Biosynthesis of the Cytochalasans. The Mode of Incorporation of L-Methionine into the Cytochalasins A and B, Protophomin and Proxiphomin*

Incorporation of [l3C,2H3-CHs]-L-methionine into cytochala-: sin A (I), cytochalasin B (II), proxiphomin (III) and protophomin (IV) by Phoma exiqua var. exiqua demonstrated that the two C-methylations occur with retention of all three H-atoms of the methyl group. Conclusions are drawn concerning the stage of the methylation reaction during the biogenetic pathway. On the basis of the results amore detailed general scheme of the cytochalasan biogenesis is proposed,