An Improved Approach to 5'-Unsubstituted 5-Formyl-dipyrromethanes
Wijesekera, T.P., Paine, J.B., and David Dolphin
J. Org. Chem.
A general synthesis of 5'-unsubstituted 5-formyl-2,2'-dipyrromethanes, precursors to centrosymmetric porphyrins, is described. Cyanovinyl groups are used to protect the aldehydes and stabilize the dipyrromethanes. 5-(Chloromethyl)-2-(2,2-dicyanovinyl)pyrroles were condensed with Et 5-unsubstituted pyrrole-2-carboxylates in warm glacial AcOH to give a series of 5'-(ethoxycarbonyl)-5-(2,2-dicyanovinyl)-2,2'-dipyrromethanes I (R1, R2, R3, R4 independently = Me, Et) in high yield. Aqueous alkali was used to deprotect the dicyanovinyl substituent to regenerate the aldehyde and saponify the Et ester in a single step. The acid group was subsequently decarboxylated thermally to give the 5'-unsubstituted 5-formyl-2,2'-dipyrromethane. An alternative route was also designed by substitution of a benzyl for an Et ester. Hydrogenolysis released the carboxylic acid without affecting the cyanovinyl group, and the subsequent decarboxylation in neat F3CCO2H occurred without rearrangement. Deprotection using aqueous alkali produced crystaline. 5'-unsubstituted 5-formyldipyrromethane. The complete lack of rearrangement during the synthesis and manipulation of the dipyrromethanes was confirmed by 13C NMR spectroscopy.