Syntheses of Phytotoxic and Phytohormone Lignan Coumarin Derivatives

Phytotoxic and/or phytohormone-like compounds were synthesized by constructions of the 8, 8'-lignan structures, followed by cyclizing one of the phenylpropanoid units into a coumarin moiety. Since these lignans have two stereogenic centers, four stereoisomers were synthesized. The configurations of the products were confirmed by X-ray and vibrational circular dichroism analyses. Examination of plant growth regulatory activities revealed that the (7'R, 8'R)-stereoisomer was a potent growth inhibitor of ryegrass seedlings. Derivatives with the (7'R, 8'R)-configuration and different substituents at the aromatic rings were also synthesized. Structure-activity relationships revealed that a 7-OCH₃ derivative was the most potent growth inhibitor of ryegrass seedlings (shoots: IC₅₀ = 15.4 μM, roots: IC₅₀ = 1.73 μM). Moreover, an 8-OCH₃ derivative was a more potent germination inhibitor than coumarin. Some synthetic derivatives displayed phytohormone-like activity, promoting growth at low concentrations and inhibiting growth at high concentrations.