Acid-Catalyzed Cyclization of 1,19-Unsubstituted a,c-Biladienes

A new route for the synthesis of corroles through cyclization of a,c-biladienes in acidic media is reported. Spectroscopic analysis of the reaction mixture shows that it proceeds with an intramolecular pathway through the formation of a totally conjugated open-chain tetrapyrrole. The presence of substituents at the 10-position of the tetrapyrrole strongly influences the fate of the reaction:  phenyl groups accelerate the cyclization to corrole, whereas in the presence of alkyl substituents this reaction does not occur. This effect has been examined by means of computational methods, and it has been demonstrated that conformational factors are dominant in driving the reaction toward the formation of the macrocycle.