(8aS)-7,8,8a,9-Tetrahydrothieno[3,2-f]indolizin-6(4H)-one

In the molecular structure of the title compound, C10H11NOS, the central six-membered ring of the indolizine unit adopts an envelope conformation, the maximum deviations from the mean plane of the ring being 0.533 (2) Å. The fused thieno ring is nearly coplanar [mean deviation = 0.007 (2) Å]. The conformation of the fused oxopyrrolidine ring is close to that of a flat-envelope, with a maximum deviation of 0.339 (3) Å. The crystal structure is stabilized by C—H⋯O hydrogen bonds.

In the molecular structure of the title compound, C 10 H 11 NOS, the central six-membered ring of the indolizine unit adopts an envelope conformation, the maximum deviations from the mean plane of the ring being 0.533 (2) Å . The fused thieno ring is nearly coplanar [mean deviation = 0.007 (2) Å ]. The conformation of the fused oxopyrrolidine ring is close to that of a flat-envelope, with a maximum deviation of 0.339 (3) Å . The crystal structure is stabilized by C-HÁ Á ÁO hydrogen bonds.
Based on these facts and in continutation of our interest in developing simple and efficient routes for the synthesis of novel indolizine derivatives, we report here the synthesis, molecular and crystal structure of the title compound, (I). The absolute configuration is known from the synthesis and is depicted in the scheme and Figure 1. The expected stereochemistry of atom C5 was confirmed as S. The central N-heterocyclic ring is not planar and adopts an envelope conformation (Nardelli, 1983).
A calculation of least-squares planes shows that this ring is puckered in such a manner that the five atoms N1, C11, C10, C7 and C6 are coplanar to within 0.032 (3) Å, while atom C5 is displaced from this plane with out-of-plane displacement of 0.533 (2) Å. The fused thieno ring is nearly coplanar [mean deviation = 0.007 (2) Å]. The oxopyrrolidine ring attached to the indolizine ring system has flat-envelope conformation with atom C5 on the flap. The deviation of atom C5 from the mean plane of the remaining four atoms N1/C2/C3/C4 is 0.339 (3) Å. The N1-C5 and N1-C11 bonds are approximately equivalent and both are much longer than the N1-C2 bond. Moreover, the N1 atom is sp 2 hybridized, as evidenced by the sum of the valence angles around it [356.1 (1)°]. These data are consistent with conjugation of the lone-pair electrons on N1 with the adjacent carbonyl and agree with literature values for simple amides (Brown & Corbridge, 1954;Pedersen, 1967).
The bond length of the carbonyl group C2=O1 is 1.224 (2) Å, respectively, is somewhat longer than typical carbonyl bonds. This may be due to the fact that atom O1 participates in intermolecular C-H···O hydrogen bonds with atom C9 ( Table 2).
The bond lengths and angles in the molecule are comparable with those in a related structure (Vrábel et al., 2004(Vrábel et al., ,Švorc et al., 2007.

Experimental
Triethylsilane (2.4 ml, 15 mmol) was added to a stirred solution of alcohol (2.1 g, 10 mmol) in trifluoroacetic acid (20 ml) at 0 °C, and the resulting yellow solution was stirred at rt for 2 h. The reaction mixture was concentrated in vacuo, diluted with water (50 ml), made alkaline with 10% Na 2 CO 3 , and then extracted with dichloromethane (3 x 50 ml). The supplementary materials sup-2 extract was washed with water, dried over magnesium sulfate, and concentrated in vacuo. The residue was purified by flash chromatography on a silica gel column eluting with dichloromethane. Recrystallization of a solid from cyclohexane gaves amide as colorless crystals (Marchalín et al. 2008).

Refinement
All H atoms were placed in geometrically idealized positions and constrained to ride on their parent atoms, with C-H distances in the range 0.93 -0.98Å and U iso set at 1.2U eq of the parent atom. The absolute configuration could not be reliably determined for this compound using Mo radiation, and has been assigned according to the synthesis. Fig. 1. Molecular structure of (I) with the atomic numbering scheme. Displacement ellipsoids are drawn at the 50% probability level (Brandenburg, 2001). Rotation method data acquisition using ω and φ scans h = −8→8 Absorption correction: analytical (Clark & Reid, 1995)  The absolute configuration could not be reliably determined for this compound using Mo-radiation, and has been assigned according to the synthesis. 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 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.