Stereoconvergent and stepwise 1,3-dipolar cycloadditions of nitrile oxides and nitrile imines

The first example of stereoconvergent 1,3-dipolar cycloaddition of nitrile oxides and nitrile imines with E/Z isomeric mixture of electron-deficient olefins is reported,delivering isoxazolines and pyrazolines bearing two vicinal stereogenic tertiary and trifluoromethylated quaternary carbon centers with perfect regioand diastereoselectivities.The possibility of concerted cycloaddition/epimerization sequence under basic condition to form the thermodynamically stable diastereomers is excluded through some control experiments and DFT calculations,and a stepwise mechanism is proposed.