NMR PARAMETERS FOR 1,3-DIOXANES - EVIDENCE FOR A HOMOANOMERIC INTERACTION

H-1 and C-13 NMR spectra have been recorded for a series of 1,3-dioxanes, 2-oxo-1,3,2-dioxathianes, 1,3,2-dioxaphosphorinanes and 2-thioxo-1,3,2-dioxaphosphorinanes, which are held in a chair conformation. In all of them, the spectral parameters are compatible with the equatorial C(5)-H bond being weaker and longer than the axial C(5)-H bond [e.g. 1J(C(5)H(5eq)) < 1J(C(f)H(5ax)) (a reversed Perlin Effect), and 3J(H(4)H(5eq)) < 3J(H(4eq)H(5ax)]. This is the reverse of what is usually observed in cyclohexanes, or at the 2-position in tetrahydropyrans, where the sequence is ascribed to an n-->sigma* (anomeric) interaction between the alpha-oxygen and the axial C(5)-H bond. It is suggested that this reversal may be due to an n-->sigma* (homoanomeric) interaction between the beta-oxygen and the equatorial C(5)-H bond, through a W-arrangement of orbitals. This interpretation is supported by ab initio 6-31G* calculations on the 1,3-dioxane molecule, which show that the equatorial C(5)-H bond is weaker and longer than the axial C(5)-H bond.