A new route to meso-formyl porphyrins

Prior syntheses of porphyrins bearing meso-formyl groups have generally employed the Vilsmeier formylation of an acid-resistant copper or nickel porphyrin. A new approach for the synthesis of free base porphyrins bearing one or two (cis or trans) meso-formyl substituents entails the use of a dipyrromethane bearing an acetal group at the 5-position, a dipyrromethane-1-carbinol bearing an acetal group at the 5-position or carbinol position, or a dipyrromethane-1,9-dicarbinol bearing an acetal group at a carbinol position. Treatment of the resulting meso-acetal-substituted free base porphyrin to gentle acidic hydrolysis yields the corresponding meso-formyl porphyrin.