Letters in Peptide Science, 9: 207–209, 2002. KLUWER/ESCOM © 2003 Kluwer Academic Publishers. Printed in the Netherlands.
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Microwave assisted facile synthesis of amino acid benzyl ester p-toluenesulfonate and hydrochloride salts Ganga-Ramu Vasanthakumar, Basanagoud S. Patil & Vommina Venkata Suresh Babu∗ Department of Studies in Chemistry, Central College Campus, Bangalore University, Dr. B. R. Ambedkar Veedhi, Bangalore, 560 001, India (∗ Author for correspondence, e-mail:
[email protected]) Received 1 July 2002; Accepted 22 August 2002
Key words: benzyl ester, esterification, hydrochloride salt, microwave irradiation, p-toluenesulfonate salt
Summary A simple method for the synthesis of several amino acid benzyl ester p-toluenesulfonate salts from the corresponding amino acid and benzyl alcohol in presence of p-toluenesulfonic acid accelerated with microwave irradiation is described. Under similar condition, the amino acid benzyl ester hydrochloride salts have also been obtained by using thionyl chloride instead of p-toluenesulfonic acid in good yield and purity.
Introduction Microwave-assisted organic synthesis [1–5], introduced independently by Giguere [6] and Gedye [7] groups, is a environmentally friendly procedure for heating materials which is distinctly different from the well tested classical thermal route. Its main advantage [8] is due to the instaneous localized superheating of materials in an homogeneous and selective manner. Further, remarkable decrease in the time necessary to carry out reactions (upto three orders of magnitude), cleaner reactions coupled with easier work-up, selectivity leading to high purity in high yields, etc., are being explored very quickly in developing alternative pathways for the synthesis of organic molecules. There are few initial reports utilizing the microwave ovens in peptide and protein chemistries during 1989–1992 [9–13]. Both the hydrolysis of proteins into amino acids to determine the amino acid analysis [9,10] and the synthesis of polypeptides like poly(GAMD), poly(GALD), poly(GAVD), poly(GAVDH), etc., [11,12] using the microwave irradiation have been demonstrated. The use of microwave ovens in the solid phase synthesis of acyl carrier protein fragment 65–74 employing Fmoc-amino acids using either symmetrical anhydrides or active esters for coupling with a significant reduction in reaction time (2–6
min.) compared to classical coupling procedures with high yield was also accomplished [13]. Although microwave ovens have already found a place in organic chemistry laboratory, their utility in peptide chemistry is yet to be fully demonstrated. This communication deals with the rapid, simple and efficient synthesis of amino acid benzyl ester p-toluenesulfonate and hydrochloride salts using an unmodified domestic microwave oven.
Materials and methods The melting points were determined using a LeitzWetzlar melting point apparatus and are uncorrected. The optical rotations were measured with an automatic AA-10 polarimeter (Optical Activity, U. K.). The I.R. spectra were recorded using a Nicolet Impact 400D I.R. spectrometer (KBr pellets, 3 cm−1 resolution). 1 H NMR spectra were recorded on a Bruker AACF 400 MHz spectrometer using Me4 Si as an internal standard. A commercial, domestic microwave oven (LG little chef model 194A) operating at 2450 MHz frequency was used in all experiments.
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Scheme 1. Synthesis of amino acid benzyl ester p- toluenesulfonate salts under microwave irradiation Table 1. Amino acid benzyl ester saltsa No
Compound
Time (sec)
Yield (%)
I.R. ν max . cm−1
Observed m.p. (◦ C)
Reported14 m. p. (◦ C)
1 2 3 4 5 6 7 8 9 10 11
GlyOBzl.TsOH AlaOBzl.TsOH ValOBzl.TsOH LeuOBzl.TsOH IleOBzl.TsOH PheOBzl.TsOH TyrOBzl.TsOH GlyOBzl.HCl AlaOBzl.HCl ValOBzl.HCl PheOBzl.HCl
60 50 50 40 40 60 50 60 50 50 60
93 95 94 96 94 97 94 92 93 94 95
1940 1944 1945 1935 1940 1945 1942 1935 1942 1940 1945
132 115–117 157 159–60 153–155 171–172 180 138–140 141–142 138 204–206
132–134 116–118 158–160 158–160 153–154 170.5 179–180.5 140 140 138–139 203–205
a All the compounds gave satisfcatory 1 H NMR data
Synthesis of amino acid benzyl ester p-toluenesulfonate salts: General procedure A mixture of amino acid (10 mmol), benzyl alcohol (3 mL) and p-toluenesulfonic acid (11 mmol) was placed in a 100 mL glass beaker and exposed to microwave irradiation operating at its 40% power. The reaction mixture, after the completion of the reaction, was cooled to room temperature and precipitated with ether (25 mL). The crystalline p-toluenesulfonate salt of amino acid benzyl ester was collected on a filter, washed with ether and dried in air. Synthesis of amino acid benzyl ester hydrochloride salts: General procedure A similar procedure used for the synthesis of amino acid benzyl ester p-toluenesulfonate salt as described above was employed for the synthesis hydrochloride salts of amino acid benzyl esters except that instead of p-toluenesulfonic acid an equimolar quantity of freshly distilled thionyl chloride was used. After the
precipitation of the hydrochloride salt, the crystalline compounds were washed using ether three to four times in order to ensure the complete removal of traces of thionyl chloride.
Results and discussion In the present studies, the synthesis of amino acid benzyl ester p-toluenesulfonate and hydrochloride (scheme 1) salts has been accomplished in a facile way utilizing microwave irradiation. The esterification reaction was carried out by exposing the slurry of amino acid, benzyl alcohol and p-toluesnesulfonic acid in a beaker to microwaves in an unmodified domestic microwave oven operating at 2450 MHz frequency at its 40% power. The benzyl ester formation, as monitored by TLC (n-butanol : acetic acid : water :: 4 : 1 : 1 ), is found to be complete in about 40 to 60 seconds. The addition of dry ether to the resulting gum in the same beaker resulted in the precipitation of the benzyl
209 ester p-toluenesulfonate salt as a crystalline solid in about 92–97% yield (Table 1). All the amino acid benzyl ester p-toluenesulfonate salts prepared have been found to have the optical rotation similar to the values reported in the literature [14] {eg, LeuOBzl.TsOH, observed: [α]25 D –1.75 (c = 1.2, methanol), reported: –1.7 (c = 1.2, methanol); PheOBzl.HCl, observed: [α]25 D –22 (c = 2, 0.25N HCl), reported: –23 (c = 2, 0.25N HCl)}. They were also characterized by the presence of a peak characteristic of the C=O stretching vibrational frequency of the benzyl ester at around 1935–1945 cm−1 . Their 1 H NMR analysis also confirmed their characterization. A similar procedure has been employed for the synthesis of several amino acid benzyl ester hydrochloride salts employing thionyl chloride (Table 1). Further, the resulting amino acid benzyl esters have been used to synthesize Fmoc-Val-Gly.OBzl and Fmoc-Phe-Leu.OBzl. A comparison of the analysis of these peptides by 1 H NMR revealed no differences among the peptides made using amino acid benzyl esters prepared by the present method with the data available in the laboratory.
Conclusions In summary, the classical method for the preparation of benzyl esters of amino acids involves refluxing a suspension of amino acid, p-toluenesulfonic acid and benzyl alcohol mixture for about 3-4 hr and to obtain better results, the esterification is carried out in a benzene solution and the water is removed by azeotropic distillation [15–20]. On the other hand, in the present study, it has been found that amino acid benzyl esters can be prepared in a facile way by microwave irradiation. The process is rapid, efficient and simple. And, the yields are also good.
Acknowledgements We thank Council of Scientific & Industrial Research, Govt. of India for financial assistance. Some of the equipments used in the execution of the work were purchased out of a DST grant which we also wish to acknowledge. VVSB thanks DBT, Govt. of India for a over-seas associateship award. We also thank Professor N. M. Nanje Gowda for permitting to use I.R. spectrometer and S.I.F., I. I. Sc., Bangalore for providing NMR data.
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