2,3-O-(S)-Benzylidene-2-C-methyl-d-ribono-1,4-lactone

The crystal structure of the title compound, C13H14O5, establishes (i) the (S) – rather than (R) – configuration at the acetal carbon and (ii) that both the acetal and the lactone form five- rather than six-membered rings; the absolute configuration is determined by the use of 2-C-methyl-d-ribono-1,4-lactone as the starting material. The compound consists of hydrogen-bonded chains of molecules running along the a axis; there are no unusual packing features. Only classical hydrogen bonding has been considered.

The crystal structure of the title compound, C 13 H 14 O 5 , establishes (i) the (S) -rather than (R) -configuration at the acetal carbon and (ii) that both the acetal and the lactone form five-rather than six-membered rings; the absolute configuration is determined by the use of 2-C-methyl-dribono-1,4-lactone as the starting material. The compound consists of hydrogen-bonded chains of molecules running along the a axis; there are no unusual packing features. Only classical hydrogen bonding has been considered.
The structure of 6 was established by X-ray crystallographic analysis (Baggett et al., 1985) which corrected the original erroneous 6 ring benzylidene acetal -5-ring lactone structure proposed (Zinner et al., 1968). The protected 1,5-lactone 6 leaves only the C-2 OH unprotected and has been widely used as a chiron (Chen & Joullie, 1984;Dho et al., 1986;Baird et al., 1987). It was hoped that the analogous reaction with 2-C-methyl lactone 3 would form the analogous lactone 4; however, treatment of 3 with benzaldehyde and concentrated aqueous hydrochloric acid gave as the major product a mixture of epimeric 1,4-lactones 1 and 2; although it was not possible to separate 1 and 2 by chromatography, suitable crystals of the major component 1 were obtained and the structure of a 5 ring benzylidene acetal -5-ring lactone, together with the (S) stereochemistry at the acetal carbon, was firmly established (Fig. 2).
The compound consists of H-O···H hydrogen bonded chains of molecules running along the a-axis (Fig. 3); there are no unusual packing features. Only classical hydrogen bonding has been considered.

Refinement
In the absence of significant anomalous scattering, Friedel pairs were merged.
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, 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