Structural factors affecting the direct exchange racemisation of benzyloxycarbonyl-S-benzyl-L-cysteine pentachlorophenyl ester and related compounds by triethylamine in chloroform

Abstract

The rates of racemisation by triethylamine in chloroform of a series of acyl-amino- and -imino-acid pentachlorophenyl esters related to benzyloxycarbonyl-S-benzyl-L-cysteine pentachlorophenyl ester have been determined. It is shown that the optical lability of benzyloxycarbonyl-S-benzyl-L-cysteine pentachlorophenyl ester is not due to a simple electron-withdrawing effect exerted by the sulphur atom on the α-CH, but that an effect involving stabilisation of negative charge at the α-carbon by a bridging interaction with the sulphur atom across the β-methylene group operates. It is pointed out that the enhanced α-CH acidity of thiol esters, which has hitherto been explained by a resonance argument involving the carbonyl group, may be open to the same interpretation. It is also proposed that the relatively high optical stability of most acylimino-acid derivatives is due to steric interference with conjugation in the carbanion formed by ionisation at the α-carbon.