1 ',2 ',3 ',4 '-tetradehydrotubulosine, a cytotoxic alkaloid from Pogonopus speciosus

Bioassay-guided phytochemical investigation of the stems of Pogonopus speci osus, using human oral epidermoid carcinoma (KB) cells as a monitor, led to the isolation of a novel alkaloid, 1',2',3',4'-tetradehydrotubulosine (I), along with tubulosine (2) and psychotrine (3) as bioactive constituents. T he structure of the novel compound was elucidated through 1D- and 2D-NMR sp ectroscopic methods. Alkaloids 1 and 3 showed weak cytotoxic activity again st a panel of human cancer cell lines, with the potency of these compounds being markedly less than that of tubulosine (2).