The solid-state and solution conformation of some 1,2-O-alkylidene xylopyranoses. Application of heteronuclear long-range coupling constants to the conformational analysis of dioxolane rings

The molecular structures of 3,4-di-O-acetyl-1,2-O-[(R)-1-cyanoethylidene]-α-D-xylopyranose (1) and its (S)-isomer (2) have been determined from X-ray diffraction data. The pyranoid ring adopts a 1C4 chair conformation for both compounds and the dioxolane ring presents E1 and 2E distorted envelope conformations for (1) and (2) respectively. The solution conformation of (1) and (2), and the ethylidene analogues (3) and (4), have been determined by n.m.r. spectroscopy. Proton–proton coupling constants, variable-temperature and n.O.e. experiments, and molecular mechanics calculations indicate that the major conformation of the pyranoid ring can be described as 1C4 for (1), ∘S2 for (2) and (4), and as an equilibrium among 4C1, 1C4, and ∘S2 forms for (3). Long-range carbon–proton coupling constants can be interpreted in terms of E1 and EO2 conformations for the dioxolane ring of (1) and (2) respectively.