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.