Potential Rearrangements in the Reaction Catalyzed by the Indole Prenyltransferase FtmPT1

The indole prenyltransferase FtmPT1 catalyzes the C-2 normal prenylation of brevianamide F (cyclo-L-Trp-L-Pro) to give tryprostatin B. A previous structural analysis and studies with alternate substrates suggest that the reaction might not proceed through a direct C-2 attack, but could involve a C-3 prenylation followed by a rearrangement. In this work we investigated the reactivity of FtmPT1 with tryptophan, 5-hydroxybrevianamide, and 2-methylbrevianamide, and isolated products that had been reverse prenylated at C-3 and normal prenylated at N-1, C-3, or C-4. The formation of these products can be rationalized through mechanisms involving either an initial C-3 normal or C-3 reverse prenylation. In addition, we demonstrate that a C-3 reverse prenylated indole can undergo a nonenzymatic aza-Cope rearrangement at 37 °C to give an N-1 normal prenylated product. Together, these studies broaden the known product scope of this interesting catalyst and suggest that alternative mechanisms might be operating.