title: The rational synthesis of chlorins via rearrangement of porphodimethenes: influence of beta-substituents on the regioselectivity and stereoselectivity of pyrroline ring formation.
abstract: The porphodimethene rearrangement methodology reported in this paper provides for a rational, step-by-step synthesis of chlorins from readily available pyrrole precursors. The intermediate porphodimethenes are furnished directly via the '2 + 2' MacDonald condensation, or by the less symmetry-constrained '3 + 1' condensation of a tripyrrane and bis-formyl pyrrole. The synthetic route is short and highly convergent, especially in the case of the '3 + 1' approach, and furnishes chlorins in good to moderate yields. The synthesis is highly regioselective and appears to be based on the ability of the beta-substituent to stabilize excess electron density, with an electron-neutral hydrogen or an electron-withdrawing carbonyl beta-substituent demonstrating the greatest influence on the formation of the pyrroline ring. The synthesis is highly stereoselective when epimerization of the pyrroline ring beta-carbons is possible, furnishing only the trans-reduced sterioisomer. Finally, there is substantial evidence that a fifth, axial ligand is involved in the transposition of peripheral hydrogens during the rearrangement of the pi-system from metalloporphodimethene to metallochlorin.
authors.affiliation: Department of Chemistry, Wichita State University, Wichita, Kansas 67260-0051, USA. Dennis.Burns@wichita.edu
authors.forename: Dennis H
authors.initials: DH
authors.lastname: Burns
authors.forename: Yue H
authors.initials: YH
authors.lastname: Li
authors.forename: Dong C
authors.initials: DC
authors.lastname: Shi
authors.forename: Timothy M
authors.initials: TM
authors.lastname: Caldwell