Zn(II)-Catalyzed Isomerization/Asymmetric [3+2] Cycloaddition of 3-Butynoyl Pyrazole with Nitrones: Subsequent Exocyclic Olefin Isomerization and Capture.

A highly efficient Zn(OTf)2-catalyzed tandem isomerization/[3+2] cycloaddition cascade of 3-butynoyl pyrazole with nitrones was reported using a C1-symmetric imidazolidine-pyrroloimidazolone pyridine ligand. The resulting cycloadducts readily isomerize with silica gel or Et3N to chiral 4-isoxazolines containing an endocyclic C═C bond in up to 93% yield with 99% ee. The exocyclic olefin intermediate is effectively trapped by 1,4-benzoquinone ester under Cu(II) catalysis to afford chiral spiroketal analogues with >20:1 dr.