Preparation, Fungitoxicity and Quantitative Structure Activity Relationship of Methyl Esters of Aryl Carboxylic Acids

Twenty eight differently substituted methyl esters of aryl carboxylic acids were prepared by reacting the corresponding aryl carboxylic acids with methanol in presence of concentrated sulphuric acid. The methyl esters of m- and p- chlorobenzoic acids were prepared by reacting the aryl acid with BF3-methanol complex. All the methyl esters were evaluated in vitro for fungicidal activity against six fungi viz. Rhizoctonia solani, Sclerotium rolfsii, Rhizoctonia bataticola, Pythium. aphanidermatum, Fusarium oxysporum and Alternaria alternata. Few of these methyl esters exhibited good fungicidal activity. Among all, the 3,5-(NO2)2, 2-OH substituted methyl ester (ED50=28 mg ml−1) against R. solani; 2-OCH3 (ED50=47 mg ml−1) against S. rolfsii, (ED50=61 mg ml−1) R. bataticola and (ED50=56 mg ml−1) F. oxysporum; 4-NO2(ED50=70 mg ml−1) against P. aphanidermatum; 2,4-(OH)2 (ED50=92 mg ml−1) against A. alternata and also 2,4-Cl2 (ED50=47 mg ml−1) against S. rolfsii exhibited the highest activity. The quantitative structure activity relationship (QSAR) study using the in vitro fungicidal activity data and physico-chemical parameters of phenyl ring substituents for hydrophobic, electronic and steric properties by means of multiple regression analysis has revealed that the fungicidal activity of the methyl esters of aryl carboxylic acids against all the test fungi was dependent on position dependent steric effect of the substituents. The fungicidal activity against R. bataticola and P. aphanidermatum also depended on electronic effect of ortho substituents, against S. rolfsii on electronic effect, against F. oxysporum on hydrophobicity and against R. solani both on electronic effect and hydrophobicity of the substituents in the phenyl ring of the methyl esters.