The cruciferous phytoalexins rapalexin A, brussalexin A and erucalexin: Chemistry and metabolism in Leptosphaeria maculans

The interactions of the cruciferous phytoalexins rapalexin A (1), brussalexin A (2) and erucalexin (3) with the fungal plant pathogen Leptosphaeria maculans were analyzed and their inhibitory activities against this pathogen were determined. The reaction of L. maculans to N-methyl S-(indolyl-3-methyl)carbamodithioate, an analogue of brussalexin A, was also investigated. Rapalexin A was resistant to metabolism and was the most inhibitory of all compounds tested, suggesting that increasing concentrations of rapalexin A in Brassica species would improve their disease resistance to L. maculans. By contrast, erucalexin was quickly detoxified by reduction to yield 3-dihydroerucalexins. The relative configurations of the diastereomeric mixture of dihydroerucalexins were established by 1D 1H nuclear Overhauser enhancement spectroscopy (NOE). Brussalexin A was chemically unstable decomposing mainly to indolyl-3-methanol, a product with anti-cancer properties. For this reason, brussalexin A might be of interest to use as a prodrug.

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► Rapalexin A is resistant to transformation by L. maculans.
► Rapalexin A is strongly inhibitory to L. maculans.
► Brussalexin A decomposes spontaneously to indolyl-3-metanol in culture media.
► Erucalexin is detoxified to a diastereomeric mixture of dihydroerucalexins.
► Dihydroerucalexins decompose to demethoxyerucalexin.