Further investigation of the South African tree Combretumcaffrum (Combretaceae) for murine P388 lymphocytic leukemia (PS) cell-growth inhibitory substances has led to discovery of three new active constituents designated combretastatins A-2 (5a, PS ED50 0.027 μg/mL), A-3 (5b, PS ED50 0.026 μg/mL), and B-2 (3b, PS ED50 0.32 μg/mL). Both combretastatins 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.
对南非树木Combretum caffrum(锥花科)进行进一步研究,以寻找对小鼠P388淋巴细胞白血病(PS)细胞生长具有抑制作用的物质,发现了三种新的活性成分,分别命名为combretastatin A-2(5a,PS ED50 0.027 μg/mL)、A-3(5b,PS ED50 0.026 μg/mL)和B-2(3b,PS ED50 0.32 μg/mL)。发现combretastatin A-2和A-3均明显抑制微管聚合。通过高分辨率(400 MHz)的1H和13C核磁共振和质谱分析以及总合成,确定了每种combretastatin的结构。通过从甲基没食子酸酯(7b)经过中间体7c→7d→7e→7a和6a→5a的转化,说明了用于获得这些物质的实用合成路线。在获得combretastatin A-3的倒数第二步中采用的Wittig反应主要产生了天然的Z异构体。