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

Abstract

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 E_O2 conformations for the dioxolane ring of (1) and (2) respectively.