Crystal structure of ethyl 2-(4-chlorophenyl)-3-cyclopentyl-4-oxo-1-propylimidazolidine-5-carboxylate

The title compound, C20H27ClN2O3, was obtained via an original synthesis method. The central heterocyclic ring adopts a shallow envelope conformation, with the N atom bearing the cyclopentane ring as the flap [deviation from the other atoms = 0.442 (2) Å]. The cyclopentane ring adopts a twisted conformation about one of the CN—C bonds: the exocyclic C—N bond adopts an equatorial orientation. The dihedral angles between the central ring (all atoms) and the pendant five- and six-membered rings are 10.3 (2) and 87.76 (14)°, respectively. In the crystal, C—H⋯O interactions link the molecules into [011] chains. A weak C—H⋯Cl interaction links the chains into (100) sheets. A mechanism for the cyclization reaction is proposed.

In the present work we have developed an efficient strategy for the synthesis of 1-cyclopenty-2-parachlorophenyl-3propyl-5-ethoxycarbonylimidazolidin-4-one (I) (Fig.1) via ring expansion of aziridine-2-carboxylate upon reaction with propylisocyanate. It should be mentioned that in a similar protocol, Gomes et al. (2006) report that aziridines rearrange under the effect of heating or radiation and transform into azomethines. The latter reacts subsequently on various electrophiles systems.
A result similar to one described by Zhang et al.(2008), but the authors did not explain the formation of the compounds obtained. To explain the formation of the imidazolidin-4-one we based on work that was performed by Gomes et al. (2006) and in which the authors suggest that aziridines rearrange under the effect of heating or irradiation and become an azomethine. The latter reacts subsequently on various electrophile systems. In our case, the attack of the isocyanate by the carbanion of azomethine, formed upon the refluxing in toluene aziridine, adequately explains obtaining imidazolidin-4-one after cyclization of the intermediate formed.

S2.1. Synthesis and crystallization
To a solution of ethyl 3-(4-chlorophenyl)-1-cyclopentylaziridine-2-carboxylate (2.20 mmol) in toluene (10 ml) under nitrogen atmosphere, were added n-Propylisocyanate (2.64 mmol). The mixture was refluxed during 20 hours. After completeness of the reaction, the mixture was concentrated under reduced pressure and the residue was purified by silica gel column chromatography using a mixture of n-hexane / EtOAc (5:5) as eluent to afford colourless prisms of the studied compound.

S2.2. Refinement
Hydrogen atoms were treated by a mixture of independent and constrained refinement. In fact hydrogen atoms from H1 to H15 were located in the difference Fourier Map. The others H atoms were located geometrically and refined using a riding model. Synthesis protocol of C 20 H 27 ClN 2 O 3 .

Figure 2
Perspective view of the title compound showing 50% displacement ellipsoids.

Figure 3
Unit cell projection of C 20 H 27 ClN 2 O 3 showing two molecules per cell.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. 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.