NMR and X-ray diffraction analysis of 3-thioamido-5-phosphono-1-cyclohexene derivatives: Conformational and stereochemical assignments

Reaction of N-(dienyl)-4-(R)-phenyloxazolidin-2-thione (1) with 1-diethoxyphosphoryl-1-buten-3-one (2a), methyl 3-(diethoxyphosphoryl)acrylate (2b) and 3-(diethoxyphosphoryl)acrylonitrile (2c) furnished the [4 + 2] cycloadducts 3a, 3b and 3c, respectively, as single stereoisomers, confirming the efficiency of the oxazolidin-2-thione chiral auxiliary in the facial discrimination of Diels–Alder reactions. The absolute configuration of the 3,4,5-trisubstituted cyclohexene derivatives (3a–c) was established by X-ray diffraction analysis. NMR studies confirmed the relative stereochemistries and showed two possible conformers in solution.