Chiral-Auxiliary-Induced Diastereoselectivity in the [4 + 2] Cycloadditions of Optically Active 2,2-Dimethyloxazolidine Derivatives of Sorbic Acid: A Model Study with Singlet Oxygen as the Smallest Dienophile

Optically active 2,2-dimethyloxazolidines, conformationally controlled chiral auxiliaries, have been applied for the first time in the [4 + 2] cycloaddition of singlet oxygen, the smallest possible dienophile. High π-facial selectivity (diastereomeric ratio ≥ 95:5) has been achieved in the attack of singlet oxygen on the 1,3-diene moiety of the chiral amides 3 of sorbic acid. Expectedly, the sterically much more imposing 4-phenyl-1,2,4-triazoline-3,5-dione dienophile [4 + 2]-cycloadds with complete stereocontrol to the amide 3d .