The catalytic enantioselective [1,2]-Wittig rearrangement cascade of allylic ethers

The catalytic enantioselective [1,2]-Wittig rearrangement of allylic ethers constitutes a recognized synthetic challenge as it is traditionally considered to arise from a non-concerted reaction pathway via formation and recombination of radical pairs. Here we show a catalytic enantioselective solution to this challenge, demonstrating that [1,2]-Wittig products are generated via an alternative reaction cascade to traditional dogma. The developed process employs a chiral bifunctional iminophosphorane catalyst to promote an initial enantioselective [2,3]-sigmatropic rearrangement. A subsequent base-promoted, stereoconvergent, fragmentation–recombination process that proceeds with high enantiospecificity and retention of configuration, formally equivalent to a Woodward–Hoffmann forbidden thermal [1,3]-sigmatropic rearrangement, generates [1,2]-Wittig products in up to 97:3 enantiomeric ratio. Supported by extensive quantum chemistry calculations, this chirality transfer process will have broad implications for fundamental stereocontrol in organic transformations. (Figure presented.)