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.
The one-step regioselective synthesis of a chlorin macrocycle from a tripyrrane and bisformylpyrrole is described for the first time; the chlorin is furnished in 46% yield and is constructed from readily prepared pyrroles.
A unified protecting-group-free approach to two stemarene and two betaerene diterpenoids through a bioinspired two-phase strategy has been developed, and three of them were obtained for the first time via chemical synthesis. Starting from a common intermediate, two distinct tetracyclic frameworks containing diastereoisomeric bridged bicycles were constructed by divergent ring reorganization strategy. Late-stage C−H functionalization through xanthylation-oxygenation protocol furnished the corresponding oxygenated stereocenters or oxo functionality in high regio-and diastereoselective fashion within a complex hydrocarbon system. The stereochemical puzzles in (-)-2-acetoxybetaer-13(17)-ene and (+)-7-acetoxybetaer-13(17)-ene were first predicted by the comparison of DFT-NMR data with the reported data and later were unambiguously addressed through the total syntheses of natural products and three diastereomers.
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