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

The asymmetric synthesis of β-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 α,β-unsaturated ester 1 by employing the Sharpless asymmetric dihydroxylation. The aziridine 7 was treated with 4-methoxybenzylthiol, indole, and acetic acid to give β-phenyl-substituted cysteine, tryptophan, and serine, respectively, in a clean S_N2 type ring opening at the C₃ position. This general approach can be used to synthesize a variety of β-substituted novel amino acids.