Ethyl 1-benzyl-1,2,3,3a,4,10b-hexahydropyrrolo[2′,3′:3,4]pyrrolo[1,2-a]benzimidazole-2-carboxylate

The title molecule, C22H23N3O2, was obtained via an intramolecular cycloaddition of an azomethine ylide and an alkene tethered by a benzimidazole unit. The benzoimidazole unit is essentially planar, with an r.m.s. deviation of 0.0087 Å from the nine constituent atoms. It has a cis fusion of the two pyrrolidine rings as well as a cis ester appendage. The two pyrrolidine rings rings have envelope conformations. The crystal packing is stabilized by aromatic π–π stacking of parallel benzimidazole ring systems, with a centroid-to-centroid distance of 3.518 (6) Å. Weak intermolecular C—H⋯O contacts may also play a role in the stability of the packing.

The title molecule, C 22 H 23 N 3 O 2 , was obtained via an intramolecular cycloaddition of an azomethine ylide and an alkene tethered by a benzimidazole unit. The benzoimidazole unit is essentially planar, with an r.m.s. deviation of 0.0087 Å from the nine constituent atoms. It has a cis fusion of the two pyrrolidine rings as well as a cis ester appendage. The two pyrrolidine rings rings have envelope conformations. The crystal packing is stabilized by aromaticstacking of parallel benzimidazole ring systems, with a centroid-tocentroid distance of 3.518 (6) Å . Weak intermolecular C-HÁ Á ÁO contacts may also play a role in the stability of the packing.

Related literature
Polycyclic nitrogen-containing heterocycles form the basic skeleton of numerous alkaloids and physiologically active compounds, see: Southon & Buckingham (1989). Conformational studies have been reported for related pyrrolidino [3,4b]pyrrolidine-2-carboxylates obtained from intramolecular cycloaddition of azomethine ylides, see: Cheng et al.   Table 1 Hydrogen-bond geometry (Å , ).  In the structure of the title compound ( Fig. 1), the benzoimidazole unit is essentially planar, with a root mean square deviation of 0.0087 Å from the nine constituent atoms. The two pyrrolidine rings have envelope forms and are cis fused, which is consistent with conventional azomethine ylide cycloadditions in similar systems (Pedrosa et al., 2006;Yang et al., 2006).
However, unlike the previously reported analogues obtained from an intramolecular azomethine ylide and alkene cycloaddition tethered by an oxazolidin-2-one (Cheng et al., 2001), the ester appendage in the title structure was unambiguously assigned as cis to the angular protons H2A and H10A by X-ray crystallography. The crystal packing (Fig. 2) exhibits π-π stacking of parallel benzimidazole ring systems, with a Cg1···Cg2 distance of 3.518 Å [Cg1 and Cg2 are the centroids of the C3A-C8A benzene ring in one molecule and the C2B/N2/C3A/C8A/N8 imidazole ring in the other molecule, respectively].
Intermolecular C-H···O contacts may also play a role in the stability of the packing.

Experimental
The title compound was prepared from 1-allyl-1H-benzimidazole-2-carbaldehyde, N-benzylglycine ethyl ester hydrochloride, and triethylamine according to the procedure reported by Meng et al. (2007). Colourless blocks of the title compound were obtained by recrystallization from EtOAc/n-hexane 1:1 with slow evaporation at room temperature. These crystals were suitable for X-ray crystallography.

Refinement
H atoms were located directly in a difference Fourier map and then allowed to refine freely throughout the final convergence stage. The final structure was refined to convergence [Δ/σ ≤ 0.001]. The final difference Fourier map was featureless, indicating that the structure is both correct and complete.  ethyl 11-benzyl-1,8,11-triazatetracyclo[7.6.0.0 2,7 .0 10,14 ]pentadeca-2(7),3,5,8-tetraene-12-carboxylate Refinement. The crystal to detector distance was 4.123 cm, thus providing a complete sphere of data to 2θ max = 55.06°. A total of 23465 reflections were collected and corrected for Lorentz and polarization effects and absorption using Blessing's method (Blessing, 1995) as incorporated into the program SADABS with 4390 unique. All crystallographic calculations were performed on a personal computer (PC) with a Pentium 3.20 GHz processor and 1 GB of extended memory. The SHELXTL program package was implemented to determine the probable space group and set up the initial files. System symmetry, systematic absences and intensity statistics indicated the centrosymmetric monoclinic non-standard space group P2 1 /n (No. 14). The structure was determined by direct methods with the successful location of a majority of the molecule within the asymmetric unit using the program XS. The structure was refined with XL. The 23465 data collected were merged based upon identical indices yielding 16536 data [R(int) = 0.0245] that were truncated to 2θ max = 55.0° resulting in 16147 data that were further merged during least-squares refinement to 4194 unique data [R(int) = 0.0373].
A single least-squares difference Fourier cycle was required to locate the remaining non-H atoms. All non-H atoms were refined anisotropically.
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.