ISOLATION, STRUCTURE, AND SYNTHESIS OF COMBRETASTATIN A-2, A-3, AND B-2

Further investigation of the South African tree Combretum caffrum (Combretaceae) for murine P388 lyrnphocytic leukemia (PS) cell-growth inhibitory substances has led to discovery of three new active constituents designated combretastatins A-2 (5a, PS ED500.027 μg/mL), A-3 (56, PS ED50 0.026 μg/mL), and B-2 (3b, PS ED500.32 μg/mL). Both cornbretastatins A-2 and A-3 were found to markedly inhibit tubulin polymerization. The structure of each combretastatin was firmly established by a combination of high resolution (400 MHz) 1H and 13C nuclear magnetic resonance and mass spectral analyses followed by total syntheses. The conversion of methyl gallate (7b) to combretastatin A-2 via intermediates 7c → 7d → 7e → 7a and 6a → 5a illustrates the practical synthetic route utilized for obtaining these substances. The Wittig reaction employed as the penultimate step in obtaining combretastatins A-3, afforded predominantly the natural Z isomer.