Chiral squaramide-catalyzed sulfa-michael/aldol cascade for the asymmetric synthesis of spirocyclic tetrahydrothiophene chromanone derivatives

A bifunctional squaramide-catalyzed sulfa-Michael/aldol cascade reaction between benzylidenechroman-4-ones and 1,4-dithiane-2,5-diol with a low catalyst loading has been developed. This reaction constitutes a facile asymmetric synthesis of chiral spirocyclic tetrahydrothiophene chromanone derivatives with three contiguous stereocenters in high to excellent yields (up to 99%) and with high enantioselectivities (up to 92% ee). Additionally, a remarkable temperature effect on reaction efficiency was observed and gram-scale syntheses were found to proceed smoothly with the same efficiency as smaller scale reactions.