Asymmetric Catalytic Ring-Expansion of 3-Methyleneazetidines with α-Diazo Pyrazoamides towards Proline-Derivatives.

We realized a highly efficient formal [1,2]-sigmatropic rearrangement of ammonium ylides generated from 3-methylene-azetidines and α-diazo pyrazoamides. The employ of readily available chiral cobalt(II) complex of chiral N,N'-dioxide enabled the ring-expansion of azetidines, affording a variety of quaternary prolineamide derivatives with excellent yield (up to 99%) and enantioselectivity (up to 99% ee) under mild reaction condition. For the rearrangement of ammonium ylides, the installation of a pyrazoamide group as a masked brick to build chiral scaffolds proved successful. The enantioselective ring expansion process was elucidated by DFT calculations.