Mesoionic Compounds as Building Blocks of Heterocycles.

Abstract

This review summarizes the authors' own works on the use of mesoionic compounds for the preparation of several heterocyclic systems. From six- to ten-membered cyclic conjugated heterocycles have been prepared by the dipolar cycloadditions of mesoionic compounds, and subsequent extrusions and ring expansions of the adducts. Mesoionic compounds react with 7-heteranorbornadiene derivatives to give two aromatic heterocycles by fragmentation of the ylide intermediates resulting from cycloaddition-extrusion. Other reactions covered in-clude [4π + 6π] cycloadditions, preparation of o-quinonoid heterocycles, cycloaddition-extrusion reactions with cumulenes, intramolecular cycloaddition-extrusion reactions, and tandem cycloadditions. Several features of mesoionic compounds as dipolar building blocks of heterocycles are briefly discussed.