Ethyl 5-oxo-2,3-diphenylcyclopentane-1-carboxylate

The title compound, C20H20O3, was prepared by an acyloin-type condensation reaction in the presence of sodium sand and dry ether using ethyl cinnamate as the starting material. The C—O bond lengths for the carbonyl groups are 1.191 (2) and 1.198 (2) Å, while the C—O bond in the ester group is 1.335 (2) Å. The C—C bond lengths in the phenyl groups average 1.375 Å, while the C—C bonds in the cyclopentanone ring average 1.525 Å, indicating single C—C bonds in the latter.

The title compound, C 20 H 20 O 3 , was prepared by an acylointype condensation reaction in the presence of sodium sand and dry ether using ethyl cinnamate as the starting material. The C-O bond lengths for the carbonyl groups are 1.191 (2) and 1.198 (2) Å , while the C-O bond in the ester group is 1.335 (2) Å . The C-C bond lengths in the phenyl groups average 1.375 Å , while the C-C bonds in the cyclopentanone ring average 1.525 Å , indicating single C-C bonds in the latter.

Support
by the NOAA-EPP award number NA06OAR4810187 to NCAT and by the ACS-PRF is gratefully acknowledged by ZA. The authors also acknowledge the National Science Foundation for their generous support (NSF-CAREER grant to RES, CHE-0846680).
Comment β-keto esters are a class of potentially useful synthetic intermediates in the preparation of some physiologically active compounds. The medicinal values of this class of compounds have been demonstrated as antitumor, antianxiety, and antihypertension agents. General methods of β-keto ester preparation have been described in several publications including by March (1985), Shiosaki et al.(1981), and Matsumoto et al. (1973). Acyloin-type condensation reactions of α, β unsaturated esters have also been demonstrated in several publications of Totton et al. (1961Totton et al. ( ), (1965Totton et al. ( ), (1967, and Singh & Totton (1981). The mechanism of this condensation reaction was first suggested by Weidlich (1938) and confirmed by the successful synthesis of several adducts. Synthesis of the title compound was first performed by Totton et al. (1965). However, the compound has not previously been characterized by X-ray diffraction and therefore these studies were undertaken in order to elucidate details of the molecular structure. The title compound, C 20 H 20 O 3 , contains three chiral centers. These correspond to atom sites C2, C3, and C4 and contain R, S, and R configurations, respectively. The C-O bond lengths for the two carbonyl groups are 1.191 (2) and 1.198 (2) Å with the ring carbonyl having the slightly longer distance. The C-O bond in the ester group is quite a bit longer than the carbonyl distances, as expected, at 1.335 (2) Å. The aromatic C-C bond lengths in the phenyl groups are not extraordinary and average to 1.375 Å, while the C-C bonds in the cyclopentanone ring have an average distance of 1.525 Å indicative of the single bond nature. The molecular nature of the compound is preserved in the solid state. No significant interactions, e.g. H-bonding interactions, etc., are observed in the structure.

Experimental
The synthesis of the (1R,2S,3R)-ethyl 5-oxo-2,3-diphenylcyclopentanecarboxylate product was accomplished by modification of the prior procedure used by Totton (1961). Into a 1 L three necked round bottom flask fitted with a reflux condenser containing a CaCl 2 drying tube was added 400 ml dry ether and 13 g of freshly prepared sodium sand, 50 g of ethyl cinnamate (287.1 mmol) was then added dropwise over a period of two hours. A series of color changes were observed where the initial light orange color changed to deep orange and finally to reddish brown. The mixture was stirred and refluxed overnight and cooled in an ice bath. While stirring, 70 ml of a 35% sulfuric acid was added carefully through an addition funnel. The reaction turned to yellow-orange color. The mixture was transferred to a large separatory funnel and the layers separated.
The aqueous layer was then extracted with two -75 ml portions of ether and combined to the original ether layer and which was then washed with four -50 ml portions of a 20 % sodium carbonate solution and 100 ml water. The ether solution was dried over 50 g of anhydrous sodium sulfate, filtered by gravity, and the solvent removed with a rotatory evaporator. The sticky residue was dissolved in 200 ml of 95% ethanol and left for 1 hr at room temperature and kept in freezer overnight.
The product was recrystallized several times from a 95% ethanol/water mixture. Yield was 10 %.
The compound is soluble in a number of organic solvents including diethyl ether, dichloromethane, methanol, ethanol etc, but found insoluble in hexane and hence single crystals for X-ray measurements were grown from an ether/hexane mixture.
The product was characterized using several spectroscopic techniques in addition to the X-ray analysis. The melting point The compound shows a bright blue unstructured emission covering the 400-600 nm s pectral region at room temperature with the emission band maximizing at 460 nm. The excitation spectrum displays two broad bands at 310 nm and 405 nm. At liquid N 2 temperature well defined bands are observed at 440 and 480 nm with a shoulder at 520 nm. The excitation band at liquid N 2 temperature is also broad, centering at 380 nm. The overall emission spectrum is unaffected upon changing the excitation wavelength.

Refinement
H-atoms were placed in calculated positions and allowed to ride during subsequent refinement, with U iso (H) = 1.2U eq (C) and C-H distances of 0.93 Å for the aromatic H atoms, with U iso (H) = 1.2U eq (C) and C-H distances of 0.98 Å for tertiary H atoms, with U iso (H) = 1.2U eq (C) and C-H distances of 0.97 Å for secondary H atoms, and with U iso (H) = 1.5U eq (C) and C-H distances of 0.96 Å for methyl H atoms. The terminal methyl group corresponding to C8 has a relatively large thermal ellipsoid corresponding to a high degree of thermal motion. Fig. 1. The molecular structure of I, with the atom-numbering scheme. Displacement ellipsoids for non-hydrogen atoms are drawn at the 50% probability level.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds 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 > 2σ(F 2 ) is used only for calculating Rfactors(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.