Palladium-Catalyzed (3 + 2) Annulation of Azaborines with Vinyl Epoxides for Constructing Polycyclic Oxazaborolidines

A palladium-catalyzed (3 + 2) annulation of azaborines with vinyl epoxides has been established. By this strategy, various polycyclic oxazaborolidines with structural diversity were synthesized in generally high yields (up to 99%). The annulation can be scaled up and the polycyclic oxazaborolidines can be further functionalized through olefin metathesis and Heck reaction, which demonstrated good feasibility for downstream transformations. Moreover, the catalytic asymmetric version of this (3 + 2) annulation has been accomplished under the catalysis of palladium/chiral phosphoramidite ligand, producing chiral oxazaborolidines in overall good enantioselectivities (up to 98:2 er). This work not only represents the first catalytic asymmetric (3 + 2) annulation of 1,2-azaborines with vinyl epoxides but also offers an efficient strategy for constructing benzooxazaborolidine skeletons, particularly those in an enantioenriched fashion.