Structure-Plant Phytotoxic Activity Relationship of 7,7′-epoxylignanes, (+)- and (−)-verrucosin: Simplification on the aromatic ring substituents

The synthesized 7-aryl derivatives of (7R,7′S,8S,8′S)-(+)-verrucosin were applied to growth inhibitory activity test against ryegrass at 1 mM. 7-(3-Ethoxy-4-hydroxyphenyl) derivative 12 and 7-(2-hydroxyphenyl) derivative 4 showed comparable activity to those of (+)-verrucosin against the root (−95%) and the shoot (−60%), respectively. The growth inhibitory activity test against lettuce using synthesized 7-aryl derivatives of (7S,7′R,8R,8′R)-(−)-verrucosin at 1 mM showed that the activities of 7-(3-hydroxyphenyl) derivative 20 and 7-(3-ethoxy-4-hydroxyphenyl) derivative 28 are similar to that of (−)-verrucosin against the root (−95%). Against the shoot, 7-(3-hydroxyphenyl) derivative 20 showed higher activity (−80%) than that of (−)-verrucosin (−60%). As the next step, (7S,7′R,8R,8′R)-7-(3-hydroxyphenyl)-7′-aryl-(−)-verrucosin derivatives, in which the most effective 3-hydroxyphenyl group is employed as 7-aromatic ring, were synthesized for the assay against lettuce. In this experiment, 7′-(2-hydroxyphenyl) derivative 37 and 7′-(3-hydroxyphenyl) derivative 38 showed similar activity to that of derivative 20. The effect of 7- and 7′-aryl structures of 7,7′-epoxylignanes on the plant growth inhibitory activity was clarified. The 7- and 7′-aryl structures were simplified to show comparable activity to or higher activity than that of (−)-verrucosin. The plant growth inhibitory activity of a nutmeg component, (+)-fragransin C3b, was estimated as −80% inhibition at 1 mM against ryegrass roots.

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