Copper Catalyzed [3 + 2] Annulation Reaction of Exocyclic Sulfonyl Enamides for the Synthesis of N,O-Spiroketal and Spiroketal.

A copper-catalyzed [3 + 2] annulation  reaction of exocyclic enamines/enol ethers with 1,4-benzoquinone esters has been developed, providing facile access to N,O- spiroketals and spiroketals under mild conditions with broad substrate scope (26 examples, 71-94% yields). Gram scale synthesis and chemical transformations demonstrated that this method is potentially useful in the synthesis of natural products and drugs containing a N,O- spiroketal moiety. The chiral N,O-spiroketal could be obtained with 98% ee after recrystallization, when a chiral SaBOX ligand was employed.