Natural Zinniol Derivatives from Alternaria tagetica. Isolation, Synthesis, and Structure−Activity Correlation

Two novel phytotoxins, 8-zinniol methyl ether (5) and 8-zinniol acetate (6), in addition to 6-(3‘,3‘-dimethylallyloxy)-4-methoxy-5-methylphthalide (2), 5-(3‘,3‘-dimethylallyloxy)-7-methoxy-6-methylphthalide (3), and the novel metabolites 8-zinniol 2-(phenyl)ethyl ether (4) and 7-zinniol acetate (7) have been identified as natural zinniol derivatives from the organic crude extract of Alternaria tagetica culture filtrates. Using zinniol as the starting material, phytotoxin 5 was synthesized, together with a number of synthetic intermediates (8−13). Both natural and synthetic zinniol derivatives were evaluated in the leaf-spot bioassay against marigold leaves (Tagetes erecta). Keywords: Alternaria tagetica; Tagetes erecta; marigold; phytotoxin; zinniol; zinniol derivatives; leaf-spot bioassay