Dimethyl 2,6-dimethyl-1,4-dihydropyridine-3,5-dicarboxylate

In the crystal of the title compound, C11H15NO4, the molecules are linked into sheets by N—H⋯O and C—H⋯O hydrogen bonds. Within the molecule, the 1,4-dihydropyridine ring exhibits a distinctive planar conformation [r.m.s. deviation from the mean plane of 0.009 (3)Å], and the other non-H atoms are almost coplanar [r.m.s. deviation = 0.021 (3) Å] with the 1,4-dihydropyridine ring. The conformation of the latter is governed mainly by two intramolecular C—H⋯O non-classical interactions.

In the crystal of the title compound, C 11 H 15 NO 4 , the molecules are linked into sheets by N-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds. Within the molecule, the 1,4-dihydropyridine ring exhibits a distinctive planar conformation [r.m.s. deviation from the mean plane of 0.009 (3)Å ], and the other non-H atoms are almost coplanar [r.m.s. deviation = 0.021 (3) Å ] with the 1,4-dihydropyridine ring. The conformation of the latter is governed mainly by two intramolecular C-HÁ Á ÁO nonclassical interactions.

Experimental
Data collection: SMART (Bruker, 1997); cell refinement: SAINT (Bruker, 1997); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL. This work was supported by the Innovation Scientists and Technicians Troop Construction Projects of Henan Province (2008IRTSTHN002). The authors are grateful to the Physiochemical Analysis Measurement Laboratory, College of Chemistry, Luoyang Normal University, for performing the Xray analysis.
In the I, interestingly, 1,4-DHP ring exhibit perfectly coplanar conformation with r.m.s. deviation from the mean plane of 0.009 (3)Å. This conformation is significantly diverse from those found in other 1,4-DHP derivatives, where each of the 1,4-dihydropyrimidine rings adopts flat-boat conformation (Quesada et al., 2006;Ramesh, et al., 2008;Zhao & Teng, 2008;Bai et al., 2009). Another point of interest in the conformation concerns the ester portion of the molecule. In each molecule, there are two short non-classical intramolecular C-H···O interactions (Table 1), and these, we think, control and stabilize the conformations of the two methoxycarbonyl fragments, which are both coplanar with the 1,4-DHP ring, as shown by the torsion angles. However, for C2-methoxycarbonyl it is carbonyl atom O2 that participates in the intramolecular hydrogen bond, and for C4-methoxycarbonyl it is ethoxy O3 atom. Within the 1,4-DHP ring, the C1-C2 and C4-C5 distances shows markedly two double bonds. The N1-C1 and N1-C5 bonds are significantly shorter than the standard N-C experimental bond length of 1.47Å (Mak, et al., 2002). These features in bond distance suggest the existence of π-delocation in the C2/C1/N1/C5/C4 fragment.
Due to the above conformational features of I, its supramolecular structure exhibits some interesting feature. The molecules of the title compounds are linked into sheets by two independent intermolecular hydrogen bonds, one of N-H···O and one C-H···O type (Table 1), the formation of which is readily analyzed in terms of two one-dimensional substructures, one formed by the the N-H···O hydrogen bond and one formed by the C-H···O hydrogen bond. For the sake of simplicity, we shall omit any further consideration of other C-H···O intermolecular interaction involving C7-methyl group, which is too weak to influence the overall dimensionality of the supramolecular structure. In the first substructure, atom N1 in the molecule at (x, y, z) acts as a hydrogen-bond donor to the methoxycarbonyl atom O4 in the molecule at (x-1, y, z), thus forming by translation a C 2 2 (6) (Bernstein et al., 1995) chain running along the [1 0 0] direction (Fig. 2). In the second substructure, methyl atom C11 in the molecule at (x, y, z) acts as a hydrogen bond donor via H11B to methoxycarbonyl atom O2 in the milecule at (x+1, y, z-1), so forming by translation a C(9) (Bernstein et al., 1995) chain parallel to the [-1 0 1] direction (Fig. 2). The combination of the two chain motifs is sufficient to link all the molecules into a two-dimensional sheet parallel to (0 1 0). Two such sheets pass through each unit cell in the domains 0 < y < 1/2 and 1/2 < y < 1, and there are no direction-specific interactions between the two sheets.
supplementary materials sup-2 Experimental Into a three-necked round-bottomed flask equipped with a stirrer were introduced methyl 3-aminobut-2-enoate (0.1 mol, 11.5 g), aqueous formaldehyde (0.05 mol, 37% 4.0 g) and ethanol (95%, 25 ml). The resulted mixture was refluxed with stirring for ca 20 min, and then the solution is cooled to room temperature. The precipitate was filtered off, washed with cool ethanol (95%), and the resulting solid product was recrystallized from hot ethanol to give crystals of I.

Refinement
All H atoms other than the C1-and C5-methyl H atoms were located in a difference map and then treated as riding atoms with C-H distances of 0.96Å (CH 3 ) or 0.97Å (CH 2 ), and N-H distance of 0.86Å with U iso (H) = 1.2U eq (C, N) or 1.5U eq (methyl C). The C1-and C5-methyl H atoms was modelled as idealized disordered methyl groups over two sets offset by 60°. Figures   Fig. 1. The molecular structure of I, showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 30% probability level. H atoms are shown as small spheres of arbitrary radius. Only one component of the disordered methyl groups is shown. Dimethyl 2,6-dimethyl-1,4-dihydropyridine-3,5-dicarboxylate

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 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 > σ(F 2 ) is used only for calculating Rfactors(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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (