N-Heterocyclic Carbene-Catalyzed Formal [3+2] Annulation of Alkynyl Aldehydes with Nitrosobenzenes: A Highly Regioselective Umpolung Strategy

N-Heterocyclic carbene-catalyzed formal [3+2] annulation of alkynyl aldehydes and nitrosobenzenes has been reported. This transformation provided the novel C–X bond formation under mild conditions in moderate to satisfactory yields. The catalytic protocol allows for a rapid construction of 2,5-disubstituted isoxazol-3(2H)-ones and 2,3-disubstituted isoxazol-5(2H)-ones from the same materials via a highly regioselectively umpolung stratgy.