X=Y-ZH systems as potential 1,3-dipoles. Part 42. Decarboxylative three carbon ring expansion of cyclic secondary α-amino acids via azomethine ylide formation.

Abstract

Cyclic 5- and 6-membered secondary α-amino acids react with formaldehyde and acetylenic dipolarophiles via azomethine ylide formation, cycloaddition and subsequent ring expansion to give 8- and 9- membered rings respectively. Ring expansion occurs by reaction of the bridgehead nitrogen of the initial bicyclic cycloadduct with a further molecule of dipolarophile to give a zwitterion which triggers the ring expansion. Dynamic p.m.r. studies show that ring inversion between pairs of mirror image conformations of the medium ring products are occurring with inversion barriers of 13-14.6 kcal/mol.