1,3-dipolar cycloaddition of azomethine ylides derived from imines and difluorocarbene to alkynes: a new active Pb-mediated approach to 2-fluoropyrrole derivatives

Domino reactions of imines with difluorocarbene in the presence of electron -deficient alkynes lead to 2-fluoropyrrole derivatives. The process involve s intermediate azomethine ylide formation, its 1,3-dipolar cycloaddition to alkyne, followed by dehydrofluorination. A modified difluorocarbene genera tion method using active lead for dibromodifluoromethane reduction is propo sed, providing shorter reaction time and improved yields of fluoropyrroles. The reactions with monoactivated acetylenes occur regioselectively. The cy cloaddition of ylides to dipolarophiles such as phenylpropynal, whose carbo nyl group is more active than the triple bond, gives rise to oxazolidine de rivatives, implying a change in the reaction site.