Synthesis of bilenes-b. Some side reactions

Condensation of a 5-ethoxycarbonyl-5′-formyl-2,2′-dipyrromethane with a fully alkyl-substituted 5-carboxy-2,2′-dipyrromethane in the presence of hydrogen bromide yielded a tripyrrene salt rather than a bilene-b salt. In the presence of trifluoroacetic acid with subsequent addition of hydrogen bromide, the bilene-6 salt obtained from the foregoing components was derived from the self-condensation of two molecules of the formyl component with elimination of one formyl group. The expected bilene-b salts were obtained from the above dipyrromethanes when the carboxy-component contained an electronegative substituent, such as acetyl or ethoxycarbonyl, on the non-carboxy-substituted ring.