Crystal structure of dimethyl 2-((2Z,5Z)-5-(2-methoxy-2-oxoethylidene)-2-{(E)-[2-methyl-5-(prop-1-en-2-yl)cyclohex-2-enylidene]hydrazinylidene}-4-oxothiazolidin-3-yl)fumarate

The crystal structure of the title compound, dimethyl 2-((2Z,5Z)-5-(2-methoxy-2-oxoethylidene)-2-{(E)-[2-methyl-5-(prop-1-en-2-yl)cyclohex-2-enylidene]hydrazinylidene}-4-oxothiazolidin-3-yl)fumarate displays a conformational disorder which inverts the configuration of the chiral C atom within the cyclohexylidene ring.


Chemical context
In recent years, the synthesis of heterocyclic systems containing nitrogen and sulfur has attracted great interest because of their broad spectrum of pharmacological activities. The thiazol nucleus is found in a large number of natural products (Nielsen et al., 2012), as well as in diverse pharmaceutical products (Le Flohic et al., 2005). Indeed, some 4-arylthiazole derivatives exhibit a strong anti-inflammatory activity (Hirai & Sugimoto, 1977) while some tetrahydrothiazolo- [4,5-b] pyridines show antioxidant properties (Uchikawa et al., 1996). The therapeutic usefulness of these heterocyclic systems prompted us to prepare a new substituted thiazole which shows important medicinal properties. The title compound 2 was synthesized by the reaction of (R)-thiosemicarbazone carvone 1 easily obtained from naturally occurring (R)-carvone] with dimethyl acetylenedicarboxylate in basic medium, using ethanol as solvent. The resulting product 2 was obtained in 65% yield.
The most striking feature of this structure is the conformational statistical disorder which affects the cyclohexylidene ring: atoms C6 0 and C5 0 are split over two positions, each of half occupancy, with respect to the mean plane of the butadiene (C1 0 -C4 0 ) fragment ( Fig.1). Such disorder inverts the configuration at C5 (R C5 0 A and S C5 0 B) and so the crystal might be considered as a racemate. Could the crystal be considered as a co-crystal built up from the combination of R and S configurations? It is difficult to answer this question.

Supramolecular features
In the crystal, there are C-HÁ Á ÁO weak hydrogen-bonding interactions (Table 1) which link the molecules, building a twodimensional network parallel to the (001) plane, as shown in Fig. 2.   The molecular view of the title compound with the atom-labelling scheme. Displacement ellipsoids are drawn at the 30% probability level. H atoms are represented as small circle of arbitrary radii. The disordered part is shown with dashed lines. Table 1 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) Àx þ 1; Ày þ 2; Àz þ 1; (ii)

Figure 2
A packing view showing the formation of layers parallel to the (001) plane.
A comparison of the main C-N, N-N, C-S distances in the title compound and the structures extracted from the CSD shows good correlation: within the C N-N C fragment, the double bonds are located on the CN, the N-N distance is that of a single bond corresponding to a hydrazono group. The C N-N C torsion angles (Table 2) indicate that in each case the four atoms are nearly planar.

Synthesis and crystallization
A solution of (1R)-thiosemicarbazone carvone 1 and dimethyl acetylenedicarboxylate (1.25 eq) in anhydrous MeCN (50 mL), was heated under reflux for 30 min. After the completion of the reaction (the progress of the reaction was monitored by TLC), the solvent was evaporated to dryness. The crude product was purified by silica gel chromatography (230-400 mesh) using hexane/ethyl acetate (95:5) as eluent. The pure thiazolic product 2 was obtained in 65% yield. Slow evaporation from an ethanolic solution of the title compound gave crystals of 2 suitable for crystallographic analysis.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The disorder was been refined using the tools available in SHELXL2014. All H atoms were initially located in a difference Fourier map but were placed in geometrically idealized positions and constrained to ride on their parent atoms with C-H = 0.95-1.0 Å and O-H = 0.84 Å , with U iso (H) = 1.5U eq (C) for methyl H atoms and 1.2U eq (C,O) for all other H atoms. Acta Cryst. (2017). E73, 296-299 research communications Table 2 Comparison of main bond lengths and C N-N C torsion angles (Å , ) in the title compound and related structures.  (3)     where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.30 e Å −3 Δρ min = −0.26 e Å −3 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.