Further Observations on Conformational and Substituent Effects in Acid-Catalyzed “3 + 1” Cyclizations of Tripyrranes with Aromatic Dialdehydes

Abstract

Tripyrranes with tert-butyl and phenyl substituents have been prepared and used to synthesize oxybenziporphyrins, oxypyriporphyrins, benzocarbaporphyrins, and azuliporphyrins with phenyl and tert-butyl substituents via a "3 + 1" methodology. The proton NMR spectra for the tripyrrane dibenzyl esters indicate that these tripyrrolic systems take on a helical conformation that favors macrocycle formation, and the NMR data can be a useful predictor on the efficiency of the "3 + 1" synthesis. Nevertheless, a tetraphenyltripyrrane proved to be susceptible to acidolytic cleavage under the usual reaction conditions and gave poor yields of porphyrinoid products. This problem could be overcome to a certain extent by carrying out the reactions in neat TFA. The presence of these substituents led to significant changes in the spectroscopic properties and diatropic character of the new porphyrinoid structures.