(6S,7S,8S,8aS)-6-Ethyl-3-oxo-1,2,3,5,6,7,8,8a-octahydroindolizine-7,8-diyl diacetate

In the molecular structure of the title compound, C14H21NO5, the six-membered ring of the indolizine moiety adopts a chair conformation. There are two independent molecules in the asymmetric unit. The oxopyrrolidine ring attached to the indolizine ring system is nearly planar, with mean deviations of 0.018 (3) and 0.010 (3) Å for the two molecules. The absolute configuration of the title compound was assigned from the synthesis.

In the molecular structure of the title compound, C 14 H 21 NO 5 , the six-membered ring of the indolizine moiety adopts a chair conformation. There are two independent molecules in the asymmetric unit. The oxopyrrolidine ring attached to the indolizine ring system is nearly planar, with mean deviations of 0.018 (3) and 0.010 (3) Å for the two molecules. The absolute configuration of the title compound was assigned from the synthesis. Heterocycles are involved in a wide range of biologically important chemical reactions in living organisms, and therefore they form one of the most important and well investigated classes of organic compounds. One group of heterocycles, indolizines, has received much scientific attention during the recent years. Indolizine derivatives have been found to possess a variety of biological activities such as antibacterial, antiinflammatory, antiviral, (Nash et al., 1988;Molyneux & James, 1982;Medda et al., 2003), anti-HIV (Ruprecht et al., 1989), anti-cancer (Liu et al., 2007Smith et al., 2007), and antitumor (Pearson & Guo, 2001). They have also shown to be calcium entry blockers (Gupta et al., 2003) and potent antioxidants inhibiting lipid peroxidation in vitro (Teklu et al., 2005). As such, indolizines are important synthetic targets in view of developing new pharmaceuticals for the treatment of cardiovascular diseases (Gubin et al., 1992). Based on these facts and in continuation of our interest in developing simple and efficient route for the synthesis of novel indolizine derivatives. We report here the synthesis, molecular and crystal structure of the title compound, (I), which crystallizes in the monoclinic space group P21 with two crystallographic independent molecules in asymmetric unit. The absolute configuration was established by synthesis. The expected stereochemistry of atoms C5, C6, C7 and C8 for molecule A and C19, C20, C21 and C22 for molecule B was confirmed for all as S, see Fig. 1. The central six-membered Nheterocyclic ring is not planar and adopts a chair conformation (Cremer & Pople, 1975). A calculation of least-squares planes shows that this ring is puckered in such a manner that the four atoms C5, C6, C8 and C9 (C19, C20, C22 and C23

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for molecule B) are coplanar to within 0.018 (2)

Experimental
The title compound was prepared according to a standard protocol described in literature (Šafář et al., 2010).

Refinement
All H atoms were positioned with idealized geometry using a riding model with C-H distances in the range 0.93 -0.98 Å. The U iso (H) values were set at 1.2 U eq (C-aromatic) or 1.5 U eq (C-methyl). The absolute configuration could not be reliably determined for this compound using Mo radiation, and has been assigned according to the synthesis.

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.

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