(4′-Acetyloxy-1,3,1′-trioxo-1,3,4,4a,4b,5,6,7,9,9a-decahydrospiro[indene-2,9′-pyrano[4,3-a]pyrrolizin]-3′-yl)methyl acetate

In the title compound, C23H23NO8, the dihedral angle between the five- and six-membered rings of the indene-dione moiety is 3.09 (13)°. The mean plane of the five-membered ring (which has a flat envelope conformation with the spiro C atom as the flap) is inclined to the mean plane of the central five-membered ring of the pyrrolizine unit by 76.48 (12)°. This central ring has a twist conformation on the N—C(spiro) bond. The outer ring of the pyrrolizine unit has an envelope conformation with the N atom as the flap. The mean planes of these two fused rings are inclined to one another by 65.28 (15)°. The pyran ring has a screw-boat conformation and its mean plane makes a dihedral angle of 29.50 (11)° with the mean plane of the central five-membered ring of the pyrrolizine unit. In the crystal, molecules are linked via C—H⋯O hydrogen bonds, forming two-dimensional networks lying parallel to the ab plane.


Comment
The pyrrolidine skeleton occurs in many families of biologically important compounds. The resulting functionality due to ease of substitution and modification at several positions has been utilized to synthesize compounds with antimicrobial and antifungal properties . Derivatives of pyrrolidine are very useful in preventing and treating rheumatoid arthritis, asthma and allergies. They also possess anticonvulsant, anti-influenza (Gayathri et al., 2005) and antivirus activities (Kumar et al., 2006). The spiro (indole-pyrrolidine) ring system is a frequently encountered structural motif in many pharmacologically important alkaloids (Seshadri et al., 2003). In view of its biological activities, the structure determination of the title compound, C 23 H 23 NO 8 was completed by X-ray diffraction. In this compound the sum of the angles at N1 of the pyrrolidine moiety (325.7°) is consistent with sp 3 hybridization (Kalyanasundaram et al., 2005). The dihedral angle between the five and six-membered rings of the indanone moiety is 3.5 (1)° (Satis Kumar et al., 2007). In the benzene ring of the indole system, the endocyclic angles at C3 and C6 are contracted to 117.5 (3) and 117.1 (3)°, respectively, while those at C4, C5 and C7 are expanded to 122.1 (3), 121.4 (4) and 121.6 (2)°, respectively. This may be due to the fusion of the indole and benzene ring systems where the strain results in angular distortion (Govind et al., 2004). The dihedral angle between the mean planes of the indole system and the pyrrolidine moiety is 77.3 (1)°. The spiro junction at C9 in the indanone group deviates from the mean plane of the C1 -C8 ring by 0.2465 Å.
Weak C-H···O intermolecular hydrogen bonds (Table 1) generate a one-dimensional chain structure extending along the a axis (Fig. 2).

Experimental
To a solution of ninhydrin (1 equiv) and proline (1.4 equiv) in dry toluene, α,β-unsaturated sugar lactone was added under a nitrogen atmosphere. The solution was refluxed for 15 h under Dean-Stark reaction conditions to give a cycloadduct.
After completion of the reaction indicated by TLC, the solvent was evaporated under reduced pressure. The residual mass was extracted with dichloromethane and water. The organic layer was dried with anhydrous sodium sulfate and concentrated in vacuo. The crude mass was purified by column chromatography using hexane/EtOAc (8:2) as an eluent.
Crystals suitable for X-ray diffraction were obtained from ethyl acetate solution using a slow evaporation method.

Refinement
The positions of all the hydrogen atoms were identified from the difference electron density map and were allowed to ride on the parent atoms in calculated positions with distances C-H = 0.93 -0.98 Å and U iso (H) = 1.2U eq (C) for nonmethyl groups and U iso (H) = 1.5U eq (C) for the methyl groups. The absolute structure was not determined since no strong anamalous scattering atoms are present. However, the configuration for the six trivially named chiral centres Friedel pairs] (Flack, 1983).

Figure 1
The molecular structure of the title compound with the numbering scheme. Displacement ellipsoids are drawn at the 50% probability level.  The packing of the title compound, viewed down the c axis.  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.46 e Å −3 Δρ min = −0.21 e Å −3 Extinction correction: SHELXL97 (Sheldrick, 2008), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.0075 (16) 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.