α-d-Tagatopyranose

The title compound, C6H12O6, also known as d-Tagatose, occurs in its furanose and pyranose forms in solution, but only the α-pyranose form crystallizes out. In the crystal, the molecules form hydrogen bonded chains propagating in [100] linked by O—H⋯O interactions. Further O—H⋯O bonds cross-link the chains.

The title compound, C 6 H 12 O 6 , also known as d-Tagatose, occurs in its furanose and pyranose forms in solution, but only the -pyranose form crystallizes out. In the crystal, the molecules form hydrogen bonded chains propagating in [100] linked by O-HÁ Á ÁO interactions. Further O-HÁ Á ÁO bonds cross-link the chains.
Arla Foods generously provided a sample of d-tagatose, obtained as described (Beadle et al., 1992) from d-galactose, for crystallization.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: FL2248).
by either chemical (Beadle et al., 1992) or biotechnological (Granstrom et al., 2004;Izumori, 2002) techniques, and it is widely investigated as a low calorie sweetener (Skytte, 2002); the potential of D-tagatose as a chiral building block is also beginning to be recognized Watkin et al., 2005;Jones et al., 2007;Jones et al., 2008;Yoshihara et al., 2008). The crystal structure of another hitherto rare diasteroisomeric ketohexose, D-psicose, has recently been published (Kwiecien et al., 2008). A previous α-D-tagatose structure solution (Takagi et al., 1969), did not report either three-dimensional coordinates or bond lengths and angles. Although in aqueous solution both furanose and pyranose forms are present, only the α-pyranose crystallizes out. The crystal structure of the title compound ( Fig. 1) consists of a network of hydrogenbonded chains running parallel to the a axis (Fig.2). Referring to Table 1, O4-H41···O10 is the only intramolecular hydrogen bond detected in the structure. O12-H121···O4 and O1-H11···O6 link the molecules into chains, and O9-H91···O1 and O10-H101···O4 stabilize the structure with inter-chain hydrogen bonds. O4 is involved as an acceptor in two hydrogen bonds and as a donor in an almost linear hydrogen bond -the latter by means of H41. The crystal structure shows three equatorial groups and two axial groups, one of which is an axial anomeric hydroxyl group; this would be expected to be the most thermodynamically stable pyranose anomer. The fairly high value of the anisotropic displacement of O12 -compared to the other C and O atoms -is probably due to thermal motion. It results also in a higher -compared to the other H atoms -isotropic displacement for H121 i.e. the hydrogen atom connected to the last atom of the flexible C7-C11-O12 chain.

Experimental
In aqueous solution the major form present is α-D-tagatopyranose (71%) (Fig.1) with 18% of the β-pyranose and small amount of the furanoses (Angyal, 1991). The title compound was recrystallized from a 1:10 mixture of water and acetone allowing the slow competetive evaporation of the solvents, after which, transparent prismatic crystals appeared.

Refinement
The data were collected with molybdenum radiation and since there were no atoms heavier than Si present, there were no measurable anomalous differences and it was admissible to merge Friedel pairs of reflections. Changes in illuminated volume were kept to a minimum, and were taken into account (Görbitz, 1999) by the multi-scan inter-frame scaling (DENZO/ SCALEPACK, Otwinowski & Minor, 1997). The H atoms were all located in a difference map, but those attached to carbon atoms were repositioned geometrically. The H atoms were initially refined with soft restraints on the bond lengths and angles to regularize their geometry (C-H in the range 0.93-0.98, N-H in the range 0.86-0.89 O-H = 0.82 Å) and U iso (H) (in the range 1.2-1.5 times U eq of the parent atom), after which the positions were refined with riding constraints. Fig. 1. The title compound with displacement ellipsoids drawn at the 50% probability level. H atoms are shown as spheres of arbitary radius. Fig. 2. Packing diagram of title compound viewed down the a axis. Hydrogen bonds are shown as dotted lines. Fig. 3. D-Tagatose and α-D-tagatopyranose.