Crystal structure of methyl 2,6-dichloro-2,6-dideoxy-3,4- O-isopropylidene-α- D-altropyranoside. A pyranoside system close to a skew-boat conformation

Abstract

The crystal structure of methyl 2,6-dichloro-2,6-dideoxy-3,4- O-isopropylidene-α- D-altropyranoside ( 1) has been determined by X-ray diffraction. The compound crystallizes in the orthorhombic system, space group P2 12 12 1, with unit-cell dimensions a  7.932, b  8.133, and c  20.447 Å. The structure was solved by the heavy-atom method and refined by the least-squares technique to an R value of 0.047 by using 736 intensities measured on a diffractometer. The pyranoside ring is close to a skew-boat conformation, with C-2 and C-5 being maximally displaced from the least-squares plane through the remaining four atoms. The H-1H-2 dihedral angle of  158° is in agreement with the J 1,2 value of 4.5 Hz. Thus the solid-state conformation appears to correspond with the conformation in solution. The dioxolane ring is in a twist form, with O-4 and, C-8 puckered on opposite sides of the plane of the other ring atoms. The pyranose-ring substituents are in equatorial and pseudoequatorial orientations. The hydrogen atoms at C-3 and C-4 are in a cis arrangement. The orientations of both the methoxyl group and the chloromethyl group with respect to the ring are gauche—trans. The exocyclic anomeric C-1O-1 bond-distance (1.39 Å) is the shortest CO bond in the structure. The intracyclic CO bonds are significantly different, C-1O-5 being less than C-5O-5.