Pyrroles and related compounds. Part XVII. Porphyrin synthesis through b-bilenes

t-Butyl 5′-formylpyrromethane-5-carboxylates condense with 5-t-butyloxycarbonylpyrromethane-5′-carboxylic acids to give crystalline, well-characterised b-bilene salts in high yield. The latter can be de-esterified and decarboxylated by cold trifluoroacetic acid to 1′,8′-unsubstituted b-bilenes which, on treatment with methyl orthoformate–trichloroacetic acid in presence of air, readily undergo cyclisation and oxidation to porphyrins. Several isomerically pure porphyrins have been prepared in this way, but there are limitations with certain complex cases, for example when an electron-withdrawing group is present in one of the pyrrole rings, and mixtures of porphyrins may then result.