Asymmetric [3 + 3] Annulation of Copper-Allenylidenes with Pyrazolones: Synthesis of Chiral 1,4-Dihydropyrano[2,3- c]pyrazoles.

The copper-catalyzed asymmetric [3 + 3] annulation of ethynyl benzoxazinanones with pyrazolones has been achieved, providing simple access to 1,4-dihydropyrano[2,3- c]pyrazole derivatives in moderate to excellent yields with excellent enantioselectivities (up to 99% ee). Compared with previous annulation reactions of copper-allenylidenes from ethynyl benzoxazinanones, the current reaction fused the three carbon atoms of the propargyl moiety into a heterocyclic framework.