enone 156 was produced. Deprotection of silylated ketone with tetrabutylammonium fluoride in anhydrous tetrahydrofuran produced alcohol 157 quantitatively, and finally, oxidation of 157 with pyridinium chlorochromate gave ketoaldehyde 158 (Scheme 50). The unsaturated alcohol 155 was also oxidized to enone 159 with Jones reagent in a moderate yield. Deprotection of silyl-ether 159 with tetrabutylammonium fluoride in anhydrous tetrahydrofuran gave alcohol 160 quantitatively, and final mild oxidation of alcohol 160 with pyridinium chlorochromate produced ketoaldehyde 161 [103]. After TBS protection of the secondary hydroxyl group of 162, the selective allylic oxidation (3,5-DMP, PCC, 0 °C) and TBS pro-

tection of primary hydroxyl group occurred to afford 163. Finally, the conversion of 163 to diol 164 was performed by reduction with LiAlH4 (Scheme 51) [104]. Oxidation of 165 with PCC at room temperature for 3 days gave 2,9,10-tribromoanthracene-1,4-dione 166. A simple short silica gel filtration and crystallization from CH2Cl2 needed to produce the expected 2,9,10-tribromoanthracene-1,4- dione 166 in high yield (Scheme 52) [105]. An oxidative rearrangement of cyclic tertiary -hydroxy allylsilanes 167 was carried out in refluxing ClCH2CH2Cl with PCC. The reaction provided a convenient method to synthesize cyclic silylenones 168 in modest to excellent yields (Scheme 53) [106].

16


Heravi al.

OH CHO

PCC

OTBDPS

PCC

OR

CH2Cl2

CH2Cl2

O

O

H

H

H

155 TBAF, THF

R= TBDPS

156

R= H

157

158

OH O

O CHO

Jones

OTBDPS

OR

acetone

PCC CH2Cl2

H

H

H

155 TBAF, THF

R= TBDPS

159

R= H

160

161

Scheme 50. HO

TBSO

TBSO

OTBS

OTBS 1) PCC, 3,5-DMP, 0

oC

OTBS

(62%)

HO

2) TBSCl, DMF/imidazole, 0 oC, 1.5 h (91%)

HO

O 162

HO 163

164

Scheme 51.

Br

OH

Br

OH

PCC Br Br

CH2Cl2, 25 oC

OH

Br Br

O 166(90%)

165 Scheme 52.

HO

SiPhMe2

SiPhMe2

n

PCC (1.5 equiv) n

ClCH2CH2Cl, refluxing, 5 min R

40-94% 167

Scheme 53.

n=0 R= H, alkyl

O

R 168



This protocol has been carried out using the Grignard 176 addition to 175 followed by PCC-oxidation to give the skeleton 2allylbenzaldehydes 177 (Scheme 57) [110].

2.2. Addition of Grignard Reagent Followed by PCC-oxidation After stereoselective synthesis of aminoalcohol 169, it was oxidized into the lactam 170 by treatment with PCC, which could be reduced to the corresponding amine and get the main target (Scheme 54) [107].

R2

X Y

R2

BnO

O

PCC

BnO

HO

Z

CHO

169

170

O R3

X= H, OMe Y= OMe Z= H, OMe R1= H, Ph R2= H, Me R3= alkyl, aryl

175

OBn

R1

Z

BnO

OBn

R2

X

2) PCC, DCM

N

R2

1) R3MgBr (176), Y THF

R1

R1 R1HN

17

177

Scheme 54.

Scheme 57.

2,3,5,6-Tetrasilyl- and 2,3,5,6-tetragermyl- 1,4-benzoquinones 172 were produced by oxidation of the corresponding 1,4dimetaloxybenzene derivatives 171 with pyridinium chlorochromate (Scheme 55) [108].

After Claisen rearrangement and O-methylation of compound 178 to afford 179 and 180, the Grignard methylation using 181 was done. Finally, PCC-mediated oxidation of them produced 182 and 183 (Scheme 58) [111].

O

As the final step for diastereoselective synthesis of 185, pyridinium chlorochromate oxidation led to produce the target structures from 184. These targets were designed as seco-structures to a musky carotol lead, and their olfactory profiles that merge violet like with musky notes to different extents, to provide interesting insight into structure–odor correlation (Scheme 59) [112].

O

X

X

X

X

1) RCl / Mg

R

R

R

R

2) PCC R= SiMe3 SiMe2H SiMe2(vinyl) GeMe3

O 171 X= Br, Cl

O

R1

R1

172

O

OH

PCC CH2Cl2

Scheme 55.

R2

R2

As an important step for the asymmetric construction of tertiary carbinols, the oxidation of 173 with pyridinium chlorochromate was done to give the alkylated ketone 175 (Scheme 56) [109].

184

185

Scheme 59. PCC, NaOAc, CH2Cl2 O 173

PCC-oxidation of cyclopentenylbutenol 186 has also led to produce dienone target 187 in 89% yield (Scheme 60) [113].

O

O

O

R1

R1 O

174

OH

Scheme 61 describes the first total synthesis of (±)-1desoxyhypnophilin A, which displayed promising antimicrobial activity. Pyridinium chlorochromate on Al2O3 oxidized enole 188 to enone 189 that was a PCC induced oxidative rearrangement. After several step the target A was synthesized [114, 115].

Scheme 56.

CHO

CHO

2) MeI, K2CO3

MeO O

MeO

1) decalin, reflux

CHO

+ MeO

R

MeO OMe

R

R 180 R= Me, Ph

179 R= H, Me

178 O

O

1) MeMgBr (181), THF

MeO +

2) PCC, DCM

MeO OMe

R

182 R= H, Me Scheme 58.

R

MeO

183 R= Me, Ph

18


OH

R

Heravi al.

organic chemists. For providing skeleton 192, compound 190 was transformed to 191 via a two-step sequence of Grignard addition, followed by oxidation of the resulting alcohols using PCC. The yields were good depending on the allylbenzaldehydes 190. Based on a the three-step strategy, papaverine 192 was the next synthetic target (Scheme 62) [110].

O

R

PCC Celite CH2Cl2 89%, 90%

186

Two different products 194 were produced from 193. Compounds 195 were provided in reasonable yields from two-steps of Grignard methylation of 194 with methylmagnesium bromide followed by the oxidation of the resulting secondary alcohols with PCC as shown in Scheme 63 [111].

187

Scheme 60. H

H PCC on Al2O3

O

CH2Cl2, r.t H

64%

OH

3. REDUCTIVE OXIDATION OF CARBOXYLIC ACIDS AND ITS DERIVATIVES TO ALDEHYDES

H

J. S. Cha and his co-workers extended the procedure to conversion of acid chlorides 196 to aldehydes 5 with aluminum hydride and PCC. It took place in high yields at room temperature. This reaction was broadly applicable and tolerated many substituents, such as chloro, methoxy, nitro and olefinic groups. It has appeared to be equally usable to both aliphatic and aromatic acid chlorides (Scheme 64a) [116].

189

188 H O A O

H

The tolerance of sodium borohydride toward a wide variety of functional groups as well as its ease of handling and low cost have combined with the mild nature of PCC as an oxidizing agent has made this method simple, general and practical (Scheme 64b) [117].

(+,-)-1-Desoxyhypnophilin

Scheme 61.

Naturally occurring products containing an isoquinoline nucleus often display the biological potency, like antimalarial, anti-HIV, antitumor, antimicrobial, and antibacterial. Thus, the synthesis of substituted isoquinolines has attracted much attention of synthetic

The reduction of acid chlorides 196 with a half equivalent of LAH at 0 oC, followed by oxidation of the resultant alkoxyalumiH

H

H

MeO

Ph

MeO

H

MeO

CHO

1) 3,4-Meo2PhCH2MgBr

Ph O

MeO

191

MeO

2) PCC, DCM, 66% of two steps

190 MeO

H

CHO

MeO N

MeO

OsO4, NaIO4

papaverine

MeO

192

NH4OAc, 77% MeO

CHO

Scheme 62.

CHO

CHO

CHO

1) decaline, reflux MeO O

R

+

MeO

2) MeI, K2CO3

OMe

R1

R MeO OMe

194

193 O

O

R1= H, Me R= Me, Ph

1) MgBr, THF + 2) PCC, DCM

OMe Scheme 63.

R

MeO

MeO R1

OMe 195


num intermediate (without isolation) with PCC actually afforded the same results as the procedure using aluminum hydride [118].


199

198

1) AlH3

RCOCl

RCOOBH2

RCOO-M+ + 2 BH3

a

2) PCC 5

5 R= alkyl, aryl M= Na, Li

b 1) NaBH4 Scheme 67.

2) PCC R= alkyl, aryl Scheme 64.

The method below showed a rapid reduction of carboxylic ester 197 with LAH, followed by oxidation of the resultant alkoxyaluminate intermediate (without isolation) with PCC or PDC at room temperature. This reaction was broadly applicable tolerating many substituents, such as chloro, nitro, and alkene groups (Scheme 65) [119]. 1) LiAlH4

R1CHO

2) PCC 197

RCHO

200

196

R1COOR2

PCC

1/3(RCH2-OBO)3

RCHO

19

5

R1= alkyl, aryl R2= alkyl

gen peroxide in 3 M sodium hydroxide at 0 oC. The resulting triol intermediate were not purified and were oxidized using two different methods depending on the nature of the triol: dichlorotris(triphenylphosphine)ruthenium(II) in benzene at 0 oC (Method A, for R2= H) PCC in CH2Cl2 at 0 oC (Method B, for R2= H) (Scheme 68) [123-126]. A useful and novel example of Cr(VI) oxidation of an alkene is the intramolecular oxidative cyclization of bishomoallylic tertiary alcohols 203, 205 by PCC to produce substituted tetrahydrofuran products 204, 206. The reactivity of these substrates toward PCC under standard conditions was dependent only on the number of R groups on the C=C, not on the degree of substitution on the most highly substituted alkene carbon. This observation suggested a symmetrical transition state in this intramolecular Cr(VI) alkene oxidation (Scheme 69) [127].

Scheme 65.

The rapid reduction of carboxylic acid 2 with LAH, followed by oxidation of the resultant alkoxyaluminate intermediate (without isolation) with PCC at room temperature for 12 h. The transformations of aromatic acids 2 into the corresponding aldehydes 5 with this procedure was quantitative. The oxidation of aromatic intermediates was much faster than that of aliphatic ones (Scheme 66) [120]. RCOOH

1) LiAlH4

PCC

203

204 R1

R1

RCHO

O

R= alkyl, aryl

5

R1=

205

The stepwise reduction-oxidation method was successfully applied to conversion of carboxylic acid salts 198 to corresponding aldehydes 5. Thus, both sodium and lithium salts of carboxylic acid 198 were readily converted to aldehydes 5 in essentially quantitative yields by stepwise reaction with borane and dimethyl sulfate, followed by oxidation with PCC (Scheme 67) [121, 122]. 4. OXIDATIVE CYCLIZATION The transformation of the unsaturated diols 201 were carried out in a three-step process involving first a hydroboration with borane in THF at 0 oC, followed by in situ oxidation with 33% hydro-

R4

R1

R3

OH

OH

H, Me R2= H, Me

206

Scheme 69.

Scheme 70 shows that PCC on silica gel in CH2Cl2 has performed oxidative cyclization of sterically crowded ,-unsaturated alcohols 207 (primary, secondary, allylic, benzylic as well as tertiary alcohols) [128]. PCC supported on silica gel led to the oxidative cyclization of structurally diverse thiophenolic and phenolic Schiff’s bases 210, 212 to 2-arylbenzothiazoles and 2 arylbenzoxazoles bases 211, 213 in good to excellent yields (Scheme 71) [129].

i) BH3.THF, 0 oC ii) 33% H2O2, 3 M NaOH, 0 oC

H R2 R1

iii) (Ph3P)3RuCl2, PhH, 0 oC (for R2= H) or PCC, CH2Cl2, 0 oC

201

R4 O

O H 202

R1= alkyl, aryl, R2= alkyl, R3= alkyl, R4= alkyl Scheme 68.

O

R2

OH

Scheme 66.

R2

R2

PCC

2) PCC 2

OH

O

OH

R3

20


A new one-pot strategy for the synthesis of a series of new Nsubstituted 3-trifluoroacetyl pyrroles 218 was presented below. These compounds have been obtained by the reaction of 3trifluoroacetyl-4,5- dihydrofuran 214 with primary amines 215 in CH2Cl2, which have generated 1,1,1-trifluoro- 3-(2-hydroxyethyl)4-alkylaminobut-3-en-2-one intermediates 216. In most cases, these

Heravi al.

intermediates were not stable enough to be isolated. Thus, in the same reaction vessel they were directly submitted to oxidation with PCC (Corey’s reagent) to furnish 1,1,1-trifluoro- 3-(2-ethanal)-4alkylaminobut-3-en-2-ones 217, which under reflux for 3 h underwent intramolecular cyclization to give the desired N-substituted 3trifluoroacetyl pyrroles 218, in moderate yields (Scheme 72) [130].



21

R O

PCC R

R

O

Silica gel

H

CH2Cl2

OH

+ O

R= H, Me, Bu, alkene, 4-MePh

207

208

209

Scheme 70.

N

N

N

PCC, Silica

Me DCM, r.t

ZH

N

Z

Z= S, O

Me

211

210 R2 R1

R3 R1

R5

N

R4

R5 PCC, Silica

N R3

DCM, r.t

ZH

R2

Z R4

213

212

R1= Cl, H, NO2, R2= H, NO2, OMe, R3= H, OBn, OH, NO2, R4= H, OMe, R5= H, Me, Cl Scheme 71.

O

O

O CF3

RNH2 (215), CH2Cl2, 0.5 h, r.t

F3C

NHR

PCC, CH2Cl2

F3C

NHR

H O

R= alkyl O

OH

217

216

214

reflux, 3 h

spontaneous conversion

O O CF3 F3C

NHR

218

219 N R

Scheme 72.

After tandem oxidative double oxidative spiroketalization sequence, a similar oxidative behavior of PCC was done to get compounds 221, 222, 223 and 224 in different steps (Scheme 73) [131]. Hetero-Diels–Alder reaction of -hydroxy-o-quinodimethanes photochemically generated from o-tolualdehydes 225 with

trifluoromethyl ketones 226 produced a mixture of hemiacetals and hydroxyaldehydes in good yields. Their subsequent oxidation with PCC provided 227 as formal oxidative [4+2] cycloaddition products (Scheme 74) [132].

22


Heravi al.

H 1

Spiroketalization

25 24

HO A 1 H

25

6

O

10

B H

26

O

C O

H

14

18

D O

H

H

28

27

E O

B O

PCC

22 30 OH

H

26 A O F O

29

H

O

H

Oxidataive Degradation

PCC chemistry

O

O

H

O

O

H

222

H O

O O

OH

H

O

H

H

O

H

221

Oxidative spiroketalization H (10%) O

O

O

H

27

220

O

O

O

PCC chemistry

O

O

H

O

O

O

H

Oxidataive Degradation

223

224

Scheme 73.

O

O O

H +

R1

O

i) hv, MeCN, r.t R3

F3C

R1

ii) PCC, MS 4A, DCM, r.t, 2-5 h

CF3

R1= 226

225

R2

Me, F R2= H, Ph, n-Hex R3= Ph, Me, Bn

R2

R3

227

Scheme 74.

O

O 308

CHO

R2

R3

O

R1

R3

R1

R2 230

228 hv

oxidation OH

OH

CHO 229 R1

O

R1

R3

R1

R2

OH R2

A

B

R3

C

Scheme 75.

Hetero-Diels–Alder reaction of a-hydroxy-o-quinodimethanes which in turn was created from the reaction of o-tolualdehydes 228 with trifluoromethyl ketones, affords a mixture of hemiacetals and hydroxyaldehydes in moderate yields. Upon treatment with PCC under standard conditions provided 1-isochromanones 230 as formal oxidative [4+2] cycloaddition products. ketone 229 (1.1 equiv) in MeCN was irradiated by Hg lamp for 3h, an inseparable mixture of hemiacetal B and hydroxyaldehyde C was obtained, which subsequently treated with PCC in DCM in the presence of molecular sieves to give only 1-isochromanone 230 in moderate yields (Scheme 75) [132].

As shown in Scheme 76, hydrolysis of (Z)-isomers 231 with conc. HBr gave two products via the intermediate 232. Aqueous workup showed that the products were mixtures of mostly cyclic hemiacetal derivatives, 4-aryl-1H-2 benzopyran-1-ols 233, and cyclic methyl acetal derivatives, 4-aryl-1-methoxy-1H-2benzopyrans 234. The mixtures were subjected to PCC oxidation to give the desired products 235 without any purification in moderate yields from (Z)-2. When the acid employed was conc. HI, rather complex mixtures of products were formed, and the yields of the desired products considerably decreased (ca. 15%) after PCC oxidation. The reactions using conc. HCl gave even more disappointing



Ar

Ar

Ar

R

Ar

R

R

R

con. HBr OMe CHO

O

OH

233

232

Z-231

+

O

OH CHO

MeCN, rt

23

OMe

234

Ar R

MeOH, H+

PCC, CH2Cl2 235 O O Scheme 76.

OH R1 O

R2

R2

OH

R2

PCC

O

R1

O

R1

O O

R1= H, alkyl, R2= alkyl, aryl

236

238

O O

O O

R

R

R

Ag2CO3/ celite

+ O

O 240

R=

O

H

O

PCC

O

239

COOH

O

237

OH

OH

H

O

241

,

Scheme 77.

R1 R2

R1

PCC OH

R2

O

R2

CHO

O 243

242

R1= alkyl R2= alkyl, aryl

R1

R1 R2

PCC O

246

R2

O

O

244

R1 R2

OH

R1

PCC OH

245

PCC O 247

O

R1 R2

O

O

248

Scheme 78.

results due to complicated mixtures of products. Surprisingly, the formation of 235 from (E)-231 could not be attained, because hydrolysis of (E)-231 gave similar results to those of hydrolysis of (Z)-231 with conc. HCl [133]. 5. OXIDATIVE CLEAVAGE The allylic alcohols 236 obtained from lithiated 1,4-dioxene and ketones or aldehydes underent regiospecifically oxidative cleavage with PCC at the dioxane double bond to give saponification, -hydroxy acids 238. Extension of this reaction to , - un-

saturated ketones 240 afforded 241 after similar oxidation reaction (Scheme 77) [134]. Treatment of a number of tetrahydrofuran methanol derivatives 242 with PCC led to the formation of the corresponding butyrolactones 245 with the loss of one or more carbon atoms in good yields under mild reaction conditions. As shown in Scheme 78, tetrahydrofuran methanol derivatives 242 have been converted to the -lactones 245 via the intermediacy of aldehyde 243 which did the oxidative cleavage with the excess of PCC reagent to form the -butyrolactone 245. In the oxidative transformation of sub-

24


Heravi al.

H+ Ph-CH=CH-CO2H + 2 PyHOCrO2Cl + (CO2H)2

PhCHO + OHCCO2H + 2 CO2 + 2 PyHCl + H2O + Cr2O3 251

250

249

252

O OO

(CO2H)2 + H+ +

O

K1

Cr

+ H2O + HCl O

O

O

250

O Cr

Cl O H

O

Ph O

Ph

Cr

Cr

+ O

HO2C

HO2C

O

K3

O Ph K4 H2O

O Cr

slow O HO2C

(C2)

O

Ph

C3

(C3)

H

OH

HO

C HO2C

O

Cr

+

C

fast

O

H

O

H

249

(C1)

O

O

H

O

H

O

H

K2

O

O

C2

O

(C1)

O

HO

OH H

O

253 Ph

O

H

OH C

HO

fast

253

fast

C

Cr (III) + CO2 + CO2-

Cr

HO2C

HO

PCHO + OHCCO2H

PCC

OH

251

H

252

O

Scheme 79.

NHOH MeO2C

NH

PCC

CO2Me

THF, 25 oC, 30 min

MeO2C CO2Me

Me

N

Me

NHCOMe

254

255

Me

H

CF3 MeO2C

Me

NHOH

N

Me

CF3

PCC THF, 25 oC, 30 min 256

MeO2C

Me

N

N

Me

O

257

Scheme 80.

strates 246 with PCC, it was believed that tertiary alcohols underwent dehydration to form enol ether intermediates 247 under the reaction conditions which were subsequently submitted to oxidative cleavage with PCC to form the -butyrolactone 248 [135]. Mechanism of oxidation of some ortho-, para- and metasubstihtted trans-cinnamic acids 249 by PCC in the presence of oxalic acid 250 was studied. The main product of oxidation was the corresponding benzaldehydes 251 and glyoxalic acid 252. The reaction was first order with respect to [PCC] and [oxalic acid]. The oxidation followed fractional order in [substrate] and [H+]. The

thermodynamic parameters were also determined. Both the electron-releasing and electron withdrawing substituents facilitated the rate of oxidation (Scheme 79) [136]. Oxidation of the 254 using PCC at 25°C yielded the unexpected 2-methoxycarbonyl-3-[(2methoxycarbonylamino-1-methoxycarbonyl-2-methyl)-vinyl]indole 255. As, electrophilic attacked by the protonated nitroso substituent of the 4-(2-nitrosophenyl)-1,4DHP intermediate product at C-3 of the DHP ring to give an unstable tetrahydroindino[2,1-c]pyridine which has undergone cleavage of the C2-C3 bond and aromatization to the isolatable indole prod-


CHO


25

CHO

PCC/CH2Cl2 O

258

259

CHO

CHO

PCC/CH2Cl2 O

OH 261 OH

260

O Scheme 81.

H

O

CHO

OH PCC, 3A mol.sieves + CH2Cl2, r.t, 8 h R

R

R 5

262

263

R= H, Cl, Br, OH, OMe, OCH2Ph, Ar R1

R1

R1

O

CHO

OH PCC, 3A mol.sieves + CH2Cl2, r.t, 8 h R

R 264

R 4

265

R= H, Cl, Br, OH, OMe, OCH2Ph, Ar R1= Me, Et, Ar Scheme 82.

uct 255. Oxidation of the C-3 hydroxylamino compound 256 with PCC also afforded the C-3 nitroso product 257 in 80% yield (Scheme 80) [137]. The unmodified retinal was oxidized by PCC in CH2Cl2 to give 5,6-dioxo-5,6-secoretinal 261 in yield of 47% predominantly. The process of the cleavage of trimethylcyclohexene ring 258 proceeded under comparatively mild conditions through the intermediate 5,6epoxide 259 and glycol 260. The presence of 5,6-dihydro-5,6epoxyretinal 259 has confirmed the reaction pathway (Scheme 81) [138]. A new PCC-mediated carbon-carbon bond cleavage reaction during oxidation of homobenzylic alcohols 262, 264 lead to the formation of benzylic carbonyl compounds. Homobenzylic alcohols with no benzylic substitution (R1= H) 262 gave benzylic aldehydes 5 without further oxidation, while those with benzylic substitution (R1= Me, Et, Ar) 264 gave benzylic ketones 4. This was one of the rare reactions of PCC where degradation of one carbon occurred and yet the end product remained an aldehyde or ketone without further oxidation. Such a reaction will be very useful in analyzing functional group compatibilities in designing oxidation reactions involving PCC (Scheme 82) [139].

Synthesis of 10-membered bislactones 270 by PCC oxidation was described. The use of PCC and PCC adsorbed on silica gel or alumina for the regioselective oxidation of vicinal diols in sugars was also reported. This intermediate 267 suffered a nucleophilic attack by the starting material. Oxidative cleavage of the indicated C–C bond led to the lactone functionality. Both bislactones showed antifungal activity against Candida albicans, and were slightly active against the bacteria Bacillus subtilis (Scheme 83) [140]. T. Caserta et al. developed the reaction of 220 in the conditions [PCC (5 equiv), AcOH (70 equiv), Celite in CH2Cl2]. This reaction led to produce a 6:1:6 mixtures (52% overall yield) of the two mono-lactones 271 and 272 and the bis-lactone 273, from the oxidative cleavage of one or both the three-carbon 2-hydroxypropyl termini. Under forcing conditions (PCC: 10 equiv, AcOH 150 equiv, 60 h) bis-lactone 273 were obtained as the sole reaction product in a 70% yield (Scheme 84) [141]. The stereoselective synthesis of simplactone B 277 through a series of nine steps in 85% overall yield was achieved. Oxidation of 274 using excess of PCC in refluxing conditions in benzene resulted in the lactone 275 in 83%. This oxidation was presumed to proceed as shown in Scheme 85 [142, 143].

26


Heravi al.

OPiv OPiv

OPiv

MeO

OH

OPiv OH

OPiv

MeO

O

MeO

O

O

HO

PCC

O

O

O

+

O OH

O

O

HO

OMe

OH

OMe

OPiv 267

266

PivO

PivO

OPiv

PivO

269

268 O

OPiv

PivO O

MeO

O

PCC

270

O OMe

O PivO

OPiv O

Scheme 83. PCC, AcOH, celite O

O

HO

O

O

O

CH2Cl2, 24 h

OH

O

O

O

HO

220

+

O

PCC, AcOH, celite CH2Cl2, 60 h

O

O

O Major monolactone

O

O +

O

O

O

O

O

O 271

O

OH

Minor monolactone

O

O

Bis-lactone

272

O

273

Scheme 84.

PCC, benzene, reflux OH O

TFA, DCM

TFA, DCM rt, 2 h, 85%

6 h, 83%

O

274

O

O

OMOM

O rt, 2 h, 85%

O

OMOM

OMOM

PCC OH

O

O

277

276

275

OMOM

OH

OH

OMOM

OMOM

+

OH

O

O

O

O

274

278

279

275

HCHO

O

Scheme 85.

S. Barradas described the first enantioselective synthesis of the natural decanolide 284 and its epimer in only the eighth and seven steps, respectively. The key formation of the macrolactone ring was carried out by the use of a high-yielding PCC mediated oxidative cleavage of a bicyclic intermediate 281 (Scheme 86) [144].

As alcohol 285 was used in the study, a mixture of 7S- and 7Rhydroxymethyl-epimers in the ratio ~6: 5, the necessary condition of the essential racemization should be the process where each of these epimers would react with PCC giving a single enantiomer 286 (Scheme 87) [145].


RO

(R)


H O

O

Me

O

RO

PCC, NaOAc

O

Me

HO

O

Me

Al (Hg), THF/EtOH/H2O, r.t, 95%

CH2Cl2, r.t, 1 h, 68% O

27

HO

OH

O

O

O 364

280

R= H

281

R= TBDMS

282

R= TBDMS

283

R= H

TBDMSOTf, 2,6-lutidine, CH2Cl2 0 oC to r.t, 30 min, 95%

Pyr· HF, CH3CN 0 oC to r.t, 2 h, 81%

Scheme 86.

, - unsaturated, cinnamaldoxime was deoximated without any difficulty (Scheme 88b) [147].

O

PCC

a

O CH2Cl2, 20

R1

oC

N

OH

R2

287

O 285

O

MW

R2

OH

R1

PCC, CH2Cl2

4 b

286

NH+ ClCrO3-

H3C

Scheme 87.

Regeneration of carbonyl functions 4 from oximes 287 could be accomplished by use of PCC under microwave irradiation within a short time without using any solvent within same reaction time and the same yield. In this method, the reaction tolerated many sensitive functional groups and even hindered oximes (camphor oxime) underwent deoximation in excellent yield and ,- unsaturated oximes were successfully deoximated too (Scheme 88a) [146].

CH2Cl2, rt Scheme 88.

A new accidental route was reported to prepare 1,2-diketone by unexpected C–C bond cleavage using PCC. The common compound 289 was not formed. Excess PCC cleaved the C–C bond of 4-keto-2-hydroxy ester 288 to provide 1,2-diketones 290. Through a [2,3]-sigmatropic rearrangement, the alpha carbon of ketone was oxidized and undergoes following decomposition eliminating two equivalents of carbon monoxide and one equivalent of methanol (Scheme 89) [148].

-Picolinium chlorochromate (-PCC) under non-aqueous conditions was also applied to aromatic aldoximes as well as aromatic ketoximes. Cyclic and acyclic aliphatic aldoximes and ketoximes were also cleaved at carbon–nitrogen double bond successfully and O

O

O OH

O

O O N H

O OCH3

N H

N H 279

288

290

O

O

O

O

O

OH PCC O OCH3

N H

N H OH

O

O

Cr O

O

O

O OMe

O O

O

O O N H Scheme 89.

N H

OMe

HO

OCH3

O

N H

OCH3

Cl OMe

O

O N H

O

Cr OH

Cr

O

O N H

28


Heravi al.

6. OXIDATION OF THIOLS AND SULFIDES TO THE CORRESPONDING DISULFIDES AND SULFOXIDES

+ 3 C2H5SC2H5 + 2 C2H5NHCrO3Cl- + 6 H+

PCC in solution and solvent free conditions performed oxidative coupling of aromatic and aliphatic thiols 291 to the corresponding disulfides 292. Omitting the solvent did not change the reaction time and product yield significantly, while the need of using the solvent was suppressed and workup procedures became easier. The symmetrical disulfides 292 was also performed in the presence of PCC and its silica gel supported form. The reactivity of silica gel supported -PCC for thiophenol derivatives was more than the reactivity of PCC and -PCC (Scheme 90) [149].

3 C2H5SOC2H5 + 2 C5H5NHCl- + 2 Cr(III) + 3 H2O

O-Py+H

O O

R= alkyl, aryl

O + O-Py+H Cr

C2H5 S

RS-SR

291

+

+

C2H5

292

H+

Cl

A or B RSH

O + O-Py+H Cr HO Cl

K

Cr

HO

A: PCC, Solvent Free, r.t B: PCC, Dichloromrthane, r.t

C2H5

Cl

O

Cr O

S

S+

O

Cr Cl

C2H5 C2

292

OH

R

R

S

O-Py+H

C2H5 S

C2

2,6-DCPCC R

O

Cr+

+

C2H5

HO

r.t, MeCN 293

294

R= alkyl, aryl

The kinetics of oxidation of diethyl sulfide (DES) by PCC was studied in 50% acetic acid-50% water (v/v) mixture. Diethyl sulfoxide was the product of the reaction. A mechanism involving the formation of a complex between protonated PCC and sulfide in a slow step was proposed (Scheme 92) [151]. 7. OXIDATIVE POLYMERIZATION S. Palaniappan and C. A. Amarnath introduced the use of pyridinium chlorochromate as an oxidizing agent in order to oxidize aniline 295 to polyaniline salts 296 via an aqueous polymerization

NH2

pathway in the presence of protic acids such as sulfuric, nitric, hydrochloric and phosphoric acid. The results of the polyaniline salts 296 prepared by pyridinium chlorochromate as an oxidizing agent were compared with those prepared by ammonium persulfate. Generally, aniline was oxidized using ammonium persulfate as an oxidizing agent, which was unstable. In this work, PCC was employed for polymerization of aniline. The optimum reaction conditions for the polymerization of aniline by PCC were: aniline, 1 ml; PCC, 2 gr; reaction temperature, 5-30 oC; reaction time, 30 min; protic acids (a) sulfuric acid (b) hydrochloric acid (c) phosphoric acid (d) nitric acid [152].

HA, H2O, PCC

NH

30 oC HA= H2SO4, HNO3, HCl, H3PO4

n (H+ A-) y

295

296 HA/SDS/35 oC/30 min NH2

NH

Cl

Scheme 92.

Scheme 91.

PCC HA= HCl, H2SO4, HNO3

NH

N

N n

(H+A-) x (DHS) y

Scheme 93.

O-Py+H

C2H5

r.t, MeCN O

S

+

K O + O-Py+H slow Cr HO Cl

RS-SR

291

R

+ H+

C2H5

2,6-DCPCC

Cl

O + O-Py+H Cr HO Cl

K

Cl

C2H5

C1 OH

Cl

HO

O-Py+H

O

Cr +

Cr+

+

C2H5

M. Tajbakhsh and his co-workers showed that 2,6-DCPCC was an efficient, rapid, mild, and inexpensive reagent for the oxidation of aliphatic, aromatic, and heteroaromatic thiols 291 and sulfides 293 to the corresponding disulfides 292 and sulfoxides 294. Thiols 291 were oxidized in acetonitrile with 2,6-DCPCC to give the corresponding disulfides 292 at room temperature. Sulfides 293 were also oxidized with this reagent to the corresponding sulfoxides 294 in 80–96% yield in acetonitrile (Scheme 91) [150].

O-Py+H

O-Py+H S

C1

RSH

S+

C2H5

Scheme 90.

O-

C2H5

K slow



O

O

THF

O

O

O

O

PCC

297

29

n

298

Scheme 94.

R

H

EtO2C

H CO2Et PCC

a H3C 299

N

EtO2C

CH2Cl2

CH3

CO2Et

R= alkyl, aryl

EtO2C

CO2Et

or H3C

N

H b

R

CH3

H3C

300

CPCC

N

CH3

301

CH3CN/reflux

Scheme 95.

The results of a research article in 2005 also showed that it was possible to prepare polyaniline salts 296 such as PANI-DHSH2SO4, PANI-DHSHNO3 and PANI-DHS-HCl with excellent yield and reasonable conductivity by chemical oxidative polymerization using PCC. SDS surfactant led to solve the polyaniline salts in organic solvents. This surfactant played a dual role of efficient surfactant and dopant (Scheme 93) [153]. The use of conductive polymers in the structure of metallic conductors and semiconductors attracted much attention in the literature. Because of chemical and electrical stability of aromatic heterocyclic polymers, they are an important class of conductive polymers. Polyfuran 298 was predicted to be a good conductive polymer but it was much less stable than polypyrrole and polythiophene. The reason was that the ring opening by nucleophilic reagents was performed especially in the oxidation conditions. R. M. McConnell and her co-workers demonstrated that polyfuran could be produced in reasonable yield through chemical polymerization and also showed that excessive ring opening could be avoided through the use of anhydrous conditions during the polymerization process. Thus, the chemical synthesis of polyfuran using a mild oxidizing agent, PCC, was described. The quality of the polymer was also improved, and the possibility of ring opening was minimized (Scheme 94) [154].

9. OXIDATION OF INDOLES AND AZAINDOLES TO ISATINS An efficient method was reported for the preparation of isatins and oxindoles 303 by oxidation of indoles 302 using PCC, catalyzed by polyaniline salt in dichloroethan. The advantages of using polyaniline salt catalyst were the ease of synthesis and handling, versatility, simple work-up procedure, mildness and recyclability (Scheme 96a) [156]. Another method was using PCC–silica gel (PCC-SiO2) in the presence of Lewis acid catalyst aluminium chloride (AlCl3) in dichloroethane. Simplicity of the reaction conditions, low cost, easy work-up procedure, and good yields were the advantages of this protocol (Scheme 96b) [157]. O R2

R2 a

O

Mohammadpoor Baltork and his co-workers reported the use of 3-carboxypyridinium chlorochromate, an inexpensive, stable and easily prepared oxidant for the effective oxidation of different types of 4-substituted 1,4-dihydropyridines 299 to their corresponding pyridines 300, 301 in refluxing acetonitrile (Scheme 95b) [146].

DCE, 80 oC

N

X 302

R1 b

PCC-SiO2 AlCl3

N

X 303

R1

DCE, 80 oC

8. OXIDATIVE AROMATIZATION Diethyl-1,4-dihydro-2,6-dimethyl-3,5-pyridine dicarboxy fates 299 could be oxidized under neutral conditions using PCC adsorbed on a solid support. J. J. V. Eynde and his co-workers found that PCC was remarkably efficient in oxidizing 1,4-DHP in dichloromethane. Because of the formation of black gums at the end of the reactions, they employed PCC in conjunction with a solid (alumina, silica gel, or montmorillonite K-10 clay) in order to obtain superior results. Therefore the work-up procedure was easier. Such supported systems were prepared by adding the solid to a solution of PCC in acetone and by removing the solvent under reduced pressure. The obtained orange powders could be dried at 100 oC and stored for several weeks without any decrease of their efficiency (Scheme 95a) [155].

PANI, PCC

Scheme 96.

10. OTHER REACTIONS The mechanism of oxidation of substituted styryl 4-biphenyl ketones with PCC was studied in 90% acetic acid-10% water (v/v) containing perchloric acid and NaClO4 at 10 o, 20 o, 30 o and 40 oC. A mechanism involved in the nucleophilic attack of PCC leading to an unsymmetric intermediate from which epoxides were formed (Scheme 97) [158]. The mechanism of oxidation of aromatic acetals 306 by PCC in the presence of acetic acid and water was also studied. By increasing the amount of acetic acid in its aqueous mixture, the rate of the reaction has increased. The reaction did not induce polymerization of acrylonitrile. The presence of electron withdrawing substituents on the benzene ring enhanced the rate of oxidation. The rate of oxidation depended on the nature of alkyl group (Scheme 98) [159].

30


Heravi al.

O C6H4

X

C H

C H

C

O PCC

R

X

C6H4

C

R

O 304

305

,

R=

O X= H, p-OH, p-OCH3, p-CH3, m-CH3, p-Cl, m-Cl, m-NO2, p-NO2

Cl

+ +

O

PCC

Cl

Cr

O Cl

OH

O

+

O-

O

PyH+

H+

Cr

Cr

+

slow OH

H

O

O

O-

+

+

OH

OH

O Cr

+

Cl

OTransition State

fast

O

O Cl

Cr

O

O H

+

Cl

O

OH

OH

Cr

+

O

O

Cl

O

O

Cr

+

O

O

Scheme 97.

O

PCC CH(OR)2

C AcOH-H2O

Y

Y

OR

+ alkene

197

306

R= n-butyl, Y= p-NO2, m-NO2, m-Cl, p-Cl, p-Me, p-OMe Y= H, R= n-propyl, iso-butyl, n-butyl, iso-amyl, benzyl Scheme 98.

R2

R2 R1

R3

PCC, CH2Cl2, 0 °C

R1

R3

30–40 min, 85–95% N

N H

307 308

Scheme 99.

O R1

CH

O

X

PCC or ZCCNH CH2Cl2 or Solvent free

R2

R1

R2 4

309 R1= alkyl, aryl R2= H, alkyl, aryl X= TBDMS, TMS Scheme 100.

In a pathway to synthesize a set of functionalized azepines, the PCC was also used to prepare morphanthridines 308 from morphanthridines dihydromorphanthridines 307 (Scheme 307 (Scheme 99) [160]. 99) [160].

Primary and secondary tert-butyldimethylsilyl and trimethylsilyl ethers 309 have been converted to their corresponding carbonyl compounds 4 with oxidizing agent such as zinc chlorochromate nonahydrate (ZCCNH) or PCC in dichloromethane or under solvent free conditions in good yields (Scheme 100) [161]. The rearrangement of tricyclo[5.3.1.0]undecanols 310, 313 were performed with PCC or 10% aqueous HCl in THF. The rearranged product 311 and ketone 312 were separated by flash chromatography. Under acidic conditions, 311 was the only isolated product. The reactivity of alcohols 313 was also examined. When 313 (R= H) reacted with PCC in dichloromethane at room temperature, ketone 315 was produced in 80% yield. On the contrary, when 313 (R= H) was treated under acidic conditions, the rearranged



R R

31

O

OH PCC + or 10% HCl

310

312

311

OH R= H, Me

R

O

R

OH PCC

+ or 10% HCl OH

313

314

315

Scheme 101.

product 288 was obtained in 92% yield. In the case of 313 (R= Me), PCC as well as acidic conditions led to produce the corresponding rearranged compound 314 in up to 90% yield (Scheme 101) [162].

OR O R1

As depicted in Scheme 102, oxidation of 7-hydroxymethylindole 316 with PCC afforded 7-formyl-indole 317 86% yield [163].

R2

PCC, CH2Cl2, rt, 1.5h N H

R3

N H

CH2OH

CH2Cl2

R2

R1, R2= alkyl R1, R3=alkyl, H R= alkyl

322

316

R1

PCC

R3

323

317

CHO

OCH2Ph

O

Scheme 102.

PCC

The allylic methylene moiety in ethers 318, 319 was also oxidized with 3 equiv of PCC and pyridine in dry dichloromethane to produce styryllactone, [7R,6S,5S,4R]-7-epi-triacetoxy-(-)- goniotriol 320, 321 (Scheme 103) [164].

R1

R2 324

O

CH2Cl2

R1

R1, R2= alkyl R1= alkyl, R2=H

R2 4

Scheme 104. OAc

O

OAc

O

PCC, Py, DCM, reflux, 8 h Ph

Ph OAc

OAc

OAc

318

OAc

320 O

S. Gowrisankar and his co-workers reported a facile method for the oxidation of the corresponding exo-methylenetetrahydrofuran derivatives 325 with PCC/Ac2O and/or CrO3/aq H2SO4 in acetone to the ,-disubstituted- or ,,-trisubstituted--methylenec- butyrolactone derivatives 326 (Scheme 105) [166]. R1 R2

R1 R2 PCC or Jones

OAc

O

OAc

PCC, Py, DCM, reflux, 8 h Ph

R3

O

Ph OAc

325

OAc 319

X

OAc

R3

X

O

326

OAc

321

Scheme 103.

J. Cossy and his co-workers demonstrated that oxidation of alkoxyallylic ethers 322 by PCC produced the corresponding , unsaturated ketones or aldehydes 323. The oxidation of benzylic alcohols 324 also produced the corresponding ketones 4 in good yields (Scheme 104) [165].

R1= CO2Me, CO2Et, CN, R2= Bn, CO2Me, CO2Et, R3= aryl, H X= O, N-Ts Scheme 105.

3,5,6-Trisubstituted -pyrones 328 were also synthesized from the Baylis–Hillman adducts. 3-Benzylidene-5,6-diphenyl-3,4dihydropyran- 2-one 327 could be oxidized to benzoyl-5,6diphenyl- -pyrone 328 in 59% yield via PCC but could not be

32


Heravi al.

O

O

Ph

O 328 (59%)

Cl 2 , CH 2 PCC

O Ph

2 r.t, 1

O

Ph

h

Ph O

Ph Ph

Ph

O

329

327

Ph Ph

Scheme 106.

OH Ts

Ts Ph CrVI rt, 1h

Ph

O 331

330

OH

chair axial Ts

OCrO3H

OH

O

Ts

Ts

OH O

Cr

CrO3 O Ph

Ph 330

Ph Ts Ts

Ph

Ts

Ph

Ph HO O

OH

O OCrO2H

Cr

O

331

O

O

Scheme 107.

converted to 329 under various acidic or basic conditions. (Scheme 106) [167]. The attempted oxidation of selected allenols 330 with PCC afforded hydroxydienones 331 instead of simple oxidation products as depicted in Scheme 107. The formation of the observed products was rationalized via a series of sigmatropic shifts, followed by hydrolysis. For an initial goal to produce allenyl vinyl ketones, alcohol 330 was treated with PCC in dichloromethane giving hydroxyketone 331 in 60% yield. The use of PCC on silica gel or alumina gave 331, but in yields lower than PCC alone. Buffering the PCC with sodium acetate or with molecular sieves had basically no effect on reaction yield. Neither PDC, Collins reagent, nor Jones reagent was superior than PCC in effecting the conversion. Accordingly the simple use of PCC gave the most acceptable yield. Interestingly, all the products from secondary allenols, derived from aldehydes, were gained as single stereoisomers. Those obtained

from tertiary systems that lacked symmetry were often 1:1 mixtures of diastereomers. the mechanistic proposal for oxidative rearrangement of allenols has been shown in Scheme 107 [168]. CONCLUSIONS In this review, we have presented the application of the pyridinium chlorochromate (PCC) in the synthesis of organic compounds. PCC has a wide range of applications in the organic synthesis as an oxidant and it has been applied to the oxidation of various primary and secondary alcohols to the corresponding aldehydes and ketones. In addition, this reagent has been used in other oxidation reactions such as, 1) reductive oxidation of carboxylic acids and its derivatives to aldehydes 2) oxidative cyclization 3) oxidative cleavage 4) oxidation of thiols and sulfides to the corresponding disulfides and sulfoxides 5) oxidative of novel antibiotics.


CONFLICT OF INTEREST

Current Organic Synthesis, 2016, Vol. 13, No. ? [25]

The authors confirm that this article content has no conflict of interest. [26]

ACKNOWLEDGEMENTS Declared none.

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Revised: March 27, 2015

Accepted: April 01, 2015