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.

Article abstract of DOI:10.1016/S0040-4020(01)90417-X

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 occuring with inversion barriers of 13-14.6 kcal/mol.