XY–ZH Systems as potential 1,3-dipoles. Part 8. Pyrrolidines and Δ<sup>5</sup>-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
Imines of α-aminoacidesters 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-Aminoacids undergo analogous cycloadditions