The X-ray crystal structure of 6-deoxy-α- l-sorbofuranose. Conformational analysis of ketohexofuranosides and their comparison with ribofuranosides and arabinofuranosides

Abstract

The crystal and molecular structure of 6-deoxy- l-sorbose have been determined by the application of multisolution methods and refined to an R-index of 0.063 for 560 reflections, using three-dimensional intensity data collected on a Picker automatic diffractometer. The compound crystallizes in the space group P2 12 12 1 with unit-cell dimensions a = 18.470 (10), b = 7.636 (10), and c = 5.371 (8) Å; Z = 4. The molecule occurs as the α-furanose form, which is also the preponderant tautomer in solution. The puckering of the furanoid ring is C-3′- exo-C-4′- endo ( 3 T 4) [equivalent to C-2′- exo-C-3′- endo ( 2 T 3) in the numbering for d-ribose], with P and τ m angles of -6.5 and 42.7° respectively. Conformational analysis of the known ketofuranosides shows that the 3 T 4 ( 2 T 3 in d-ribose numbering) puckering mode, which is typical of α-nucleosides, is favored, in contrast to the favored 3 T 2 or 2 T 3 puckering mode for the β- d-ribonucleosides and β- d-arabinonucleosides. The conformational differences among furanoid rings are mainly influenced by the configuration at the anomeric carbon atom. The favored orientation about the C-2′-C-1′ bond (O-5′-C-2′-C-1′-O-1′)of the ketofuranosidesis — gauche. All four hydroxyl groups are involved in donor-acceptor hydrogen bonding, and O-4′-8 appears to be involved in a bifurcated hydrogen bond to O-2′ and O-3′ of neighboring molecules.