Wittig Reaction with N-Protected 3-(Triphenylphosphonio) alaninates: Synthesis of Optically Active (E)-(2-Arylvinyl) glycine Derivatives.

Abstract

(R)-[2-Carboxy-2-[(methoxycarbonyl)amino]ethyl]triphenylphosphonium chloride (1) was converted by treatment with anion exchange resin (HCO-3) into the inner salt 13h, which gave a better yield (43%) than 1 in the Wittig reaction with benzaldehyde to afford the [S-(E)]-(2-phenylvinyl)glycine derivative 24. The inner salt 13i bearing an N-benzyloxycarbonyl group was prepared by hydrogenolysis of (R)-[3-benzyloxy-2-[(benzyloxycarbonyl)amino]-3-oxopropyl]triphenylphosphonium chloride (11e) over palladium on carbon, followed by dehydrochlorination. Hydrogenolysis of 11e over Pearlman's catalyst afforded the unprotected phosphonium chloride 12 (X=Cl). N-tert-Butoxycarbonylation of 12 followed by dehydrochlorination afforded 13j, which was more efficiently prepared through hydrogenolysis of (R)-[3-benzyloxy-2-[(tert-butoxycarbonyl)amino]-3-oxopropyl]triphenylphosphonium chloride (11f).The usefulness of 13h-j as building blocks for the synthesis of configurationally labile (2-arylvinyl)glycine derivatives was exemplified by the Wittig reactions with piperonal, which exclusively afforded the (E)-isomers 18h-j with high optical purity in 28-39% yield.