(2Z,2′Z)-Diethyl 3,3′-[butane-1,4-diylbis(azanediyl)]bis(but-2-enoate)

The whole molecule of the title β-enaminoester, C16H28N2O4, is generated by a crystallographic inversion center, situated at the mid-point of the central C—C bond of the 1,4-diaminobutane segment. There are two intramolecular N—H⋯O hydrogen bonds that generate S(6) ring motifs. This leads to the Z conformation about the C=C bonds [1.3756 (17) Å]. The molecule is S-shaped with the planar central 1,4-diaminobutane segment [maximum deviation for non H-atoms = 0.0058 (13) Å] being inclined to the ethyl butylenonate fragment [C—C—O—C—C=C—C; maximum deviation = 0.0710 (12) Å] by 15.56 (10)°. In the crystal, molecules are linked via C—H⋯O interactions, leading to the formation of an undulating two-dimensional network lying parallel to the bc plane.

The tile compound was prepared by condensation of ethyl 3-oxobutanoate with 1,4-diaminobutane using a catalytic amount of Ca(CF 3 COO) 2 under solvent-free conditions according to the procedure we have previously described (Harrad et al., 2010). The β-enaminoester was typically characterized by 1 H, 13 C NMR, IR and mass spectroscopy. The characteristic broad singlet for the amine proton appears at 8.50 p.p.m., the singlet corresponding to the proton of the double bond at 4.29 p.p.m. The triplet and the quartet for the ethyl moiety appeared at 1.11 and 3.98 p.p.m., respectively.
The molecular structure of the title molecule is illustrated in Fig The bond distances and angles are normal and similar to those in related compounds (Harrad et al., 2011a,b;Amézquita-Valencia et al., 2009). There are two intramolecular N-H···O hydrogen bonds (Table 1), that generate S(6) ring motifs (Bernstein et al., 1995). This leads to the Z conformation about the C4═C5 and C4a═C5a bonds (Fig. 1).
In the crystal, molecules are linked via C-H···O interactions leading to the formation of an undulating two-dimensional network that lies parallel to the bc plane (Table 1 and Fig. 2).

Experimental
In a typical experiment 1.0 mmol of Ethyl acetoacetate, 0.5 mmol of 1,4-diaminobutane and 0.1 mmol of Ca(CF 3 COO) 2 were stirred at room temperature for 30 min under solvent free conditions. At the end of the reaction, 10 ml of distilled water was added to the residue and it was extracted with diethyl ether (3 × 25 ml). The organic layer was dried over Na 2 SO 4 . The solvent was removed under reduced pressure, and pure β-enaminone was obtained by column chromatography over silica gel using hexane/ethyl acetate (

Refinement
All the H-atoms were located in a difference Fourier map. In the final cycles of refinement the NH H-atom was freely refined, while the C-bound H-atoms were included in calculated positions and treated as riding atoms: C-H = 0.95, 0.99 and 0.98 Å for CH(allyl), CH 2 and CH 3 , respectively, with U iso (H) = k × U eq (C), where k = 1.5 for CH 3 H-atoms, and = 1.2 for all other H-atoms.

Special details
Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su's are estimated from the variances of the (full) variance-covariance matrix. The cell e.s.d.'s are taken into account in the estimation of distances, angles and torsion angles Refinement. The NH H-atom was located in a difference electron-density map and freely refined. The C-bound H-atoms were included in calculated positions and treated as riding atoms: C-H = 0.95, 0.99 and 0.98, Å for CH(allyl), CH 2 and CH 3 H-atoms, respectively, with U iso (H) = k × U eq (parent C-atom), where k = 1.5 for CH 3 H-atoms and = 1.2 for other H atoms.