6-[Bis(ethoxycarbonyl)methyl]-6-deoxy-1,2;3,4-di-O-isopropyl-idene-d-galacto-pyran-ose.

The title compound, C(19)H(30)O(9), was prepared by substitution at the C6 position in 1,2;3,4-di-O-isopropyl-idene-6-O-trifluoro-methane-sulfonyl-d-galactose using sodium eth-oxy-malonate in dimethyl-formamide. The conformation is skew-boat (0)S(2), slightly distorted towards boat B(2,5). The inflexible pyran-ose structure makes the title compound a suitable inter-mediate for further synthetic work by keeping stereogenic carbon atoms safe from inversion. Several short intra-molecular C-H⋯ O contacts may stabilize the conformation of the mol-ecule. Inter-molecular C-H⋯O inter-actions also occur.

The title compound, C 19 H 30 O 9 , was prepared by substitution at the C6 position in 1,2;3,4-di-O-isopropylidene-6-Otrifluoromethanesulfonyl-d-galactose using sodium ethoxymalonate in dimethylformamide. The conformation is skewboat 0 S 2 , slightly distorted towards boat B 2,5 . The inflexible pyranose structure makes the title compound a suitable intermediate for further synthetic work by keeping stereogenic carbon atoms safe from inversion. Several short intramolecular C-HÁ Á Á O contacts may stabilize the conformation of the molecule. Intermolecular C-HÁ Á ÁO interactions also occur.

Experimental
The absolute structure of 6-deoxy-6-(diethylmalonyl)-1,2;3,4-di-O -isopropylidene-D-galactopyranose is certain from the synthetic route which does not affect asymmetric atoms of the starting compound. Nevertheless, we preferred to receive a direct experimental confirmation using X-ray diffractometry data. Because there are no heavy atoms in a chiral molecule of title compound, Cu Kα radiation was necessary for determination of the absolute structure.
No classic hydrogen bonds are possible for the title compound. However, several short C-H··· O contacts were detected that possibly stabilize the existing conformation of the molecule (Table 1).
The inflexible pyranose structure makes the title compound a suitable intermediate for further synthetic work by keeping the stereogenic carbon atoms C1-C5 safe from inversion. For the same reason, it is very probable that in solution this molecule will keep almost the same geometry as in the molecular crystal.

Refinement
The chirality of the title compound was known from the synthetic route; it was also examined using anomalous scattering.
Analysis of the absolute structure using likelihood methods (Hooft et al., 2008) was performed using PLATON (Spek, 2003); 1570 Bijvoet pairs were employed. The results confirmed that the absolute structure had been correctly assigned: the probability that the structure is inverted is smaller than 10 -9 with probability of racemic twinning at 0.002. Because no atom heavier than O is present, the standard deviation of the Flack parameter is relatively high. All H atoms were positioned geometrically and refined using a riding model, with C-H = 0.99-1.03 Å and U iso (H) = 1.2 or 1.5 U eq (C). The rotating group model was applied for the methyl groups.
Figures Fig. 1. The title compound with displacement ellipsoids drawn at the 50% probability level. H atoms are shown as spheres of arbitary radius.