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)A, 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.