Highly Chemo- and Diastereoselective Dearomative [3 + 2] Cycloaddition Reactions of Benzazoles with Donor-Acceptor Oxiranes.

A Sc(OTf)3-catalyzed dearomative [3 + 2] cycloaddition of benzazoles with donor-acceptor oxiranes through chemoselective C-C bond cleavage of oxiranes was developed under mild conditions. This reaction provides an efficient method to construct benzazolo[3,2- c]oxazole compounds in good yields and with high diastereoselectivity. The reaction has a general substrate scope, and the donor-acceptor oxiranes with electron-donating and electron-withdrawing groups on the aromatic ring afforded the desired cycloadducts.