Cytochalasins Z4, Z5, and Z6, three new 24-oxa[14]cytochalasans produced by Phoma exigua var. heteromorpha

Cytochalasins Z4, Z5, and Z6, three new cytochalasans, were isolated from the wheat culture of Phoma exigua var. heteromorpha together with the known cytochalasins A, B, F, T, Z2, and Z3, 7-O-acetylcytochalasin B, and deoxaphomin. Z2 and Z3, together with Z1, have recently been described as new 24-oxa[14]cytochalasans produced by Pyrenophora semeniperda, a fungus proposed for biological control of grass weeds. All three new cytochalasins were characterized as 24-oxa[14]cytochalasans by extensive use of NMR and MS techniques. Cytochalasins Z4 and Z5 proved to be structurally related to cytochalasin B, whereas Z6 was related to cytochalasin F. When assayed on tomato seedlings at 10(-4) M, Z6 caused a very low inhibition of rootlet elongation (30%), whereas Z4 and Z5 were almost inactive. On brine shrimp, at the same concentration, only Z5 had a minor effect, with 20% mortality, whereas the other two metabolites proved to be inactive.