Conformational study of six-membered phostones. cis- and trans-3-(Methoxycarbonyl)-2-methoxy-2-oxo-1,2-oxaphosphorinane

The conformations of cis- and trans-3-(methoxycarbonyl)-2-methoxy-2-oxo-1, 2-oxaphosphorinane have been investigated by variable temperature ³¹P, ¹H and ¹H{³¹P} NMR spectroscopy and semiempirical calculations. An X-ray diffraction study of the trans isomer has also been performed. The ³¹P NMR spectra of both isomers did not change with temperature over the range of 183-333 K. The temperature dependence of the C(3)-H spectral pattern in the cis isomer suggested an important change in the conformer distribution with temperature. For the trans isomer, no changes were detected. The crystal structure of trans-3-(methoxycarbonyl)-2-methoxy-2- oxo-1,2-oxaphosphorinane was solved by direct methods and refined to R = 0.046. The space group is P2₁/n, a = 8.644(1), b = 7.432(1), c = 15.718(2) Å, β = 105.38(1)°. The molecule in the asymmetric unit adopted a chair conformation with equatorial phosphoryl and methoxycarbonyl groups. The conformation in the crystal agreed well with the calculated global energy minimum conformation.