Methyl 6-oxo-4-phenyl-2-[(Z)-2-(pyridin-2-yl)ethenyl]-1,4,5,6-tetrahydropyridine-3-carboxylate

In the title molecule, C20H18N2O3, an intramolecular N—H⋯O hydrogen bond leads to a cis conformation of the pyridinyl-vinyl fragment. The phenyl and pyridine rings are inclined to one another by 77.3 (1) °. In the crystal, molecules are linked via pairs of C—H⋯O hydrogen bonds, forming inversion dimers. The dimers are linked by C—H⋯O hydrogen bonds and C—H⋯π interactions, forming a three-dimensional structure.

In the title molecule, C 20 H 18 N 2 O 3 , an intramolecular N-HÁ Á ÁO hydrogen bond leads to a cis conformation of the pyridinyl-vinyl fragment. The phenyl and pyridine rings are inclined to one another by 77.3 (1) . In the crystal, molecules are linked via pairs of C-HÁ Á ÁO hydrogen bonds, forming inversion dimers. The dimers are linked by C-HÁ Á ÁO hydrogen bonds and C-HÁ Á Á interactions, forming a threedimensional structure.  Table 1 Hydrogen-bond geometry (Å , ).

Rufus Smits, Sergey Belyakov, Brigita Vigante and Gunars Duburs Comment
Dihydropyridones are important intermediates for the synthesis of natural products, particularly alkaloids (Dong et al., 2005;Elias et al., 2008). They have been extensively investigated as valuable building blocks for the construction of piperidines, perhydroquinolines, indolizidines, quinolizidines and other alkaloid systems, with a wide range of biological and pharmacological activities. Herewith we present the title compound (I).
The main feature of (I) (Fig. 1) is cis-conformation of the pyridinylvinyl fragment, see Table 1 for selected torsion angles. A search of the Cambridge Structural Database (CSD, Version 5.33; November, 2012) (Allen, 2002) indicates that there is no entry containing pyridinylvinyl substituent in cis-conformation. Molecular cis-conformation is stabilized by strong intramolecular hydrogen bond of NH···N type (Table 2). By means of this bond the additional seven-membered cycle is formed in the molecular structure. In the molecule there is also an intramolecular hydrogen bond of CH···O type (Table 2). This bond leads to formation of the additional six-membered cycle in the molecule.
In the crystal structure there are shortened C···O contacts. These contacts can be described as weak CH···O type intermolecular hydrogen bonds. Also it should be noted a weak CH···π type H-bond. The geometrical parameters of these H-bonds are given in Table 2.

Experimental
In a 50 ml RB was placed 0.59 g (0.001 mol) of the DHPOD 6-methyltriphenylphosphonium bromide and dissolved in 25 ml dry THF. Under an Ar atmosphere while stirring magnetically 0.22 g (0.001 mol) of tBuOK was added. The orange solution was stirred for 30 min and 0.11 g (0.001 mol) of 2-pyridinecarboxaldehyde was added. The solution was allowed to stir at RT overnight, 3 ml of aqueous solution containing 0.6 g NH 4 Cl was added and after stirring 15 min the layers were separated. The THF was removed under reduced pressure and the sticky reaction product was dissolved in min.
EtOAc. After addition of hexane the precipitated triphenylphosphine oxide was filtered off and the solvent removed to leave 0.55 g of product. The product was purified using prep. HPLC with 50% EtOAc / DCM as eluent. The solvent was removed providing 0.21 g of product (62% yield) which was recrystallized from EtOH giving 100 mg of light green needles.

Refinement
Atoms H1, H4, H18 and H19 were located on a difference map and isotropically refined. All other H-atoms were positioned geometrically (C-H = 0.93-0.97 Å) and allowed to ride on their parent atoms, with U iso (H) = 1.2 U eq (parent atom).  (Mackay et al., 1999).

Figure 1
The molecular structure of (I) showing 50% probability displacement ellipsoids and the atom numbering scheme. Dashed lines denote intramolecular hydrogen bonds. 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 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq N1 0.3841 (