3,4-O-Isopropylidene-2-C-methyl-d-galactonolactone

X-ray crystallography unequivocally confirmed the stereochemistry of the 2-C-methyl group in the title molecule, C10H16O6, in which the 1,5-lactone ring exists in a boat conformation. The use of d-galactose in the synthesis determined the absolute stereochemistry. The crystal exists as O—H⋯O hydrogen-bonded layers in the ab plane, with each molecule acting as a donor and acceptor for two hydrogen bonds.

X-ray crystallography unequivocally confirmed the stereochemistry of the 2-C-methyl group in the title molecule, C 10 H 16 O 6 , in which the 1,5-lactone ring exists in a boat conformation. The use of d-galactose in the synthesis determined the absolute stereochemistry. The crystal exists as O-HÁ Á ÁO hydrogen-bonded layers in the ab plane, with each molecule acting as a donor and acceptor for two hydrogen bonds.
Data collection: COLLECT (Nonius, 2001); cell refinement: DENZO/SCALEPACK (Otwinowski & Minor, 1997); data reduction: DENZO/SCALEPACK; program(s) used to solve structure: SIR92 (Altomare et al., 1994); program(s) used to refine structure: CRYSTALS (Betteridge et al., 2003); molecular graphics: CAMERON (Watkin et al., 1996); software used to prepare material for publication: CRYSTALS. Comment 2-C-Methyl branched sugars constitute a class of rare sugars with chemotherapeutic potential Booth et al., 2008) as well as being chirons for the enantiospecfic synthesis of complex targets (Hotchkiss et al., 2006;Hotchkiss et al., 2007;Booth et al., 2009) including 2'-C-methyl nucleosides . In a project to investigate the physical and biological properties of 2-C-methyl-D-galactose 4, D-galactose 1 [the use of which determines the absolute stereochemistry of the product] was converted by a number of steps to the lactols 2 ( Fig.   1) . The reaction of 2 with sodium cyanide in water gave a chain extension to afford a single isolated crystalline product 3 (Fig. 2). 3,4-O-Isopropylidene-1,5-lactones, such as 3, invariably crystallize in a boat conformation (Baird et al., 1987;Bruce et al., 1990;Punzo et al., 2005); the diastereoselectivity may be rationalized by the formation of the galactono-lactone 3 with less steric congestion (Punzo et al., 2006;Booth et al., 2007a;Booth et al., 2007b) than in the epimeric talono-lactone. The structure of 3 is confirmed by the X-ray crystallographic analysis reported in this paper. The lactone 3 is an intermediate for the unambiguous synthesis of 2-C-methyl-D-galactose 4.
The 6-membered lactone ring adopts a boat conformation with the hydroxy group rather than the methyl group in the flagpole position (Fig. 2). The title compound exists as O-H···O hydrogen bonded layers of molecules in the ab-plane ( Fig. 3, Fig. 4). Each molecule acts as a donor and acceptor for 2 hydrogen bonds. Only classical hydrogen bonds have been considered.

Refinement
In the absence of significant anomalous scattering, Friedel pairs were merged and the absolute configuration was assigned from the starting material.
One outlying reflection was omitted for the refinement as it was thought to be partially occluded by the beam stop.
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 (