X=Y-ZH Systems as potential 1,3-dipoles: Part 19. Intramolecular cycloadditions of non-stabilised azomethine ylides generated via the decarboxylative route from α- amino acids

Abstract

Heating a range of acyclic and cyclic secondary α-amino acids with aryl aldehydes containing a proximate terminal alkene or alkyne results in consecutive condensation and decarboxylation, followed by intramolecular cycloaddtion of the resultant non-stabilised azomethine ylides. Evidence is produced for syn-anti dipole equilibration and it is found that intramolecular cycloadditions to a terminal alkyne involve only the anti-dipole and proceed via an exotransition state to give a single cycloadduct. In contrast, intramolecular cycloaddition to a terminal alkene involves both anti- and syn-dipoles, with trapping of the former being slightly energetically preferred resulting in mixtures of two stereoisomeric cycloadducts. Intramolecular cycloadditions of terminal alkenes to anti-dipoles proceed via exo-transition states whilst analogous reactions of the syn-dipole involve endotransition states.