Neighboring group participation of the indole nucleus : An unusual DAST-mediated rearrangerment reaction

A rearrangement reaction involving the indole nucleus was investigated using stereochemical markers and low-temperature NMR experiments. Treatment of(3S,4S)-3-hydroxy-4-(2-phenyl-1H-indol-3-yl)-piperidine-1-carboxylic acid benzyl ester (> 90% ee) with diethylaminosulfur trifluoride gave stereospecifically (3S,4S)-4-fluoro-3-(2-phenyl-1H-indol-3-yl)-piperidine-1-carboxylic acid benzyl ester (> 90% ee) with complete regioselectivity. The initial formation of a reactive spirocyclopropyl-3H-indole intermediate is believed to be responsible for the stereo- and regiochemical outcome of the reaction.