Proxiphomin und Protophomin, zwei neue Cytochalasane

Two new cytochalasans, proxiphomin and protophomin have been isolated from cultures of a Phoma species (strain S 298). They are shown to possess the same skeleton as deoxaphomin ( 4 ), which has been isolated recently. Thus they represent lower states of oxidation of 4 . On the basis of the spectral data, the structures 16‐methyl‐10‐phenyl‐[13]cytochalasa‐6 (7), 13 t ,21 t ‐triene‐1,23‐dione ( 1 ) and 16‐methyl‐10‐phenyl‐[13]cytochalasa‐5 (6)13 t ,21 t ‐triene‐1,7,23‐trione ( 2 ) are proposed for proxiphomin and protophomin respectively. On treatment with perchloric acid, substance 1 is isomerized to give 16‐methyl‐10‐phenyl‐10‐pheny‐10‐phenyl‐[13]cytochalasa‐5(6)13 t ,21 t ‐triene‐1,23‐dione ( 3 ). The assumption that the [13]cytochalasans are potential biogenetic precursors of the 24‐oxa‐[14]catochalasans is discussed.