Treatment of Phoma betae with P-450 inhibitors caused accumulation of biosynthetic precursors 2, 3 and 4 of aphidicolin (1). Their structures were elucidated by spectroscopic analysis and they were confirmed by chemical transformations from 1. Isotopically labeled precursors were synthesized by either reductive deoxygenation of aphidicolin derivatives or microbial conversion of [1-14C] acetate in the presence of the P-450 inhibitor. Feeding ...