threo-Diethyl 2-ethyl-2-hydroxy-3-(4-methylbenzenesulfonamido)succinate

The asymmetric unit of the title compound, C17H25NO7S, contains two independent molecules, which are enantiomers forming a hydrogen-bonded dimer associated with two R 2 2(7) patterns. In each molecule, one ethyl group from the two available ethyl ester functional groups is disordered. In one molecule, the ethyl group of the ester function from an α-carboxylic acid is positionally disordered over two sets of sites with occupancies of 0.66:0.34. In the second molecule, it is the ethyl group in the γ-ester function that is disordered over two sets of sites with occupancies of 0.58:0.42.

The asymmetric unit of the title compound, C 17 H 25 NO 7 S, contains two independent molecules, which are enantiomers forming a hydrogen-bonded dimer associated with two R 2 2 (7) patterns. In each molecule, one ethyl group from the two available ethyl ester functional groups is disordered. In one molecule, the ethyl group of the ester function from ancarboxylic acid is positionally disordered over two sets of sites with occupancies of 0.66:0.34. In the second molecule, it is the ethyl group in the -ester function that is disordered over two sets of sites with occupancies of 0.58:0.42.

Comment
In the present work, as a part of an on-going study of asymmetric syntheses of optically pure β-substituted β-hydroxy aspartates (Wehbe et al., 2003a,b,c;Mekki et al., 2011a,b), the structure of a new compound, threo-diethyl 2-ethyl-2-hydroxy-3-(4-methylphenylsulfonamido)succinate, is described. The key step of the synthesis is the regiospecific Sharpless aminohydroxylation on an ethyl fumarate derivative.
The crystal structure is made up by racemic dimers formed by two independent homochiral molecules ((2S,3S) and (2R,3R) for (I) and (II), respectively). They are bonded by non-covalent NH···O and OH···O hydrogen bonds ( Fig. 1) forming two R 2 2 (7) patterns (Etter, 1990;Bernstein et al., 1995), where the H···O distances range from 2.10 (2) Å to 2.232 (19) Å and the D-H···O angles from 142 (2) to 161.9 (18) ° (Table 1). In order to get an idea of the relative strength of the NH···O and OH···O hydrogen bonds the intersection of the Van der Waals surfaces of donor hydrogen and acceptor was calculated using the program Jmol (Jmol, 2011;'contact' command with 'full' and 'hbond' options). The resulting Fig. 2 shows clearly that the Van der Waals interaction zones between the hydroxyl groups and the carbonyl ester O atoms are more important than those between the hydroxyl groups and the secondary amine group. The latter interaction zones are much smaller than the former ones. A calculation based on the electron density and its derivatives (Johnson et al., 2010; calculation done in Jmol using the 'contact' command with 'nci' and 'hbond' as options) gives slightly different results (Fig. 3), in the sense that one of the OH···O interactions appears to be negligible. The relevant Van der Waals surfaces may be inspected in the enhanced Jmol picture in Fig. 4. This pictorial view of the non-covalent interaction regions is not completely in agreement with what could be concluded from the directionality of the interaction which is greater for nitrogen as hydrogen bond donor than for oxygen ( Table 1). The dimeric structure bears much similarity with those reported recently for the two concomitant β-benzyl β-hydroxy aspartate analogue polymorphs (Mekki et al., 2011a).
The two independent homochiral molecules are very approximately related by a local inversion center between the two molecules. That this local center is only very approximate, can be clearly seen in Fig. 5, which shows the best superposition of the (2S,3S) molecule (I) and the (2S,3S) inversion center related molecule (II) as calculated with Olex2 (Dolomanov et al., 2009). The root-mean-squared deviation (considering the majority disordered parts only) between the two molecules is 0.780 Å. The main conformational differences between molecules (I) and (II) stem from the orientation of the ethyl ester moiety in both residues. This is well illustrated by the torsion angles C9-O5-C4-C3 (-4.2 (2)° and 173.6 (3)° for molecules (I) and (II), respectively) and C1-01-C7-C8 (-165.5 (3)° and -88.6 (2)° for molecules (I) and (II), respectively).

supplementary materials sup-2
After 1 h stirring at room temperature a second fraction of diethyl 2-ethylfumarate (180 mg, 0.9 mmole) in CH 3 CN (1.25 ml) was added to the reaction mixture. After 5 h, a solution of Na 2 SO 3 (357 mg) in water (5.4 ml) was added an the reaction mixture was extracted 3 times with AcOEt (5.4 ml). The fraction was then washed with brine and dried under MgSO 4 . The solvent was removed and the title compound was recrystallized in cyclohexane by slow evaporation at ambient temperature yielding colourless crystals in the form of relatively large prisms.

Refinement
All N-bound and O-bound H atoms were located in a difference Fourier maps and later restraint to a distance O-H = 0.82 (2) Å with U iso (H)=1.5U eq (O) and N-H = 0.88 (2) Å with U iso (H)=1.2U eq (N) in order to stabilize their coordinates during the final step of the refinement. All other H atoms were introduced at calculated positions and refined as riding atoms with C-H = 0.96-0.98 Å, with displacement parameters U iso (H) equal to 1.5U eq (C) for methyl and 1.2U eq (C) for all other H atoms.