Porphyrins with Exocyclic Rings: Part 9 [1] Synthesis of Porphyrins by the ‘3 + 1’ approach

Tripyrrane dibenzyl esters are readily available from the reaction of two equivalents of 5-acetoxymethylpyrrole-2-carboxylates with 2,5-diunsubstituted pyrroles in refluxing acetic acid-ethanol. Hydrogenolysis of the benzyl ester moieties affords the corresponding dicarboxylic acids, and subsequent condensation with 2,5-diformylpyrroles in 5% trifluoroacetic acid-dichloromethane, followed by neutralization with triethylamine and oxidation with DDQ, gives porphyrin products in good to excellent yields. The isomeric purity of these products has been rigorously demonstrated by proton and carbon-13 NMR spectroscopy. This chemistry allows the direct synthesis of porphyrin structures that would otherwise only be accessible via multistep procedures. In addition, this approach provides a convenient synthetic entry to the geochemically important tetrahydrobenzoporphyrins and cycloalkanoporphyrins. The first examples of tripyrranes with tetrahydroindole subunits are reported and their utility in the synthesis of novel cycloalkanoporphyrins is disclosed.