An Improved Synthesis of Covalently Strapped Porphyrins. Application to Highly Deformed Porphyrin Synthesis
Wijesekera, T.P., Paine, J.B. III, and David Dolphin
J. Org. Chem.
The title porphyrins I (n = 1, 2, 3) were prepared α,ω-Dicarboxyalkyl dichloride, was treated with 2 equivalents of 2-(ethoxycarbonyl)-3,5-dimethylpyrrole, and the chain-linked bis[5-(ethoxycarbonyl)pyrrole] so obtained was transformed into the pyrrole-2-carboxaldehyde by using standard methodology. Protection of the formyl groups as the dicyanovinyl derivative and the activation of the 2-Me substituents with SO2Cl2 gave the bis[2-(chloromethyl)pyrrol)], which on reaction with a 5-unsubstituted 2-pyrrole-carboxylate, in warm AcOH, afforded the chain-linked dipyrromethane dimer in high yield. Regeneration of the formyl substituents and removal of the ester group produced the 5-formyldipyrromethane dimer II, which was cyclized intramolecularly, under high dilution, to give I. II (n = 0) failed to cyclize.