Stereoselective synthesis of beta-benzyl-alpha-alkyl-beta-amino acids from L-aspartic acid

A stereoselective synthesis of beta-benzyl-alpha-alkyl-beta-amino acids 1 and 2 from L-aspartic acid 3 has been developed. Methyl 5-phenyloxazolidin-2-one-4-acetate 4 was prepared from L-aspartic acid 3 through the acylation of benzene or phenyllithium with alpha-amino carboxyl group of L-aspartic acid skeleton. Alkylation of a dianion of 4 with alkyl halides and subsequent hydrogenation afforded anti-disubstituted beta-amino acids 1b and 1c in high stereoselectivities. Complete reversal of the stereoselection was realized by the alkylation of 4-phenyl-3-tert-butoxycarbonylamino-4-butanolide 6 which was obtained in a single step from 4. The 2,3,4-trisubstituted amino lactone 7 thus obtained was hydrogenated to give a syn-disubstituted beta-amino acid 2a. The syn-products 2b, 2c, and 2dwere alternatively prepared via aldol condensation of 6 with aromatic or aliphatic aldehydes followed by stereoselective reduction of the double bond with nickel chloride-sodium borohydride.