One-Pot Organocatalytic Michael Addition/I2-Mediated Cyclization Sequence: Metal-Free Synthesis of Spiropyrazolones from 1,3-Diketones and Unsaturated Pyrazolones

A highly efficient organocatalytic Michael addition/I2-mediated cyclization reaction sequence between 1,3-diketones and unsaturated pyrazolones was achieved in the presence of a chiral cinchonidine-derived squaramide catalyst and I2 under mild reaction conditions. A variety of optically active spiro-dihydrofuran pyrazolones were obtained in good to high yields (71–97 %) with excellent diastereoselectivities and high to excellent enantioselectivities (up to >99:1 dr and 99 % ee). A gram-scale synthesis of chiral spiro-dihydrofuran pyrazolones was also successfully realized.