Studies in sigmatropic rearrangements of N-prenylindole derivatives – a formal enantiomerically pure synthesis of tryprostatin B?

Organic & Biomolecular Chemistry Pub Date: 2006-09-28 DOI: 10.1039/B606457D

Abstract

Rearrangement of Na-prenyl-Nb-acetyltryptamine, induced by BF3·Et2O at low temperature, leads to a 2-prenyl derivative, and thence to the tricyclic tryptamine 7 and the indoline 8. Similarly, Na-prenyl-Nb-phthaloyl-L-tryptophan methyl ester furnished the corresponding 2-prenyl derivative 16, a known advanced precursor of tryprostatin B. Density functional (B3LYP) calculations for the putative rearrangement transition state for N-prenylskatole show that prior coordination of BF3 to the indolic nitrogen changes the character of the subsequent sigmatropic pericyclic shifts from being entirely covalent to acquiring a significant degree of ionic character. The shifting prenyl group favours the endo over the exo mode of the transition state by 4.1 kcal mol?1.

Graphical abstract: Studies in sigmatropic rearrangements of N-prenylindole derivatives – a formal enantiomerically pure synthesis of tryprostatin B
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