Crystal structure of methyl-3, 4-O-ethylidene-β-L- arabinopyranoside

Abstract

The acid-catalyzed ethylidenation of several methyl pentopyranosides has been studied and syntheses of many cyclic alkyl orthoacetates from pentopyranosides have been reported. In a study of the scope and limitation of the reduction of sugar epoxides with diborane/sodium borohydride, methyl-4-O-acetyl-2,3-anhydro-β-L-lyxopyranoside was reduced, yielding two products: methyl-2,3-anhydro-β-L-lyxopyranoside and methyl-3,4-O-ethylidene-β-L-arabinopyranoside. The latter product was characterized by X-ray diffraction and its structure indicates that the epoxide was not reduced as expected; rather, the epoxide was opened by the hydroxyl group of the reduced acetyl. Crystals of the title compound are monoclinic, space group P21 with the following cell dimensions: a = 5.818(l) A, b = 10.842(2) A, c =7.251(1) A, β = 91.47(1)°, Z = 2. Complete three-dimensional data (2θmax = 150° for Cu Kα) were collected on a CAD-4 diffractometer by the ω−2θ scan method. The structure was solved by the application of direct methods and refined by full-matrix least-square methods to a final reliability index of 0.04 for the observed 1031 reflections (I ≥ 3σ). The ethylidene ring has a twist conformation with a torsion angle of −42.8° about the C4–O bond. The pyranose ring is in a flattened chair conformation with the C2–C3–C4–C5 torsion angle of 33.5°. Of the acetal bond lengths, the C5–O5 length, (1.423 A) is shorter than the values found in other methyl pyranosides. O2–H is involved in a bifurcated hydrogen bond intramolecularly to O1 and intermolecularly to O4.