Diastereoselective synthesis of spiropyrazolones via 1,3-dipolar [3+2] cycloadditions between pyrazolone-based olefins and N, N’-cyclic azomethine imines

Under the catalysis of PhCO2H, the 1,3-dipolar [3+2] cycloaddition between pyrazolone-based olefins and N,N'-cyclic azomethine imines proceeded readily, thus delivering structurally novel spiropyrazolones with up to 98% yield and >20:1 dr. The relative stereochemical configuration of the obtained spiropyrazolones was unambiguously assigned by X-ray single-crystal structure analysis. The diastereoselective formation of the title spiropyrazolones was interpreted by the hypothesized reaction mechanism.