Crystal structure of ethyl 2-cyano-3-[(1-ethoxyethylidene)amino]-5-(3-methoxyphenyl)-7-methyl-5H-1,3-thiazolo[3,2-a]pyrimidine-6-carboxylate

In the title compound, C22H24N4O4S, the central pyrimidine ring adopts a sofa conformation with the ring-junction N atom displaced by 0.2358 (6) Å from the mean plane of the remaining ring atoms. The 3-methoxyphenyl ring, at the chiral C atom opposite the other N atom, is positioned axially and is inclined to the thiazolopyrimidine ring with a dihedral angle of 83.88 (7)°. The thiazole ring is essentially planar (r.m.s. deviation = 0.0034 Å). In the crystal, pairs of weak C—H⋯O hydrogen bonds link molecules related by twofold rotation axes to form R 2 2(8) rings, which in turn are linked by weak C—H⋯N interactions, forming ribbons along [-110]. In addition, π–π stacking interactions [centroid—centroid distance = 3.5744 (15) Å] connect the ribbons, forming slabs lying parallel to (001).

Pyrimidine has been subjected to a variety of structural modifications in order to synthesize derivatives (Singh et al., 2011) with different biological properties, among which, a thiazole ring fused to a pyrimidine ring, viz. a thiazolopyrimidine, has been found to be more active (Ozair et al., 2010a,b;Sayed et al., 2010). Thiazolo[3,2-a]pyrimidine derivatives act as potential enzyme inhibitors and are novel therapeutic entities for severe neurodegenerative diseases (Zhi et al., 2008). In continuation of our research interests on thiazolo[3,2-a]pyrimidine derivatives , we attempted to synthesize tricyclic thiazolopyrimidine derivatives (Mobinikhaledi et al., 2005). The title compound, an intermediate, was isolated and we report herein on its crystal structure.
The molecular structure of the title compound is shown in Fig. 1. The 3-methoxy phenyl ring at chiral carbon C5 is positioned axially and exactly bisects the thiazolopyrimidine ring with a dihedral angle of 83.88 (7)°. The pyrimidine ring adopts a flattened sofa conformation with atom N1 displaced by 0.2358 (6) Å from the mean plane of the other five atoms (C5/C6/C7/N2/C9). The carbonyl group of the exocyclic ester at C6 adopts a cis orientation with respect to C6-C7 double bond. The 3-methoxy phenyl ring adopts a syn periplanar conformation with respect to C5-H5 bond of the pyrimidine ring. The thiazole ring is essentially planar (r.m.s. deviation = 0.0034 Å).

S3. Refinement
The H atoms were placed at calculated positions in the riding model approximation: C-H = 0.95 -1.00 Å with U iso (H) = 1.5U eq (C) for methyl H atoms and = 1.2U eq (C) for other H atoms.

Figure 1
The molecular structure of the title compound, with the atom labelling. Displacement ellipsoids are drawn at the 50% probability level.

Figure 2
Crystal packing of the title compound viewed along the b axis, showing the intermolecular interactions as dashed lines (see Table 1). H-atoms not involved in hydrogen bonding have been omitted for clarity.  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.49 e Å −3 Δρ min = −0.31 e Å −3 Special details Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.