Asymmetric synthesis of N-protected syn and anti (E)-3-amino-2-hydroxy-4-hexenoate:: A practical method for the C-α epimerization of anti β-amino-α-hydroxy acids

A practical method to convert anti beta-amino-alpha-hydroxy acids into the corresponding syn esters via the DCC/DMAP . HCl mediated esterification was devised, and methyl (2R,3S)-(E)-3-t-butoxycarbonylamino-2-hydroxy-4-hexenoate 15 was synthesised from the corresponding (2S,3S)-isomer 9 using the epimerization procedure developed. The alpha- and beta-stereogenic centres of 9 were constructed by the Michael addition of a homochiral lithium amide to t-butyl sorbate and subsequent oxidation of the enolate intermediate in one pot. (C) 1998 Elsevier Science Ltd. All rights reserved.