Heterocyclic mesomeric betaines. Part 5. Synthesis and cycloaddition reactions of hetero derivatives of the 2-methylene-1,2-dihydro-1,3-phenalenylene dianion

Abstract

Conjugated heterocyclic mesomeric betaines (5; R = H, Me, Ph) which are isoconjugate with the 2-methylene-1,2-dihydro-1,3-phenalenylene dianion (2) have been synthesized. These heterocyclic mesomeric betaines could not be isolated, but they have been characterized by 1,3-dipolar cycloaddition with olefinic dipolarophiles. Cycloadducts (7a) and (8a) underwent an acid-catalysed retro-Michael reaction.