Enantioselective synthesis of unsaturated amino acids using p-methoxybenzylamine as an ammonia equivalent

A versatile, non-alkylative enantioselective synthesis of unsaturated alpha-amino acids based on the Sharpless asymmetric epoxidation has been developed. Enantiomerically enriched trans epoxy alcohols bearing unsaturated substituents were prepared and submitted to regio- and stereospecific ring-opening with p-methoxybenzylamine as a nucleophile, leading to anti-3-(p-methoxybenzylamino)- 1,2-diols which were further protected by reaction with Boc(2)O. The 1,2-diol fragment was then oxidatively cleaved by a sequential treatment with sodium periodate and sodium chlorite to afford the corresponding amino acid. Using this methodology, doubly N-protected (p-methoxybenzyl and Boc) allyl glycine, 3-butenyl glycine and 4-pentenyl glycine have been prepared in three synthetic steps from the corresponding allyl alcohols. As a demonstration of the orthogonal nature of the nitrogen protection, both protecting groups have been selectively removed from the fully protected amino ester. (C) 1999 Elsevier Science Ltd. All rights reserved.