Metal-catalyzed synthesis of isocoumarin derivatives

Chemistry of Heterocyclic Compounds 2016, 52(3), 149–151

Metal-catalyzed synthesis of isocoumarin derivatives (microreview) Zaman Ashraf1,2 1

Department of Biology, College of Natural Sciences, Kongju National University, Gongju 314-701, Republic of Korea 2 Department of Chemistry, Allama Iqbal Open University, Islamabad 44000, Pakistan; е-mail: [email protected] Published in Khimiya Geterotsiklicheskikh Soedinenii, 2016, 52(3), 149–151

Submitted December 16, 2015 Accepted February 2, 2016

of a number of medicinally important heterocycles.9–11 A number of conventional methods have been used for the synthesis of isocoumarins starting from homophthalic acids,12,13 but metal-catalyzed synthesis of isocoumarins has attracted considerable attention due to its economic advantages and good functional group tolerance.14,15 Recent reports on the Cu-, Pd-, Ru-, and Rh-catalyzed synthesis of isocoumarins are reviewed here. Besides, an example of Re catalysis has appeared recently.16

Isocoumarins (1H-isochromen-1-ones), an important class of naturally occurring unsaturated lactones, exhibit a broad range of pharmacological activities including cytotoxicity toward HeLa cervical cancer cells,1 antioxidant activity in HepG2 cells exposed to oxidative stress conditions induced by H2O2,2 antiangiogenic,3 anticancer,4,5 antifungal,6,7 and antimicrobial activity.8 Isocoumarins are also important building blocks for the synthesis Cu catalysis An efficient strategy for the synthesis of a variety of 3-substituted isocoumarins was illustrated by Cai et al.17 o-Halobenzoic acids 1 react with 1,3-diketones 2 in a copper(I)-catalyzed domino process in the presence of K3PO4 without added ligands to afford the corresponding 3-substituted isocoumarins 3 in 60–96% yields. A cascade intramolecular Ullmann-type coupling–rearrangement reaction of 1-(2-halophenyl)-1,3-diones 4 catalyzed by a copper(I) complex furnished the corresponding 3-substituted isocoumarins 3 in 61–87% yields.18 The Cu-catalyzed synthesis of 3-substituted isocoumarins 3 by coupling–cyclization of o-iodobenzoic acid 5 (R = H) with terminal alkynes 6 through a green chemistry approach was described by Chary et al. in 2013.19 The combination of CuI, K2CO3, and polyethylene glycol PEG400 facilitated the coupling–cyclization reaction under ultrasound agitation, and isocoumarins were obtained with remarkable regioselectivity. 3-Aryl-substituted isocoumarins were also obtained in a reaction of substituted 2-(trimethylsilyl)phenyl triflates with terminal alkynes catalyzed by an N-heterocyclic carbene copper complex.20

Zaman Ashraf was born in Gujranwala, Pakistan, in 1977. He obtained PhD degree in chemistry from the Quaid-iAzam University, Islamabad, in 2011. He is awarded the prestigious BK-21 PLUS postdoctoral fellowship by the Kongju National University, South Korea, in 2014. At present he is assistant professor and research group leader at the Allama Iqbal Open University, Islamabad. His research interests are isocoumarin synthesis, liquid crystal synthesis, and drug development through enzyme-based assay. 0009-3122/16/52(3)-0149©2016 Springer Science+Business Media New York

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Pd catalysis α-(o-Haloaryl)-substituted ketones 7 in arylhalide–enolate tautomeric form 8 are transformed into isocoumarins 9 through palladium-catalyzed carbonylation followed by intramolecular o-acylation.21 Substituted cyclic ketones have also been converted into the corresponding isocoumarins by using this strategy.

R2

R

Jiang et al. have demonstrated a three-component Pd-catalyzed process involving cyclization of 2-alkynylbenzoic esters and subsequent coupling of isocoumarin intermediates with alkenes producing 2,3-disubstituted isocoumarins in yields up to 90%.22

R2

R1 Cs2CO3 PhMe O X 110°C, 16 h R 7

R2 R1 O R

R1 O– X 8 CO, Pd2(dba)3 DPEphos R2 R1 O– PdL

R O O 9 R = H, F; R1 = Ar, R2 = Me; R1 + R2 = (CH2)n; X = Br, Cl

Cherry et al. described the synthesis of 3-substituted isocoumarins by the reaction of 2-iodobenzoic acids 5 with various allenyltributyltin reagents 10 in the presence of palladium acetate, triphenylphosphine, and tetrabutylammonium bromide in dimethylformamide. The substituted o-iodobenzoic acids can also be used as substrates to furnish the substituted isocoumarins 11 in 57–87% yields.23 2-Iodobenzoic acids 5 can also be converted into substituted isocoumarins 3 by a reaction with terminal alkynes 6 using a catalyst system of 10% Pd/C–Et3N–CuI–PPh3. The 3-substituted isocoumarins 3 were obtained in 60–78% yields with good regioselectivity when reaction was performed in ethanol.24 Fei et al. reported a novel and highly efficient strategy for the synthesis of isocoumarins through a palladium(0)catalyzed incorporation of tert-butyl isocyanide. First step is the cyclization reaction of α-(o-haloaryl)-substituted ketone 7 (R2 = H, X = Br) with tert-butyl isocyanide in the presence of palladium catalyst and base. The second step is a simple acid hydrolysis of the intermediate 12 leading to the corresponding 3-substituted isocoumarins 3 in 60–90% overall yields.25 Substituted benzoic acids 13 can be converted into isocoumarins 15 in 56–72% yields via ligandless palladiumcatalyzed oxidative coupling reaction with vinylarenes 14.26

Ru catalysis An efficient Ru-catalyzed synthesis of isocoumarin derivatives 18 (yields 65–89%) has been accomplished by the decarbonylative addition reaction of anhydrides 16 with alkynes 17 under thermal conditions.27 There are very few reports on the construction of isocoumarin heterocycle through a transition metal-catalyzed decarbonylative addition of anhydrides and alkynes. 150


Rh catalysis In 2014, a mild, efficient, and regioselective redox-neutral C–H bond activation–annulation protocol for the synthesis of isocoumarins was developed. The O–N bond was employed as the internal oxidant in the Rh(III)-catalyzed annulation, and the coupling of benzoates 19 and alkynes 20 was carried out to

construct the isocoumarin nucleus in compounds 9 (yields 70– 90%).28 Rhodium-catalyzed intermolecular cyclization of benzamides 21 and diazo compounds 22 for the construction of isocoumarins 9 (yields 63–93%) through C–H activation were demonstrated for the first time by Li et al. in 2015.29

Very recently 3-phenylisocoumarins 15 (R1 = H) were synthesized in 35–78% yields by Rh(III)-catalyzed oxidative coupling of benzoic acids 13 with electron-rich substituted vinyl acetates 23 by Zhang et al.30 The same catalyst was applied to a coupling of benzamides with cyclic alkenyl carbonates with following deamination– cyclization producing isocoumarins in 66–85% yields.31

Isocoumarins 25 were synthesized in 60–85% yields by Li et al. by using green synthesis approach. The substituted benzoic acids 23 were reacted with phenyl-disubstituted alkyne 24 under microwave acceleration in the presence of water as solvent and Rh/Cu as catalyst.32

References

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