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Incorporation of [1‐13C]‐, [2‐13C]‐ and [1,2‐13C2]‐acetate, [1‐13C]‐propionate, [13C‐CH3]‐L‐methionine and [3‐14C]‐DL‐tryptophan into chaetoglobosin A (1) and 19‐O‐acetylchaetoglobosin A (2) by Chaetomium globosum demonstrated that the building blocks of 1 and 2 are 9 and 10 units of acetate/malonate respectively, 3 units of methionine and 1 unit of tryptophan. Propionate is incorporated indirectly after several biological transformations. Using [2‐13C, 2‐2H3]‐acetate as precursor, the starter unit of the polyketide‐chain was identified. Experiments which [13C, 2H3‐CH3‐L‐methionine demonstrated that the three C‐methylations occur with retention of all three H‐atoms of the methyl group. Incorporation experiments with various 14C‐ and 3H‐labelled tryphtophan samples and with [2‐2H]‐ and [2‐15N]‐L‐tryptophan showed that the amino acid is incorporated intact with retention of both the α‐H‐ and the α‐N‐atom. On the basis of these results a more detailed general scheme of the cytochalasan biogenesis is proposed.

Two new metabolites have been isolated from cultures of Chaetomium globosum. The structures of 19‐O‐acetylchaetoglobosin B (4) and 19‐O‐acetylchaetoglobosin D (5) are assigned. The 13C‐NMR. spectra of chaetoglobosin A (1), 19‐O‐acetylchaetoglobosin A (2), chaetoglobosin C (3), 19‐O‐acetylchaetoglobosin B (4), 19‐O‐acetylchaetoglobosin D (5) and of cytochalasin G (6), a (3‐indolyl)‐[11]cytochalasan isolated from Pseudeurotium zonatum, have been interpreted.