Diethyl 2,6-dimethyl-4-p-tolyl-1,4-dihydropyridine-3,5-dicarboxylate

In the title compound, C20H25NO4, the 1,4-dihydropyridine ring adopts a flattened-boat conformation and forms a dihedral angle of 89.77 (8)° with the benzene ring. Intermolecular N—H⋯O hydrogen bonds result in the formation of extended chains parallel to the b axis.

In the title compound (I, Fig. 1), the bond lengths and angles have normal values and are comparable to closely related structures (Teng et al., 2008;Bai et al., 2009). The dihedral angle between the benzene ring and the mean plane of 1,4-dihydropyridine ring is 89.77 (8)°, indicating that both the rings are perpendicular to each other. The 1,4-dihydropyridine ring adopts a flattened-boat conformation (Boeyens, 1978;Cremer & Pople, 1975) with ring distortions at the nitrogen (Nl) and the tetrahedral carbon (C7). Both atoms are displaced in the same direction from the ring with distances of 0.115 (1) Å and 0.151 (2) Å, respectively, from the plane defined by C8, C9, C10, and C11, and form the apexes of a boat-type conformation (Fossheim et al., 1982).
The crystal packing ( Fig. 2) is consolidated by intermolecular N1-H1···O1 hydrogen bonds, Table 1, linking the molecules into chains parallel to the b-axis.

Experimental
Compound (I) was prepared according to the Hantzsch pyridine synthesis. A mixture of p-tolylaldehyde (10 mmol), ethylacetoacetate (20 mmol) and ammonium acetate (10 mmol) were heated at 353 K for 2 h (monitored by TLC). After completion of the reaction, the mixture was cooled to room temperature and kept for 2 days to obtain the solid product.
The solid that formed was washed using diethyl ether. Solid was collected separately and liquid was kept for solidification.
The purity of the crude product was checked through TLC and recrystallized using acetone and ether.

Refinement
The N-bound H atom was located in a difference Fourier map and constrained to ride with the parent atom with N-H = 0.84 Å, and with U iso (H)= 1.2 U eq (N). C-bound H atoms were positioned geometrically [C-H = 0.93-0.98 Å] and refined using a riding model, with U iso (H) = 1.2U eq (C) and 1.5U eq (methyl-C). A rotating group model was used for the methyl groups.