Discovery of anti-phytopathogenic fungal activity of a new type of (<i>S</i>)-coumarin bearing a phenylpropanoid unit at the 3-position

The enantiospecific anti-phytopathogenic fungal activity of a new type of coumarin bearing a phenylpropanoid unit at the 3-position was found. (<i>S</i>)-3-[1-Methoxy-3-(4-methoxyphenyl)prop-2-yl]coumarin ((<i>S</i>)-<b>5</b>: EC₅₀=16.5 µM) was 30 times more effective than the (<i>R</i>)-form against the <i>Alternaria alternata</i> Japanese pear pathotype. Derivatives bearing different substituents on the 7'-aromatic ring and the coumarin ring were synthesized to discover the more potent compounds. The 3'-CF₃ and 4'-CF₃ derivatives, <b>39</b> and <b>40</b>, respectively, had the lowest EC₅₀ values (1-2 µM) in this project, suggesting that the size of the electron-withdrawing and hydrophobic substituents at these positions gave an advantage. On the coumarin ring, the presence of the OCH₃ or CH₃ group at the 5-position accelerated the activity, as the (4'-OCH₃, 5-OCH₃) derivative <b>41</b> and (4'-OCH₃, 5-CH₃) derivative <b>45</b> were, respectively, 4-5 times more potent than the 4'-OCH₃ derivative (<i>S</i>)-<b>5</b>.