Regioselective and stereoselective nucleophilic ring opening reactions of a phenyl-substituted aziridine: enantioselective synthesis of beta-substituted tryptophan, cysteine, and serine derivatives

The asymmetric synthesis of beta-phenyl-substituted cysteine, tryptophan, and serine derivatives was successfully developed. In this approach, the key intermediate, enantiomerically pure 3-phenylaziridine-2-carboxylic ester 7, was prepared from alpha,beta-unsaturated ester 1 by employing the Sharpless asymmetric dihydroxylation. The aziridine 7 was treated with 4-methoxybenzylthiol, indole, and acetic acid to give beta-phenyl-substituted cysteine, tryptophan, and serine, respectively, in a clean S(N)2 type ring opening at the C3 position. This general approach can be used to synthesize a variety of beta-substituted novel amino acids.