Ethyl 3-ethoxycarbonylmethyl-7-methyl-5-phenyl-5H-thiazolo[3,2-a]pyrimidine-6-carboxylate

In the title compound, C20H22N2O4S, the central pyrimidine ring incorporating a chiral C atom is significantly puckered and adopts a slight boat conformation with C atom bearing the phenyl ring and the N atom opposite displaced by 0.367 (2) and 0.107 (2) Å, respectively, from the plane formed by the remaining ring atoms. The benzene ring is positioned axially to the pyrimidine ring, making a dihedral angle of 88.99 (5)°. The thiazole ring is essentially planar (r.m.s. deviation = 0.0033 Å). In the crystal, pairs of C—H⋯O interactions result in centrosymmetric dimers with graph-set motifs R 1 2(7) and R 2 2(8). A weak C—H⋯π contact is also observed.

In the title compound, C 20 H 22 N 2 O 4 S, the central pyrimidine ring incorporating a chiral C atom is significantly puckered and adopts a slight boat conformation with C atom bearing the phenyl ring and the N atom opposite displaced by 0.367 (2) and 0.107 (2) Å , respectively, from the plane formed by the remaining ring atoms. The benzene ring is positioned axially to the pyrimidine ring, making a dihedral angle of 88.99 (5) . The thiazole ring is essentially planar (r.m.s. deviation = 0.0033 Å ). In the crystal, pairs of C-HÁ Á ÁO interactions result in centrosymmetric dimers with graph-set motifs R 1 2 (7) and R 2 2 (8). A weak C-HÁ Á Á contact is also observed.
NSB is thankful to the University Grants Commission (UGC), India, for financial assistance and HN thanks for the fellowship.
In the title molecule ( Fig. 1), the benzene ring is positioned axially and lies almost perpendicular to the pyrimidine ring (N1/N2/C5/C6/C7/C9) with dihedral angle of 88.99 (5)°. The pyrimidine ring substituted with C5 chiral carbon atom is significantly puckered and adopts a slight boat conformation with N2 and C5 atoms displaced by 0.107 (2) and 0.367 (2) Å, respectively, from the plane formed by the remaining ring atoms. The thiazole ring (S1/N1/C2/C3/C9) is essentially planar with r.m.s.d 0.0033 Å for the fitted atoms. The ethyl carboxylate at C6 is almost co-planar with the thiazolopyrimidine ring with a dihedral angle of 13.40 (5)°, where as the other ethyl carboxylate group at C17 is inclined at an angle of 80.57 (4)° with the thiazolopyrimidine ring and is positioned almost parallel to the benzene ring. This is because of intramolecular carbonyl-π interaction of aryl ring with the ethyl carboxylate group (Gautrot et al., 2006). The exocyclic ester at C8 adopts a cis orientation with respect to C8═C9 double bond. The N1-C3 bond length (1.403 (2) Å) in the thiazole ring is longer than that of a typical C═N bond but shorter than a C-N single bond, indicating electron delocalization in the ring. The bond distances and angles in the title compound agree very well with the corresponding bond distances and angles reported in a closely related compound (Nagarajaiah & Begum, 2011).
The crystal structure is stabilized by C-H···O intermolecular interactions involving carbonyl O2 atom, resulting in centrosymmetric dimers; the seven and eight membered rings thus resulting from these interaction can be described as R 2 1 (7) and R 2 2 (8) motifs in graph-set notations (Bernstein et al., 1995). In addition π-ring interaction of the type C-H···Cg (Cg being the centroid of the thiazolopyrimidine ring) is also observed in the crystal structure (Table 1 and Fig. 2).

Experimental
The synthesis of the title compound has already been reported (Nagarajaiah et al., 2012). The crystals suitable for X-ray crystallographic analysis were grown from a solution of ethylacetate.

Refinement
The H atoms were placed at calculated positions in the riding model approximation with C-H = 0.93, 0.96, 0.97 and 0.98 Å for aryl, methyl, methylene and methyne H-atoms, respectively, with U iso (H) = 1.5U eq (C) for methyl H atoms and U iso (H) = 1.2U eq (C) for other H atoms.

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.