Methoxycamalexins and related compounds: Syntheses, antifungal activity and inhibition of brassinin oxidase.

The phytoalexin camalexin is a competitive inhibitor of brassinin oxidase, an enzyme that detoxifies the phytoalexin brassinin and is produced by an economically important plant pathogen. For this reason, the camalexin scaffold has guided the design of inhibitors of brassinin detoxification. To further understand the structure-activity relationships of camalexin related compounds, the syntheses of monomethoxy and dimethoxycamalexins were undertaken. Four monomethoxy camalexins together with 4,6-dimethoxy and 5,7-dimethoxy camalexins were prepared from the corresponding methoxyindoles using the Ayer's method. The dimethoxy derivatives were prepared from the corresponding dimethoxyindole-3-thiocarboxamides using the Hantzsch reaction; however, this method did not work for the syntheses of 4,6-dimethoxy and 5,7-dimethoxycamalexins due to the lower reactivities of the corresponding indole-3-thiocarboxamides. The antifungal activity and brassinin oxidase inhibitory activity of all methoxycamalexins and ten camalexin related compounds were investigated. Among the 20 compounds evaluated, monomethoxycamalexins were stronger antifungals than the dimethoxy derivatives. However, remarkably, 5,6-dimethoxycamalexin, 6,7-dimethoxycamalexin and 5-methoxycamalexin displayed the strongest inhibitory activity against brassinin oxidase, while 4,5-dimethoxycamalexin displayed no inhibitory effect. Altogether the structure-activity relationships of camalexin related compounds suggest that the targets for fungal growth inhibition and brassinin oxidase inhibition are unrelated and emphasize that brassinin oxidase inhibitors do not need to be antifungal.