Novel hexacyclic camptothecin derivatives. Part 1: synthesis and cytotoxicity of camptothecins with an A-ring fused 1,3-oxazine ring.

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 potent than topotecan toward HEPG-2. Furthermore, it was also found that the N-alkyl substituted derivatives were more potent than the N-aryl and N-benzyl substituted compounds against most cell lines.