A novel series of A-ring modified hexacyclic camptothecin derivatives containing a 1, 3- oxazine ring were first designed and synthesized. All of the hexacyclic camptothecins were assayed for in vitro cytotoxicity against nine human cancer cell lines. Among these compounds, 9b and 9c showed most potent cytotoxicity against several cell lines. Particularly, 9c was about 13-fold more potent than camptothecin, and about sixfold more ...