Crystal structure of (1S*,2R*)-7-benzyloxy-2-methyl-3-tosyl-2,3,4,5-tetrahydro-1H-3-benzazepin-1-ol: elucidation of the relative configuration of potent allosteric GluN2B selective NMDA receptor antagonists

Tetrahydro-3-benzazepines with a hydroxy group in the 1-position and a methyl group in the 2-position were designed as conformationally restricted ifenprodil analogues. The enantiomerically pure 3-benzazepine (S,R)-4 representing a constitutional isomer of ifenprodil shows high affinity towards the ifenprodil binding site (Ki = 26 nM) and high antagonistic activity at the NMDA receptor (IC50 = 9.0 nM). The crystal structure analysis of the intermediate sulfonamide (S,R)-2 was performed in order to assign unequivocally the relative configuration of the methyl and hydroxy groups.


Chemical context
Inhibition of overactive N-methyl-d-aspartate (NMDA) receptors represents a promising strategy for the treatment of acute (e.g. stroke, epilepsy, traumatic brain injury) and chronic neuronal disorders (e.g. neuropathic pain, depression, Alzheimer's and Parkinson's disease) (Brä uner-Osborne et al., 2000;Kew & Kemp, 2005;Paoletti et al., 2013;Wu & Zhou, 2009). The NMDA receptor consists of four proteins (heterotetramer), which form a cation channel allowing the penetration of Ca 2+ and Na + ions into the neuron (Furukawa et al., 2005). In particular, NMDA receptors containing the GluN2B subunit are an attractive target for the development of innovative drugs, since the expression of the GluN2B subunit is limited to only a few regions of the central nervous system, including cortex, striatum and hippocampus (Borza & Domá ny, 2006;Layton et al., 2006;Mony et al., 2009). Moreover, the GluN2B subunit can be addressed selectively by ligands interacting with the so-called ifenprodil binding site, which is formed at the interface between GluN2B and GluN1 subunits (Karakas et al., 2011;Paoletti et al., 2013).
The 2-piperidino-1-phenylpropan-1-ol derivative ifenprodil (Paoletti et al., 2013;Williams, 2001) (Fig. 1) represents the first ligand interacting with this binding site at the NMDA receptor. As a result of its poor selectivity and low bioavailability, ifenprodil has not been developed as a drug for clinical use. In order to improve the selectivity and metabolic stability, the flexible -aminoalcohol substructure of ifenprodil has been incorporated into a rigid tetrahydro-3-benzazepine ring ISSN 2056-9890 (Tewes et al., 2010a,b;Schepmann et al., 2010;Falck et al., 2014).

Elucidation of the relative configuration
For the synthesis of 3-benzazepine analogs of ifenprodil, we developed a chiral pool synthesis starting with (R)-alanine. In a five step synthesis (Fig. 1), the central intermediate ketone (R)-1 was prepared from (R)-alanine (Tewes et al., 2015).
The reduction of the ketone (R)-1 with NaBH 4 led to the diastereomeric alcohols (S,R)-2 and (R,R)-3, which were further transformed into potent GluN2B antagonists by reductive removal of the tosyl group, alkylation with 1-chloro-4-phenylbutane and finally, hydrogenolytic cleavage of the benzyl ether. For example, the phenol (S,R)-4 displays very high affinity towards the ifenprodil binding site of the NMDA receptor (K i = 26 nM) and, moreover, (S,R)-4 is able to reduce the glutamate-and glycine-induced cytotoxicity with an IC 50 value of 9.0 nM (Tewes et al., 2015).
The diastereomeric alcohols (S,R)-2 and (R,R)-3 were separated by flash column chromatography and isolated in 50% and 23% yield, respectively. However, as a result of flexibility of the seven-membered tetrahydro-3-benzazepine ring, it was not possible to assign the relative configuration of the methyl and hydroxy moiety. Therefore, the main diastereomer (1S,2R)-2 was crystallized and we report herein on its crystal structure.

Structural commentary
The molecular structure of the title compound (1S,2R)-2 is illustrated in Fig. 2. Since the starting material was not enantiomerically pure, the compound crystallized as a racemate. However, the relative trans-configuration of the OH and CH 3 groups in the 1-and 2-positions on the azepine ring is clearly shown, leading to a trans-configuration for compound (S*,R*)-2. The CH 3 and the OH groups adopt an axial orientation in the seven-membered azepine ring which has a chair conformation. The phenyl group of the benzyl moiety Synthesis of GluN2B antagonists including the lead compound ifenprodil and the target compound (S,R)-4. Reagents and reaction conditions: (a) NaBH 4 , CH 3 OH, (S,R)-2 50%, (R,R)-3 23%.

Figure 2
The molecular structure of the title compound (1S,2R)-2 with atom labelling. Displacement ellipsoids are drawn at the 30% probability level.

Supramolecular features
In the crystal, molecules are linked via O-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds, forming double-stranded chains along the a-axis direction (Table 1 and Fig. 3). The chains are linked via C-HÁ Á Á interactions (Table 1), forming a three-dimensional architecture.

Synthesis and crystallization
Details of the synthesis of the title compound are illustrated in Fig. 1.
As described for the synthesis of (R,S)-2 and (S,S)-3 (Tewes et al. (2015), the ketone (R)-1 (5.20 g, 12.0 mmol) was reduced with NaBH 4 (909 mg, 23.9 mmol) in CH 3 OH (125 ml). Crystals of the title compound, suitable for X-ray diffraction analysis, were obtained by recrystallization from EtOAc. A view along the a axis of the crystal packing of the title compound (1S,2R)-2. The hydrogen bonds are shown as dashed lines (see Table 1); for clarity, H atoms not involved in these interactions are omitted. Table 1 Hydrogen-bond geometry (Å , ).

Bastian Tewes, Bastian Frehland, Roland Fröhlich and Bernhard Wünsch
Computing details Data collection: COLLECT (Nonius, 1998); cell refinement: COLLECT (Nonius, 1998); data reduction: DENZO-SMN (Otwinowski & Minor, 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: XP in SHELXTL (Sheldrick, 2008) and Mercury (Macrae et al., 2008); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008) and PLATON (Spek, 2009). where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.64 e Å −3 Δρ min = −0.27 e Å −3 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. 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 > 2sigma(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.