XY–ZH Systems as potential 1,3-dipoles. Part 8. Pyrrolidines and Δ5-pyrrolines (3,7-diazabicyclo[3.3.0]octenes) from the reaction of imines of α-amino acids and their esters with cyclic dipolarophiles. Mechanism of racemisation of α-amino acids and their esters in the presence of aldehydes

Abstract

Imines of α-amino acid esters with aromatic, heterocyclic, and aliphatic aldehydes generate azomethine ylides stereospecifically by a prototropic shift on heating in toluene. The azomethine ylides undergo cycloaddition to N-phenylmaleimide, maleic anhydride, and p-naphthoquinone via an endo-transition state to give racemic, single diastereoisomeric, cycloadducts. α-Amino acids undergo analogous cycloadditions, without decarboxylation, in hot acetic acid. Mechanisms of racemisation of α-amino acids and their esters in the presence of aldehydes are discussed. The pyrrolidine cycloadducts (22) are smoothly oxidised to the corresponding Δ5-pyrrolines (33) by dichlorodicyano-p-benzoquinone.