CONFORMATIONAL-ANALYSIS OF 2-OARYL-2-OXO-4,6-DIMETHYL-DIOXAPHOSPHORINANE AND -4-METHYL-1,3,2-LAMBDA(5)-DIOXAPHOSPHORINANE - SPECTROSCOPIC, X-RAY, AND SOLID-STATE C-13 AND P-31 STUDIES

Results of IR and H-1, C-13, and P-31 NMR studies of the anancomeric t itle compounds (2-5) and compound 1 (Scheme I) ave analyzed to search for the existence of high-energy boat or twist-boat conformations in t he equatorial epimers. While the difference in frequencies (Delta v)(P =0) between the axial and equatorial compounds and C-13 NMR J and anti J(POCC) values suggest the participation of twist-boat conformations for the equatorial isomers, coupling constants in H-1 NMR J(H4H5a) or J(H6H5a) and J(H4H5e) or J(H6H5e) of the equatorial isomers 2e-4e alon g with the lack of a large (3)J(PH) in P-31 NMR are consistent with pr edominant chair conformations. In addition, an X-ray structure of the equatorial xy-2-oxo-cis-4,6-dimethyl-1,3,2-dioxaphosphorinane (4e) sho wed that the molecule adopts a chair conformation with no severe ring flattening in the OPO region in the solid state. X-ray structures of t rans-4 and trans-5 displayed chair conformations with mild ring flatte ning especially in the axial methyl region, presumably as a result of the steric methyl-oxygen interaction. CPMAS C-13 and P-31 NMR spectra of 4a and 4e provide evidence against the presence of a significant co ntribution of a twist-boat conformation in solid equatorial 4e. The NM R spectral analysis of le, on the other hand, suggests a substantial c ontribution of a twist conformation as well as, possibly, some contrib ution of the inverted chair. (C) 1997 John Wiley & Sons, Inc.