Sc(OTf)₃-catalyzed tandem (3+3)-annulation/lactonization of cyclopropanes with salicylaldehyde nitrones to access polycyclic 1,2-oxazines

A Sc(OTf)(3)-catalyzed tandem (3 + 3)-annulation/lactonization of donor-acceptor cyclopropane 1,1-dieters with salicylaldehyde nitrones has been reported, affording a wide range of tetrahydrochromeno[4,3-c][1,2]oxazin-5(1H)-ones in moderate yields with excellent diastereoselectivities. Besides, the gram-scale reaction has also been explored to demonstrate the utility of this protocol. Moreover, the mechanism was investigated through control experiments, which show the reaction proceeds via a tandem (3 + 3)-annulation/lactonization reaction.