Phosphine-catalyzed [4 + 2] cycloaddition of unsaturated pyrazolones with allenoates: a concise approach toward spiropyrazolones

A MePPh2-catalyzed [4 + 2] cycloaddition reaction of unsaturated pyrazolones with allenoates was described. The reaction affords the spiropyrazolone derivatives in moderate to excellent yields with moderate to good diastereoselectivities under mild conditions. Using thiourea-based bifunctional phosphines as chiral catalysts, an asymmetric variant of this [4 + 2] cycloaddition reaction has been achieved, giving chiral spiropyrazolone derivatives in moderate to excellent yields with moderate to excellent diastereoselectivities and excellent enantioselectivities (89–95% ee).