Synthesis of an azabicycloalkane amino acid scaffold as potential rigid dipeptide mimetic

Short syntheses are presented of the pseudo-dipeptide (3S,6S)-6-[(benzyloxy)carbonyl]amino-5-oxo-1,2,3,5,6,7-hexahydro-3-indolizinecarboxylic acid (1a) and of its (3S,6R) diastereoisomer (1b). The key step involves adding vinylogous β-enaminoester derived from pyroglutamic acid on an acrylate derivative. The 6,5-fused bicyclic lactam obtained may be viewed as a conformationally restricted Ala-Pro mimetic.