Rational synthesis of trans-substituted porphyrin building blocks containing one sulfur or oxygen atom in place of nitrogen at a designated site

The use of heteroatom-substituted porphyrins in bioorganic and materials chemistry requires the ability to position a variety of substituents in a controlled manner about the porphyrin periphery. We describe a rational route to trans-AB(2)C-type porphyrins bearing one oxygen atom (N3O) or one sulfur atom (NBS) in a designated location in the porphyrin core. The synthesis involved four stages: (1) Acid-catalyzed condensation of a furyl- or thienylcarbinol in excess pyrrole afforded the aryl-substituted furyl- or thienylpyrromethane in high yield. (2) Treatment of the furyl- or thienylpyrromethane with an acid chloride catalyzed by SnCl4 or AlCl3 afforded the corresponding diketo product. (3) Reduction with NaBH4 in alcoholic solvents gave the furyl- or thienylpyrromethanediols. (4) Reaction of a furylpyrromethanediol, thienylpyrromethanediol, or dipyrromethanediol with a dipyrromethane in a one-flask process of condensation followed by oxidation gave the corresponding porphyrin. Reaction conditions previously identified to minimize scrambling in a dipyrromethane-aldehyde condensation were found to be effective in this application. Thus, reaction with 10 mM reactants in acetonitrile at 0 degrees C containing BF3 . Et2O and NH4Cl followed by oxidation with DDQ resulted in the desired porphyrin(10-20% yields) without acidolysis. In this manner, N3O-, N3S-, or Na-porphyrins bearing 5-(p-iodophenyl), 15-[4-(2-(trimethylsilyl)ethynyl)phenyl], and 10,20-di-p-tolyl groups have been made. This set of trans-substituted porphyrin building blacks is expected to be useful in the synthesis of biomimetic energy transduction systems.