THE SOLID STATE AND SOLUTION CONFORMATION OF 1,2-O-ALKYLIDENE-α-D-RIBOPYRANOSES. CHIRALITY ASSESSMENT OF PENTOSES FROM CRYSTALLOGRAPHIC TORSION ANGLES

Article abstract of DOI:10.1016/0040-4020(86)80019-9

The structure of 3,4-di-O-acetyl-1,2-O-(S)-(1-cyanoethylidene)-α-D-ribopyranose (1) has been determined from X-ray diffraction data and refined to a conventional R value of 0.050 for the 1259 observed reflexions (3 ? (I) criterion).The compound packs in the P2₁2₁2₁ space group with Z = 4 and lattice parameters of a = 20.0157 (7), b = 10.0552 (2), and c = 7.0870 (1) Angstroem.The conformation of the pyranoid ring in the solid state can be described as chair ¹C₄ (D) and that of the dioxolane ring, as envelope E₁.The use of configurational angles for characterization of pentoses is presented.NMR studies have been carried out to determine the major conformation in solution of 1 and its analogues 2 - 5.The results seem to indicate in all cases the presence of a conformational equilibrium between the chair-like, ¹C₄, and the boat-like, ^oS₂ conformation, in agreement with the results obtained from molecular mechanics calculations.