BF3-Mediated cis-Selective Cycloaddition of O-Silyloxime with Alkenes.

A C-amide-substituted O-silylated oxime, (E)-(tert-butyldimethylsiloxyimino)acetic acid N,N-dimethylamide (8b), on treatment with 2.2 equiv of BF3·OEt2, in situ generated boracyclic nitrone-type intermediate BF3·14, which underwent cycloaddition with alkenes to give 3,5-cis-isoxazolidines as the major products. The mechanism was strongly supported by isolation of the reaction intermediate 14 that was characterized by X-ray diffraction and its further reaction. This cycloaddition was successfully applied to the synthesis of syn-HPA-12 known as an inhibitor of CERT that mediates the transport of ceramide.