A biosynthetic precursor 4 of the antitumor alkaloid camptothecin (1) has been isolated from alkaloid enriched extracts of Camptotheca acuminata. The structure of 4, which was determined both in the solid state by x-ray diffraction and in solution by NMR analysis, provides strong evidence in support of the proposed biosynthetic scheme for camptothecin (1). © 1990.