A new mechanistic proposal for the origin of alpha-homoallylic alcohols inindium-mediated allylation reactions in water

A new mechanism is proposed for the alpha -regioselective indium-mediated a llylation reaction in water. Based on the results and observations obtained from an NMR study, a cross-over experiment and the complete inversion of t he stereochemistry of 22 beta gamma -adduct homoallylic sterols to the 22 a lpha alpha -adduct homoallylic sterols, it is suggested that the initially formed gamma -adduct undergoes a bond cleavage to generate the parent aldeh yde in situ followed by a concerted rearrangement, perhaps a retro-ene reac tion followed by a 2-oxonia [3,3]-sigmatropic rearrangement to furnish the alpha -adduct. (C) 2001 Elsevier Science Ltd. All rights reserved.