rac-Ethyl (2Z)-3-{2-[(Z)-4-ethoxy-4-oxobut-2-en-2-ylamino]cyclohexylamino}but-2-enoate

The asymmetric unit of the title compound, C18H30N2O4, contains two independent molecules. In each molecule, the cyclohexane ring adopts a chair conformation with equatorial orientation of the substituents, and the conformation is stabilized by two intramolecular N—H⋯O hydrogen bonds, forming rings of S(6) graph-set motif. One ethoxy group and one ethyl group are disordered over two sets of sites with refined occupancy ratios of 0.704 (2):0.296 (2) and 0.505 (3):0.495 (3), respectively. In the crystal, a weak intermolecular C—H⋯O hydrogen interaction is observed, involving the O atom of the major component of the disordered ethoxy group.


Related literature
Financial support from the Universitá degli Studi di Parma is gratefully acknowledged.
for the preparation of a large number of heterocyclic derivatives (Elaridi, Thaqi et al., 2005) and novel organometallic complexes (Cornils & Herrmann, 1996;Venter et al., 2009). Furthermore, chiral β-amino acid derivatives were prepared by enantioselective hydrogenation of β-enaminoesters (Elaridi, Jackson & Robinson, 2005). In the course of our studies in this field, we have recently reported an efficient method for the synthesis of various β-enaminoesters (Harrad et al., 2010). Following our catalysis objective on the coupling of amines and keto-ester compounds, we describe herein the crystal structure of a new β-enaminoester which has been prepared using our previously mentioned method by the dicondensation of trans-cyclohexane-1,2-diamine with 3-oxo-butyric acid ethyl ester under solvent-free conditions.

Experimental
To a stirred mixture of 3-oxo-butyric acid ethyl ester (1.7 mmol), trans-cyclohexane-1,2-diamine (0.85 mmol) and Ca(CF 3 COO) 2 (0.17 mmol) at room temperature 10 ml of distilled water was added, and the residue extracted with diethyl ether (3 × 25 ml). The organic layer was dried over Na 2 SO 4 and the solvent removed under reduced pressure. The title β-enaminoester was obtained by column chromatography over silica gel using a mixture of n-hexane/ethyl acetate (5:95 v/v) as eluent (yield 90%; m. p. 160 °C). Crystals suitable for X-ray analysis were obtained on slow evaporation of the solvent at room temperature.

Refinement
The amine H atoms were located in a difference Fourier map and refined freely. All other H atoms were placed at calculated positions and refined using a riding model approximation, with C-H = 0.93-0.98 Å, and with U iso (H) = 1.2 U eq (C) or 1.5 U eq (C) for methyl H atoms. One ethoxy group (O6/C29-C30) and one ethyl group (C35-C36) are disordered over two orientations (called A and B) with refined site occupancy ratios of 0.704 (2):0.296 (2) and 0.505 (3):0.495 (3), respectively.
During the refinement, the O-C and C-C bond distances within the disordered groups were restrained to be 1.45 (1) and 1.49 (1) Å, respectively, and the anisotropic displacement parameters of the pairs of the disordered atoms were set equal by the command EADP (Sheldrick, 2008). Fig. 1. The asymmetric unit of the title compound, with displacement ellipsoids drawn at the 30% probability level. Only the major components of disorder are shown.