Mechanistic studies on CymD: a tryptophan reverse N-prenyltransferase

The prenyltransferase CymD catalyzes the reverse N-prenylation of tryptophan using dimethylallyl diphosphate (DMAPP) in the biosynthesis of the cyclic peptides cyclomarin and cyclomarazine. The mechanism is of interest because a non-nucleophilic indole nitrogen must be alkylated in this process. Three mechanisms were initially considered, including (A) a direct addition of a carbocation to the nitrogen, (B) an addition of a carbocation to C-3 followed by an aza-Cope rearrangement, and (C) deprotonation of the indole followed by an S(N)2' addition to DMAPP. The use of 4-fluorotryptophan and 6-fluorotryptophan revealed that the reaction kinetics are only modestly affected by these substitutions, consistent with the notion that positive charge does not accumulate on the indole ring during catalysis. When (E)-3-(fluoromethyl)-2-buten-1-yl diphosphate was used in place of DMAPP, the maximal rate was reduced by a factor of 100, consistent with the development of positive charge on the dimethylallyl moiety. Positional isotope exchange (PIX) experiments show that the reaction with Trp proceeds without isotopic scrambling of the label in the starting material [1-(18)O]DMAPP. However, in the case of 4-fluorotryptophan, significant isotopic scrambling is observed (v(PIX)/v(rxn) = 1.1). This is consistent with a mechanism involving a discrete carbocation intermediate. Finally, a significant solvent kinetic isotope effect of 2.3 was observed in D(2)O, indicating that a proton transfer step is rate-limiting. Taken together, these observations support a mechanism that is a hybrid of mechanisms A and C. Ionization of DMAPP generates a dimethylallyl carbocation, and deprotonation of the indole nitrogen accompanies or precedes the nucleophilic attack.

Indole prenylation in alkaloid synthesis

Important biologically active indole alkaloids are decorated with prenyl (3,3-dimethylallyl) and tert-prenyl (1,1-dimethylallyl) groups. Covering the literature until the end of 2010, this review article comprehensively summarises and discusses the currently available technologies of prenylation and tert-prenylation of indoles, which have been applied in natural products total syntheses or could be applied there in the near future. We focus on those procedures which introduce the C(5) units in one step, organised according to the indole position to be functionalised. Key strategies include electrophilic and nucleophilic prenylation and tert-prenylation, prenyl and tert-prenyl rearrangements, transition metal-mediated reactions and enzymatic methods.

Brief total synthesis of the cell cycle inhibitor tryprostatin B and related preparation of its alanine analogue

Tryprostatin B was synthesized in 32% overall yield from the readily available dipeptide anhydride cyclo-(l-Trp-l-Pro). Its tandem C-3 prenylation/cyclization gave the corresponding pentacyclic pyrroloindole systems bearing a prenyl group at the indole C-3 position. These compounds were then submitted to acid-catalyzed opening of the newly formed ring, with concomitant migration of the prenyl group to the indole C-2 position. The alanine analogue of tryprostatin B was also prepared using a similar sequence. The successful implementation of this strategy strengthens the case for a biosynthetic route for the tryprostatins along similar lines.