Crystal and molecular structure of two C-phenyl pentitol derivatives

Abstract

Grignard addition of phenylmagnesium bromide to 3,5-O-benzylidene-1-deoxy-D-erythro-2-pentulose (1) affords C-phenyl branched-chain pentitols having exclusively either the D-arabino (2) or D-ribo (5) stereochemistry according to the mode of substitution of 1. In order to determine the chirality of these products conclusively, the crystal and molecular structure of the unsubstituted pentitol 6 (from 2) has been determined. Crystals of 1-deoxy-2-C-phenyl-D-arabinitol (6; C11H16O4) are orthorhombic, space group P212121, with cell dimensions a = 8.9420(6), b = 18.910(1), c = 6.4263(6) Å, and Z = 4. The structure was solved by multi-solution direct methods and refined to a final discrepancy index of 5.6%. The molecule has an extended, planar zigzag conformation with the phenyl ring nearly perpendicular to this plane. Compound 6 forms one intramolecular (between 2-OH and O-4) and three intermolecular hydrogen bonds. Palladium-catalyzed hydrogenolysis of the 2-phenylcarbamate of 2 results in deoxygenation of the benzylic center with complete-inversion of configuration, as established by crystal-structure determination of the product 7. Crystals of 2(S)-4-O-acetyl-3,5-O-benzylidene-1,2-dideoxy-2-C-phenyl-D-erytro-pentitol (7; C20H22O4) are monoclinic, space group C2, with cell dimensions a = 19.102(1), b = 6.1925(5), c = 16.098(1) Å, β = 108.325(9)°, and Z = 4. The structure was solved by multi-solution direct methods and refined to a final discrepancy index of 5.9%. The molecule has an almost ideal planar, zigzag backbone conformation which incorporates the 1,3-dioxane ring. Comparison of the conformation observed in the crystal with that calculated from proton–proton coupling constants shows good agreement