Enantioselective synthesis of spiro[cyclohexane-1,3′-indolin]-2′-ones containing multiple stereocenters via organocatalytic Michael/aldol cascade reactions

Organocatalytic reactions of 3-olefinic oxindoles and pentane-1,5-dial were investigated to provide access to substituted spirocyclohexane oxindoles via Michael/aldol cascade reactions. Of particular interest, we have examined the stereochemical outcome of electron withdrawing and electron-donating groups on the oxindole ring nitrogen. Interestingly, we have observed that the N-protecting group on the oxindole has critical effect on aldol ring closure leading to ultimate stereochemical outcome of the hydroxyl center. The overall process is quite efficient and afforded products with multiple stereocenters in high yields and excellent enantioselectivities (>99% ee).