3 , 5O-Isopropylidene-2-C-methyl-D-xylonolactone

Almost all carbohydrate scaffolds contain linear carbon chains (Lichtenthaler & Peters, 2004). The two exceptions that do provide carbohydrates with branched carbon chains are (i) the Kiliani reaction on ketoses which provides efficient access to a set of 2-C-hydroxymethylaldonolactones (Hotchkiss et al., 2004; Soengas et al., 2005), and (ii) the treatment of sugars with base to give 2-C-methyl aldonic acids, also known as saccharinic acids (Bols, 1996). However, the reaction of base with sugars is complex: glucose gives a mixture of more than 50 compounds on treatment with calcium hydroxide, of which branched sugars comprise a very small percentage (Yang & Montgomery, 1996). Better yields are obtained from ketoses; however, even the optimized conditions (several weeks under careful control in a laborious procedure) for treatment of dfructose with calcium hydroxide afford 2-C-methyl-d-ribonolactone in only 11% yield (Whistler & BeMiller, 1963). Very low yields of branched lactones have been isolated from similar treatment of l-sorbose (Ishizu et al., 1972). A further ketohexose, d-tagatose (1), has recently become available in quantity as a new food additive (Skytte, 2002); (1) has the potential for making 2-C-methyl-d-xylonolactone as a branched-sugar building block under green environmentally friendly conditions. Treatment of d-tagatose with aqueous calcium hydroxide produces a very complex mixture of products. In order to identify the branched-chain sugar products, it was necessary to make authentic samples of easily crystallized derivatives.

The ring size of both the lactone and the ketal protecting group in the title compound, C 9 H 14 O 5 , have been established by X-ray crystallographic analysis. The crystal structure consists of hydrogen-bonded spirals parallel to the b axis.

Comment
Almost all carbohydrate scaffolds contain linear carbon chains (Lichtenthaler & Peters, 2004). The two exceptions that do provide carbohydrates with branched carbon chains are (i) the Kiliani reaction on ketoses which provides efficient access to a set of 2-C-hydroxymethylaldonolactones (Hotchkiss et al., 2004;Soengas et al., 2005), and (ii) the treatment of sugars with base to give 2-C-methyl aldonic acids, also known as saccharinic acids (Bols, 1996). However, the reaction of base with sugars is complex: glucose gives a mixture of more than 50 compounds on treatment with calcium hydroxide, of which branched sugars comprise a very small percentage (Yang & Montgomery, 1996). Better yields are obtained from ketoses; however, even the optimized conditions (several weeks under careful control in a laborious procedure) for treatment of dfructose with calcium hydroxide afford 2-C-methyl-d-ribonolactone in only 11% yield (Whistler & BeMiller, 1963). Very low yields of branched lactones have been isolated from similar treatment of l-sorbose (Ishizu et al., 1972). A further ketohexose, d-tagatose (1), has recently become available in quantity as a new food additive (Skytte, 2002); (1) has the potential for making 2-C-methyl-d-xylonolactone as a branched-sugar building block under green environmentally friendly conditions. Treatment of d-tagatose with aqueous calcium hydroxide produces a very complex mixture of products. In order to identify the branched-chain sugar products, it was necessary to make authentic samples of easily crystallized derivatives.
A crystalline acetonide was obtained from treatment of 2-C-methyl-d-xylonolactone with acetone in the presence of acid. The absolute stereochemistry of (2) is determined by using d-tagatose (1) as the starting material; however, there are ambiguities in the synthesis with regard to the relative stereochemistry at C-2 of the lactone, the ring size of the lactone and the ring size of the ketal. X-ray crystallographic analysis removed all the ambiguities and firmly established the structure of the acetonide as (2).
In the absence of significant anomalous scattering, Friedel pairs were merged, and the absolute configuration assigned from the known staring materials.
The H atoms were all located in a difference map, but 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 in the range 0.93-0.98 and O-H = 0.82 Å ) and displacement parameters [U iso (H) in the range 1.2-1.5 times U eq of the parent atom], after which they were refined with riding constraints.
Financial support from the EPSRC (to DH) is gratefully acknowledged.

Figure 1
The title compound, with displacement ellipsoids drawn at the 50% probability level. H atoms are shown as spheres of arbitary radius.

S1. Comment
Almost all carbohydrate scaffolds contain linear carbon chains (Lichtenthaler & Peters, 2004). The two exceptions that do provide carbohydrates with branched carbon chains are (i) the Kiliani reaction on ketoses which provides efficient access to a set of 2-C-hydroxymethyl-aldonolactones (Hotchkiss et al., 2004;Soengas et al., 2005), and (ii) the treatment of sugars with base to give 2-C-methyl aldonic acids, also known as saccharinic acids (Bols, 1996). However the reaction of base with sugars is complex: glucose gives a mixture of more than 50 compounds on treatment with calcium hydroxide, of which branched sugars comprise a very small percentage (Yang & Montgomery, 1996). Better yields are obtained from ketoses; however, even the optimized conditions (several weeks under careful control in a laborious procedure) for treatment of D-fructose with calcium hydroxide afford 2-C-methyl-D-ribonolactone in only 11% yield (Whistler & BeMiller, 1963). Very low yields of branched lactones have been isolated from similar treatment of L-sorbose (Ishizu et al., 1972). A further ketohexose, D-tagatose (1), has recently become available in quantity as a new food additive (Skytte, 2002); (1) has the potential for making 2-C-methyl-D-xylonolactone as a branched-sugar building block under green environmentally friendly conditions. Treatment of D-tagatose with aqueous calcium hydroxide produces a very complex mixture of products. In order to identify the branched-chain sugar products, it was necessary to make authentic samples of easily crystallized derivatives.
A crystalline acetonide was obtained from treatment of 2-C-methyl-D-xylonolactone with acetone in the presence of acid. The absolute stereochemistry of (2) is determined by using D-tagatose (1) as the starting material; however, there are ambiguities in the synthesis with regard to the relative stereochemistry at C-2 of the lactone, the ring size of the lactone and the ring size of the ketal. X-ray crystallographic analysis removed all the ambiguities and firmly established the structure of the acetonide as (2).

S3. Refinement
In the absence of significant anomalous scattering, Friedel pairs were merged, and the absolute configuration assigned from the known staring materials.
The H atoms were all located in a difference map, but 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 in the  The title compound, with displacement ellipsoids drawn at the 50% probability level. H atoms are shown as spheres of arbitary radius.