Heterocyclic [3,2-b]-Fused Pentalenes and Their Benzoannellated Derivatives

Abstract

The methods of preparation of heterocyclic [3,2-b]-fused pentalenes and their benzoannellated derivatives are summarized. Tricyclic systems were prepared by linear or convergent syntheses, tetracyclic derivatives by convergent methods. Only the base-catalyzed cyclization seems to be a generally applicable method. Intramolecular Friedel-Crafts acylation together with acid- catalyzed dehydration are used for the annellation of thiophene ring in some more stable systems. Higa's reaction and other cyclizations of sulphenyl and selenyl intermediates are used for the closure of thiophene and selenophene rings, whereas cyclization of nitrene intermediates, Fischer indol synthesis, and cyclodehydration of gama-aminoketones are used for the formation of pyrrole ring.