HETEROCYCLIC [3,2-B]-FUSED PENTALENES AND THEIR BENZOANNELLATED DERIVATIVES

The methods of preparation of heterocyclic [3,2-b]fused pentalenes and their benzoannellated derivatives are summarized. Tricyclic systems w ere prepared by linear or convergent syntheses, tetracyclic derivative s by convergent methods. Only the base-catalyzed cyclization seems to be a generally applicable method. Intramolecular Friedel-Crafts acylat ion together with acid-catalyzed dehydration are used for the annellat ion of thiophene ring in some more stable systems. Higa reaction and o ther 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 gamma-aminoketones are used for the formation of pyrrole ring.