Studies in terpenoid biosynthesis. Part VIII. The formation of the trichothecane nucleus

The incorporation of [2-3H2]-, (4R)-[4-3H]-, and [5-3H2]-mevalonate, [2-3H]geranyl pyrophosphate, and [1-3H2]-and [2-3H]-farnesyl pyrophosphate into the trichothecane nucleus is described. The results define the manner of folding of the farnesyl pyrophosphate and exclude a γ-bisabolene intermediate in the biosynthesis. The central prenyl (4R)-[4-3H]-label is retained in the biosynthesis and is transferred to C-2 of the trichothecane. The relative numbers of [2-3H2]- and [5-3H2]-mevalonoid labels that are incorporated suggest that the carbonyl group of trichothecin might arise through the rearrangement of an epoxide.