Heterocycles by cycloaddition. Part 9. Bridged heteroannulenes by cycloaddition–extrusion–ring-expansion reactions of mesoionic compounds with benzocyclopropene. A methanothiazonine, a methanothionine, and a methanothiecinone

Abstract

A methanothiazonine (13) was formed by cycloaddition–extrusion–ring expansion of benzocyclopropene with a mesoionic oxathiazoliumolate (12). The reaction with a dithioliumolate (14) gave the cycloadduct (15), from which a methanothionine (16) and a methanothiecinone (19) were prepared. Attempts at similar reactions with several other mesoionic systems failed to give the cycloadducts, or the cycloadducts did not form the desired extrusion products. The methanothionine (16) isomerised thermally to a cyclohepta[c]thiophene (17). The degree of electron delocalisation of these bridged annulenes is discussed.