Barrenazine A and B; two new cytotoxic alkaloids from an unidentified tunicate.

[structure: see text] Two novel compounds, barrenazine A (1) and B (2), were isolated from an unidentified tunicate collected at Barren Islands, Madagascar. The two new compounds are of an unprecedented heterocyclic skeleton, namely 1,3,4,6,8,9-hexahydrodipyridino[3,4-b:3',4'-e]pyrazine. The structures of the two alkaloids were elucidated by interpretation of MS, COSY, HMQC, HMBC, NOESY, and (15)N-HMBC data. Barrenazine A exhibits mild cytotoxicity against LOVO-DOX colon carcinoma (with a GI(50) value of 0.9 g/mL)