Crystal structures of the two epimers from the unusual thermal C6-epimerization of 5-oxo-1,2,3,5,5a,6,7,9b-octahydro-7,9a-epoxypyrrolo[2,1-a]isoindole-6-carboxylic acid, 5a(RS),6(SR),7(RS),9a(SR),9b(SR) and 5a(RS),6(RS),7(RS),9a(SR),9b(SR)

The two epimers from the unusual thermal C6-epimerization of 5-oxo-1,2,3,5,5a,6,7,9b-octahydro-7,9a-epoxypyrrolo[2,1-a]isoindole-6-carboxylic acid, 5a(RS),6(SR),7(RS),9a(SR),9b(SR) and 5a(RS),6(RS),7(RS),9a(SR),9b(SR) have similar geometries but differ in their hydrogen-bonded crystal-packing modes


Chemical context
The intramolecular Diels-Alder furan (IMDAF) reaction between ,-unsaturated acid anhydrides and hydrogenated heterocycles, containing a furfurylamine moiety, has been studied for a long time (see, for example, Parker & Adamchuk, 1978;Blokzijl et al., 1991;Varlamov et al., 2006;Groenendaal et al., 2008;Nakamura et al., 2011;Zubkov et al., 2011Zubkov et al., , 2012Zubkov et al., , 2014Toze et al., 2015) and used for diastereospecific synthesis of diverse fused-ring systems. It is arguable that the pathway with a simultaneous controlled formation of four or five new stereogenic centers is the best approach to epoxyisoindoles and affords target adducts under mild conditions with satisfactory yields. However, the simplest 2-furyl azaheterocycles (azetidine, pyrrolidine, piperidine, perhydroazepine ) have not yet been studied in this reaction. One of the goals of our work is to fill the gap. Here we report on the ISSN 2056-9890 utilization of 2-furyl pyrrolidine as an initial reagent in the IMDAF reaction.
The interaction between 2-furyl pyrrolidine and maleic anhydride at room temperature leads to the mixture of cyclic (Ia) and open-chain (Ib) tautomers, the crystallization of which results in the cyclic form (Ia) only ( Fig. 1). In contrast, the same reaction at 413 K leads to the maleic amide fragment isomerization and affords a mixture of the adduct (IIa) and the amide (IIb) (Fig. 2). Similarly, the mixture crystallization gives rise the cyclic tautomer (IIa) only. The crystal structures of both (Ia) and (IIa) using synchrotron X-ray diffraction data have been determined and are reported herein.

Figure 3
Molecular structure and atom-numbering scheme for epimer (Ia). Displacement ellipsoids are drawn at the 50% probability level. H atoms are presented as small spheres of arbitrary radius.

Figure 4
Molecular structure and atom-numbering scheme for epimer (IIa). Displacement ellipsoids are drawn at the 50% probability level. H atoms are presented as small spheres of arbitrary radius. ment with those observed in a related structure (Lu et al., 2013).

Supramolecular features
Although the similarity of the molecular geometries might lead to similar packing motifs, this is not found in the case of (Ia) and (IIa). The intermolecular interactions, namely strong O-HÁ Á ÁO and weak C-HÁ Á ÁO hydrogen bonding, combined in a different way, give rise to different packing networks. In the crystal of (Ia), molecules form zigzag-like hydrogenbonded chains extending along [010] through strong O12-H12Á Á ÁO5 i hydrogen bonds, which are further linked by weak C5A-H5AÁ Á ÁO12 ii hydrogen bonds into complex two-tier layers lying parallel to (100) ( Table 1, Fig. 5).

Figure 5
Crystal structure of (Ia) showing the two-tier layers parallel to (100).

Refinement
Crystal data, data collection and refinement details are summarized in Table 3. X-ray diffraction studies were carried out on the 'Belok' beamline ( = 0.96990 Å ) of the National Research Center "Kurchatov Institute" (Moscow, Russian Federation) using a MAR CCD detector. The hydrogen atoms of the hydroxyl groups were localized in the difference-Fourier maps and refined in an isotropic approximation with fixed displacement parameters [U iso (H) = 1.5U eq (O)] [for (Ia)] or included in the refinement with fixed positional (riding model) and isotropic displacement parameters [U iso (H) = 1.5U eq (O)] [for (IIa)]. Other hydrogen atoms were placed in calculated positions with C-H = 0.95-1.00 Å and refined in the riding model with fixed isotropic displacement parameters [U iso (H) = 1.2U eq (C)].
The insufficient data completeness of 94.1% in the case of (IIa) is the result of the low (triclinic) crystal symmetry, making it very difficult to obtain good data completeness using      (9) 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq C1 0.8345 (2) (17) (11) 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.