An Improved Approach to 5'-Unsubstituted 5-Formyldipyrromethanes

Article abstract of DOI:10.1021/jo00220a030

A general synthesis of 5'-unsubstituted 5-formylpyrromethanes, 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 5-unsubstituted pyrrole-2-carboxylate ethyl esters in warm glacial acetic acid to give a series of 5'-(ethoxycarbonyl)-5-(2,2-dicyanovinyl)-2,2'-dipyrromethanes in high yield.Aqueous alkali was used to deprotect the dicyanovinyl substituent to regenerate the aldehyde and saponify the ethyl 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 ethyl ester.Hydrogenolysis released the carboxylic acid without affecting the cyanovinyl group, and the subsequent decarboxylation in neat trifluoroacetic acid accurred without rearrangement.Deprotection using aqueous alkali produced crystalline 5'-unsubstituted 5-formyldipyrromethane.The complete lack of rearrangement during the synthesis and manipulation of the dipyrromethanes was confirmed by 13C NMR spectroscopy.