Crystal structure of the thalidomide analog (3aR*,7aS*)-2-(2,6-dioxopiperidin-3-yl)hexahydro-1H-isoindole-1,3(2H)-dione

The title compound, C13H16N2O4, consists of a six-membered unsaturated ring bound to a five-membered pyrrolidine-2,5-dione ring and N-bound to a six-membered piperidine-2,6-dione ring and thus has the same basic skeleton as thalidomide, except for the six-membered unsaturated ring substituted for the aromatic ring.

Thus, over the years, there has been increasing interest in thalidomide and its derivatives for the treatment of various hematologic malignancies (Singhal et al., 1999;Raje & Anderson, 1999), solid tumors (Kumar et al., 2002), and a variety of inflammatory and autoimmune diseases (Tseng et al., 1996). Recent studies have uncovered a variety of mechanisms of thalidomide action. It was reported in 1991 that thalidomide is a selective inhibitor of tumor necrosis factor-(TNF-) production in lipopolysaccharide (LPS) stimulated human monocytes (Moreira et al., 1993;Sampaio et al., 1991). TNF-a is a key pro-inflammatory cytokine, and elevated levels have been linked with the pathology of a number of inflammatory and autoimmune diseases including rheumatoid arthritis, Crohn's disease, aphthous ulcers, cachexia, graft versus host disease, asthma, ARDS and AIDS (Eigler et al., 1997). Taken together, the immunomodulatory properties of thalidomide, which are dependent on the type of immune cell activated as well as the type of stimulus that the cell receives, provide a rationale for the mechanism of thalidomide action in the context of autoimmune and inflammatory disease states. Other pharmacologic activities of thalidomide include its inhibition of angiogenesis (D'Amato et al., 1994) and its anti-cancer properties (Bartlett et al., 2004). In the late 1990 0 s it was reported that thalidomide is efficacious for the treatment of multiple myeloma (MM), a hematological cancer caused by growth of tumor cells derived from the plasma cells in the bone marrow (Singhal et al., 1999;Raje & Anderson, 1999).
A medicinal chemistry program to optimize the immunomodulatory properties of thalidomide and reduce its sideeffects led to the discovery of lenalidomide (2), which is a potent immunomodulator that is $800 times more potent as an inhibitor of TNF-in LPS-stimulated hPBMC (Muller et al., 1999;Zeldis et al., 2011). In the US, lenalidomide was approved by the FDA in 2005 for low-or intermediate-1-risk myelodysplastic Structural optimization of thalidomide, 1 also led to the discovery of pomalidomide (3), which is tenfold more potent than lenalidomide as a TNF-a inhibitor and IL-2 stimulator (Muller et al., 1999;Zeldis et al., 2011). Pomalidomide is currently undergoing late-stage clinical development for the treatment of multiple myeloma and myeloproliferative neoplasm-associated myelofibrosis (Galustian & Dalgleish, 2011;Begna et al., 2012). In clinical trials for multiple myeloma, pomalidomide has been shown to be effective in overcoming resistance to lenalidomide and thalidomide, as well as the proteosome inhibitor bortezomib (Schey & Ramasamy, 2011).
These studies have shown the efficacy of a continued search for more pharmacologically active analogs of thalidomide and its derivatives. Focus has previously been on modifying the basic thalidomide skeleton by changing its substituents. However, there have been very few studies on related derivatives where the six-membered ring is changed from an aromatic to an unsaturated ring. In view of the wide interest in these types of compounds for their pharmacological activities, the structure of (3aR,7aS)-2-(2,6-dioxopiperidin-3-yl)hexahydro-1H-isoindole-1,3(2H)-dione, 4, is reported where the only change to thalidomide is the substitution of an unsaturated six-membered for the aromatic ring.
As a result of this interest in thalidomide, the crystal structure of this molecule in both the racemic and enantiomerically pure forms have been determined multiple times (Lovell, 1970(Lovell, , 1971Reepmeyer et al., 1994;Allen & Trotter, 1971;Caira et al., 1994;Suzuki et al., 2010;Maeno et al., 2015). Two polymorphs of the racemic derivative have been determined crystallizing in the space groups P2 1 /n (Allen & Trotter, 1971;Suzuki et al., 2010;Maeno et al., 2015) and P2 1 /c (Lovell, 1970) or C2/c (Reepmeyer et al., 1994;Caira et al., 1994). The crystal packing in the C2/c structure is determined by intermolecular N-HÁ Á ÁO hydrogen bonding that is more extensive than that reported for the racemate of thalidomide crystallizing in space group P2 1 /n.

Supramolecular features
Similarly to the hydrogen-bonding patterns found in both the enantiomerically pure form of thalidomide (Lovell, 1971;Maeno et al., 2015) and the racemic P2 1 /n polymorph (Allen & Trotter, 1971;Suzuki et al., 2010;Maeno et al., 2015), the molecules of the title compound are linked into inversion dimers by R 2 2 (8) (Etter et al., 1990) hydrogen bonding (Table 1) involving the N-H group as shown in Fig. 2. In addition, there are bifurcated C-HÁ Á ÁO interactions involving O2 with graph-set notation R 1 2 (5). These interactions, along with C-HÁ Á ÁO interactions involving O4, link the molecules into a complex three-dimensional array.

Database survey
A search of the Cambridge Structural Database (CSD version 5.39; Groom et al., 2016) using a skeleton containing the three rings as in thalidomide but without the ketone substituents gave 39 hits but not a single example where the six-membered aromatic ring in the isoindoline moiety is changed to an unsaturated six-membered ring.

Synthesis and crystallization
Some details of the synthesis have been previously reported (Benjamin & Hijji, 2017). cis-1,2-Cyclohexane dicarboxylic acid anhydride (0.10 g, 0.65 mmol), glutamic acid (0.095 g, 0.65 mmol), DMAP (0.02 g, 0.16 mmol), and ammonium chloride (NH 4 Cl) (0.040 g, 0.75 mmol) were mixed thoroughly in a CEM-sealed vial with a magnetic stirrer. The sample was heated for 6 min at 423 K in a CEM Discover microwave powered at 150 W. It was then cooled rapidly to 313 K and dissolved in 15 ml of (1:1) ethyl acetate:acetone. The organic layer was washed with 2Â 10 ml of distilled water and dried over sodium sulfate (anhydrous). The organic layer was concentrated under vacuum and precipitated with hexanes (30 ml) affording a white solid. Crystals suitable for X-ray experiments were grown by slow evaporation of an ethyl acetate/acetone (1:1)

(3aR*,7aS*)-2-(2,6-Dioxopiperidin-3-yl)hexahydro-1H-isoindole-1,3(2H)-dione
Crystal data 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. Refinement. Refined as a two-component twin