3-(7′-Aryl-9′-hydroxyprop-8′-yl)­coumarin, which is a structural isomer of a Z-2-hydroxybenzylidene-γ-butyrolactone-type lignan, was stereoselectively synthesized and subjected to plant growth regulation examination. (R)-4′-Methoxyphenyl derivative 3 showed stereospecific plant growth suppressive activity. The significance of the presence of hydroxy group at the 9′-position for the activity was clarified. The effect of the substituent at the 7′-aryl group was also shown. The 3′-methoxy, 4′-methoxy, and 4′-trifluoromethyl derivatives 10, 3, and 22 led to the most significant growth suppression of Italian ryegrass roots. The 2′-methoxy derivative 9 and 4′-methoxy derivative 3 provided the most growth suppressive activity against lettuce shoots and roots, respectively.

Novel derivatives of E-β-benzyl-α-benzylidene-γ-butyrolactone (3-benzyl-2-benzylidene-4-butanolide) with lignano-9,9′-lactone structures were developed as anti-phytopathogenic fungal compounds. Their regiospecific and enantiospecific characteristics were determined, with the E-form and 3R-configuration showing higher activities against the Alternaria alternata Japanese pear pathotype. By the syntheses of benzyl compounds instead of benzylidene and aromatic derivatives, followed by an bioassay experiment, the importance of the benzylidene structure and effects of the substituents of the aromatic ring were clarified. The (2-OCH 3 , 4′-CH 3 /4′-CF 3 )-derivatives, 19 and 25, and (2-OCH 3 , 6-CH 3 /6-F/6-Br, 4′-OCH 3 )-derivatives, 34, 38, and 42, were more effective with EC 50 values of 0.1−0.3 μM. It was assumed that the 2-OCH 3 group, a hydrophobic group at the 6-position, and some size of the hydrophobic group at the 4′-position were necessary for the increased activity.

Derivatives of the coumarin ring in (R)-3-[3-(4-methoxyphenyl)-1-hydroxyprop-2-yl]coumarin 2, which is a lignan structure, were synthesized to clarify their structure-phytotoxicity relationships. The growth-inhibitory activity of the 8-OCH 3 derivative 8 (IC 50 =228 µM) was more potent against the roots of lettuce seedlings than the compound without substituents 2. As for the roots of Italian ryegrass seedlings, the presence of the methoxy group at the 7-or 8-position was extremely effective for inhibiting growth (7-OCH 3 7: IC 50 =121 µM, 8-OCH 3 8: 56.7 µM). Methyl derivatives at the 5-or 8-position showed activity levels similar to those of the compound without substituents 2 (5-CH 3 13: IC 50 =214 µM, 8-CH 3 16: IC 50 =225 µM). The activities of OH-and F-derivatives were not observed or were lower.