1,3-Dipolar cycloaddition of azomethine ylides generated from ketimines and difluorocarbene to symmetrically substituted olefins

Iminiodifluoromethanides generated from difluorocarbene and benzophenone or fluorenone imines enter into reaction of 1,3-cycloaddition with electron-deficient alkenes to furnish pyrrolidone derivatives. The generation of iminiodifluoromethanides from alkyl N-benzhydrylidene glycinates in the presence of dipolarophiles is liable to complication by a concurrent proton shift in the initial imine giving NH-azomethine ylide also capable of 1,3-dipolar cycloaddition resulting in a side product of pyrrolidone series. The use of active lead instead of lead powder as reductant for dibromodifluoromethane in generation of difluorocarbene permits suppressing formation of the side products in these reactions.