(1S)-1,2-O-Benzylidene-α-d-glucurono-6,3-lactone

X-ray crystallographic analysis has established that the major product from the protection of d-glucoronolactone with benzaldehyde is (1S)-1,2-O-benzylidene-α-d-glucurono-6,3-lactone, C13H12O6, rather than the R epimer. The crystal structure exists as O—H⋯O hydrogen-bonded chains of molecules lying parallel to the a axis. The absolute configuration was determined by the use of d-glucuronolactone as the starting material.

X-ray crystallographic analysis has established that the major product from the protection of d-glucoronolactone with benzaldehyde is (1S)-1,2-O-benzylidene--d-glucurono-6,3lactone, C 13 H 12 O 6 , rather than the R epimer. The crystal structure exists as O-HÁ Á ÁO hydrogen-bonded chains of molecules lying parallel to the a axis. The absolute configuration was determined by the use of d-glucuronolactone as the starting material.
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.  (Fig. 1), the only cheaply availably uronic acid, reacts with acetone in the presence of an acid catalyst to form the acetonide 4 (Sheldrick et al., 1983). With only a single unprotected hydroxyl group, the lactone 4 provides convenient access to C-5 of D-glucose and has long been used as a versatile intermediate for the synthesis of other sugars Dax et al., 1991;Ke et al., 2003;Masaguer et al., 1997), imino sugars (Dax et al., 1990), sugar amino acids (Bashyal et al., 1986(Bashyal et al., , 1987 and many other bioactive targets (Kitahara et al., 1974;Austin et al., 1987;Witty et al., 1990;Shing & Tsui, 1992;Yoda et al., 2002). Reaction of 3 with benzaldehyde in the presence of zinc chloride gives a high yield of the benzylidene protected lactones in which the epimers are formed in a ratio of approximately 5:1 (Macher et al., 1979;Shah, 1969). The configuration of the benzylidene acetal has previously been investigated by NMR experiments which suggest that 1, which is the major product, has the 1,2(S)-configuration (Csuk et al., 1984). The crystallographic analysis confirms that this assignment is correct and that the major product is 1. Although as yet there have been no examples of the use of the benzylidene acetals 1 and 2 as synthetic intermediates, it is likely there will be cases where the use of a benzylidene group, which can be removed by hydrogenation, will have a significant advantage over the acetonide 4, where strong acid must be used to remove the protecting group.
The title compound (Fig. 2) exists as alternating layers of hydrogen bonded chains of molecules lying parallel to the a-axis (Fig. 3, Fig. 4). Only classical hydrogen bonding has been considered. The absolute configuration was determined by the use of D-glucuronolactone as the starting material.

Refinement
In the absence of significant anomalous scattering, Friedel pairs were merged and the absolute configuration was assigned from the starting material.
The H atoms were all located in a difference map, but those attached to C atoms were repositioned geometrically. The