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.