A number of 3-alkylthio-4-ary 1-4H- 1,2,4-triazoles were synthesized and evaluated for their bleaching activity by using a lettuce seedling test. In a series of 4-phenyl-4H- 1,2,4-triazoles, 3-propylthio and 3-propargylthio analogs caused complete bleaching symptoms at 50 ppm, while the activity fell off with increasing or decreasing size of the alkylthio group at the 3-position of the triazole ring. The introduction of a chloro, trifluoromethyl, methyl or ethoxy substituent at the meta- or para-position on the benzene ring increased the activity in comparison with that of the phenyl analog. 4-(3-Chlorophenyl and 4-chlorophenyl)-3-propargylthio-4H-l,2,4-triazole (16 and 17) markedly reduced ß-carotene content as well as chlorophyll in plants. Inhibition of ß-carotene formation was accompanied by an accumulation of large amounts of phytoene, suggesting that these 1,2,4-triazoles inhibit the desaturation of phytoene.
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