2- C -Methyl- D -lyxono-1,4-lactone

The title compound, C6H10O5, has been crystallized for the first time, allowing the stereochemistry at C-2 and the ring size of the lactone to be firmly established.

The title compound, C 6 H 10 O 5 , has been crystallized for the first time, allowing the stereochemistry at C-2 and the ring size of the lactone to be firmly established.

Comment
The Kiliani ascension of ketoses (Hotchkiss et al., 2004;Soengas et al., 2005) provides ready access to a new class of branched carbohydrate scaffolds (Lichtenthaler & Peters, 2004;Bols, 1996) with branched carbon chains. Although saccharinic acids, which are 2-C-methyl aldonic acids, are formed in very low yields from treatment of aldoses or ketoses with aqueous calcium hydroxide (Whistler & BeMiller, 1963), it has been shown that significantly higher yields may be obtained from the reaction of lime with ketoses (Hotchkiss et al., 2006) derived from the Amadori rearrangement (Hodge, 1955). d-Galactose reacted with dibenzylamine to form the Amadori ketose, (2) (Grunnagel & Haas, 1969), in which the -configuration at the anomeric position of the pyranose ring has been proved by X-ray crystallographic analysis (Harding et al., 2005). Treatment of (2) with aqueous calcium hydroxide allowed the isolation of a mixture of two epimeric lactones.
Racemic lactone (4) has only been obtained as an oil (Lopez et al., 1984); the enantiomer of (4) has been prepared in low yield from l-sorbose (Ishizu et al., 1972). The absolute configuration of (4) was determined from the use of d-galactose (1) as the starting material.
In the absence of significant anomalous scattering, Friedel pairs were merged. H atoms were located in a difference density map. Those attached to C atoms were repositioned geometrically. H atoms were initially refined with soft restraints on the bond lengths and angles to regularize their geometry (C-H = 0.93-0.98 Å and O-H = 0.82 Å ) and isotropic displacement parameters [U iso (H) = 1.2-1.5U eq (parent atom)], after which their positions were refined with riding constraints.
Financial support from EPSRC (to DH) is acknowledged. The molecular structure of (4), with displacement ellipsoids drawn at the 50% probability level. H-atom radii are arbitrary.

Figure 2
Packing diagram of (4), viewed down the c axis. Hydrogen bonds are displayed with dashed lines.

S1. Comment
The Kiliani ascension of ketoses (Hotchkiss et al., 2004;Soengas et al., 2005) provides ready access to a new class of branched carbohydrate scaffolds (Lichtenthaler & Peters, 2004;Bols, 1996) with branched carbon chains. Although saccharinic acids -which are 2-C-methyl aldonic acids -are formed in very low yields from treatment of aldoses or ketoses with aqueous calcium hydroxide (Whistler & BeMiller, 1963), it has been shown that significantly higher yields may be obtained from the reaction of lime with ketoses (Hotchkiss et al., 2006) derived from the Amadori rearrangement (Hodge, 1955). d-Galactose reacted with dibenzylamine to form the Amadori ketose (2) (Grunnagel & Haas, 1969), in which the α-configuration at the anomeric position of the pyranose ring has been proved by X-ray crystallographic analysis (Harding et al., 2005). Treatment of (2) with aqueous calcium hydroxide allowed the isolation of a mixture of two epimeric lactones. Table 1.
The structure of the minor isomer was confirmed as 2-C-methyl-d-xylono-1,4-lactone (3) by an X-ray structure of its 3,5-acetonide . The major product 2-C-methyl-d-lyxono-1,4-lactone (4), initially isolated as an oil, slowly crystallized allowing the relative configuration at C-2 and the ring size of the lyxonolactone to be unambiguously assigned by X-ray crystallographic analysis. Racemic lactone (4) has only been obtained as an oil (Lopez et al., 1984); the enantiomer of (4) has been prepared in low yield from l-sorbose (Ishizu et al., 1982(Ishizu et al., or 1972. The absolute configuration of (4) is determined from the use of dgalactose (1) as the starting material.

S2. Experimental
The lactone (4) (m.p. 379-380 K, [α] D 23 +70.4 (c 0.87 in acetone)) was crystallized by dissolving it in acetone and allowing the slow evaporation of the solvent until colourless block-shaped crystals formed. The multi-scan technique was used to correct for changes in the illuminated volume.

S3. Refinement
Because the data were collected with molybdenum radiation, there were no measurable anomalous differences, as a consequence of which it was admissible to merge Friedel pairs of reflections. H atoms were seen in a difference density synthesis. Those attached to C 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 = 0.96-0.98, O-H = 0.81-0.88 Å), after which they were refined as riding, with U(H) = 1.2U eq (C) for those bonded to carbon, and U(H) = 0.05 Å 2 for the hydroxy group.  The asymmetric unit of (4), with displacement ellipsoids drawn at the 50% probability level. H-atom radii are arbitrary.

Figure 2
Packing diagram of (4), viewed down the c axis. Hydrogen bonds are displayed with dashed lines.