A 3,5-dinitrobenzoyl derivative of a stereoisomer of glycerol menthonide

The title compound, [(2S,5R,6S,9R)-6-isopropyl-9-methyl-1,4-dioxaspiro[4.5]dec-2-yl]methyl 3,5-dinitrobenzoate, C20H26N2O8, was synthesized as part of a study of three-carbon stereochemical systems. The crystallographic assignment of the absolute stereochemistry is consistent with having started with (−)-menthone, the acetal carbon is R and the secondary alcohol is S. This brings the dinitrobenzoate into approximately the same plane as the menthyl ring and anti to the isopropyl group. Close intermolecular C=O⋯NO2 contacts between neighboring molecules [2.8341 (16) Å] contribute to the packing arrangement. The structure was refined as a pseudo-merohedral twin (monoclinic space group P21 emulating the orthorhombic space group C2221). Application of the twin law 100, 00, 0 gave a 2:1 ratio of twin moieties [refined BASF value = 0.3790 (7)].


Related literature
For the synthesis of glycerol menthonide, see: Greenberg (1999). For the synthesis and NMR spectra of the title compound, see: Kiessling et al. (2009). Glidewell et al. (2003) report a related structure with a very short C O Á Á Á NO 2 distance. Allen et al. (1998)  Data collection: APEX2 (Bruker, 2008); cell refinement: SAINT (Bruker, 2008); data reduction: SAINT; program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL. stereochemistry of the third owing to the steric bulk of the menthone.
The starting material, glycerol menthonide, was originally prepared as an additive to spearmint gum by reaction of menthone with glycerol under acid catalysis. (Greenberg, 1999) No further chemical analysis of the menthonide has been reported in the literature.
Glycerol menthonide exists in as many as six isomers which are difficult to separate. However, conversion of the hydroxy group to an ester by reaction with 4-bromobenzoyl chloride yields a mixture of esters that are separable by flash chromatography.
One stereochemically pure ester was hydrolyzed back to the free alcohol then converted to the 3,5-dinitrobenzoate.
The crystallographic assignment of the absolute stereochemistry is consistent with having started with (-)-menthone, and provides the stereochemistry of the acetal carbon and the esterified secondary alcohol of the glycerol chain. Specifically, the acetal carbon, C5, is R and the secondary alcohol, C2, is S. This brings the dinitrobenzoate into approximately the same plane as the menthyl ring and anti to the isopropyl group.
There is a close contact between the carbonyl oxygen, O25, and one of the nitro groups on a 2 1 screw-related molecule, specifically N28 in the molecule at (1 -x, -0.5, 2 -z). The orientation of the carbonyl group is nearly perpendicular to the plane of the nitro group and the O25 ··· N28 distance is 2.8341 (16) Å. A search of the Cambridge Structural Database (Allen, 2002) for intermolecular C=O ··· NO 2 -benzene groups found 360 observations for C=O ··· NO 2 distances of 3.07 or less. Of these, only seventeen observations were shorter than that reported here, and each of these had a similar perpendicular orientation.
The simplest structure in this set is that of 3-nitrophthalic acid (Glidewell et al., 2003) wherein the C=O ··· NO 2 distance was reported as 2.807 (2) Å, which the authors attributed to the electrostatic interaction between the partially negative oxygen of the carbonyl and the partially positive nitrogen of the nitro group and analogous to the short intermolecular C=O ··· C=O contacts frequently found between carbonyl groups.
In a study (Allen, et al., 1998) of these intermolecular C=O ··· C=O interactions based on a combination of a detailed analysis of structures from the Cambridge Structural Database as well as ab initio molecular-orbital calculations the authors conclude that, although these intermolecular forces are only a fraction of that of hydrogen bonds, they are significant contributors to the stabilization of the solid state structures. It appears a similar argument could be made for C=O ··· NO 2 interactions.
supplementary materials sup-2 Experimental Details on the synthesis of the title compound and its NMR spectra have been published separately. (Kiessling et al., 2009)