Reactivity and Diastereoselectivity in the Thermal and Lewis Acid-Catalyzed Diels−Alder Reactions of N-Sulfinylphosphoramidates1

The [4 + 2] cycloaddition reactions of N -sulfinylphosphoramidates, prepared from the corresponding phosphoramidates by treatment with N -(chlorosulfinyl)imidazole, and 1,3-cyclohexadiene were found to be diastereoselective in the absence (>90:10) and presence (>95:5) of Lewis acid. The sulfur configuration of the major adduct from the cycloaddition reaction has been established unambiguously by X-ray crystallography. The use of Lewis acids improved the diastereoselectivity and yield, as well as shortened reaction times. Based on the intermediacy of a tin chelate, and the absence of phosphoryl secondary orbital interactions, a mechanism for the cycloaddition reactions is discussed.