Synthesis of unnatural enone-containing α-amino acids : precursors to chiral N-heterocycles

摘要

A fast and efficient synthetic route was developed for the synthesis of a novel class of enone-containing alpha-amino acid. An amino acid-derived beta-ketophosphonate ester was subjected to Horner-Wadsworth-Emmons conditions using a variety of aldehydes to produce a diverse library of alpha,beta-unsaturated amino acids. E-Configured enone-containing amino acids were also deprotected using a two-stage approach to give the parent alpha-amino acids. A minor modification to the route enabled the synthesis of Z-configured enones via the Still-Gennari reaction. A small library of Z-enones was produced using various aldehydes. Enone-functionalised alpha-amino acids were employed as substrates for an intramolecular cyclisation reaction to generate 6-substituted-4-oxopipecolic acids. A diastereoselective one-pot reductive amination/cyclisation strategy was developed to gain access to the anti-diastereomer of the chiral N-heterocycles. A small selection of 6-substituted-4-oxopipecolic acids was synthesised. 6-Substituted-4-oxopipecolic acids were also reduced diastereoselectively to generate 4- hydroxypipecolic acid analogues.