Synthesis, crystal structures and docking studies of 2,7-diphenyl-1,4-diazepan-5-one derivatives

Background 1,4-Diazepine derivatives are the seven membered, nitrogen containing heterocyclic ring systems possessing a wide range of therapeutic applications. 1,4-Diazepines attracted the attention of chemists and druggists due to their biological and medicinal properties, such as antimicrobial, anti-HIV and anticancer activities. Herein, we report the preparation, crystal structure determined by X-ray crystallographic methods and docking of the molecules with the potential target protein NS5B RNA polymerase. Results The crystal structures and conformational studies of 1,4-diazepine [t-3, t-6-dimethyl-r-2,c-7-diphenyl-1,4-diazepan-5-one(DIAZ1)] and its nitroso derivative [t-3, t-6-dimethyl-1-nitroso-r-2,c-7-diphenyl-1,4-diazepan-5-one(DIAZ2)] are reported. The analyses of the molecules reveal that the seven membered diazepine ring systems adopt chair and boat conformations in compounds DIAZ1 & DIAZ2, respectively. In DIAZ2, the oxygen O2A is disordered over two positions with the refined occupancies of 0.792(7): 0.208(7) in the nitroso group. In both DIAZ1 & DIAZ2, the symmetry related molecules form a hetero/homo-dimer through N-H…O hydrogen bonds. Conclusion In this study, the crystal structures of two new 1,4-diazepines, namely t-3, t-6-dimethyl-r-2,c-7-diphenyl-1,4-diazepan-5-one and t-3, t-6-dimethyl-1-nitroso-r-2,c-7-diphenyl-1,4-diazepan-5-one were synthesized and characterized by X-ray crystallographic methods. The docking studies show that the compounds inhibit at the active site of the target protein and can be utilized as potential drug molecules. In both the compounds, N-H…O hydrogen bonds lead to dimer formation. In DIAZ2, additionally a couple of C-H…O interactions are noted between the molecules. Graphical Abstract Structure and docking studies of 1,4-diazapine derivatives. Electronic supplementary material The online version of this article (doi:10.1186/s13065-015-0094-3) contains supplementary material, which is available to authorized users.

Background 1,4-Diazepine derivatives are the seven membered, nitrogen containing heterocyclic ring systems possessing a wide range of therapeutic applications. These derivatives are predominantly used in the inhibition of signals in the central nervous system which is useful for the synthesis of psychoactive drugs [1]. 1,4-Diazepines are widely used in the field of peptidomimetics as potential mimetic and molecular scaffolds [2,3]. Analogs of 1,4-diazepine nucleoside with the protected sugar moiety have been made as possible agents against HIV-1 and HIV-2 viruses [4]. 1,4-Diazepines attracted the attention of chemists and druggists for their biological and medicinal activities, such as antimicrobial [5], anti-HIV [6], herbicidal [7], psychotropic [8] and anticancer [9] activities. Also 1,4diazepines act as antagonists of platelet activation factor (PAF) [10]. 1, 4-Diazepines play prominent roles in the field of medicinal chemistry because it is the core moiety used for the synthesis of various drug molecules like, dibenzepine, clozapine, brotizolam and zometapine. In view of above said importance, 1,4-diazepine derivatives were synthesized and the crystal structures were determined. The docking studies of the above derived molecules were carried out with the targeted protein NS5B RNA polymerase.

Experimental
Noller and Baliah [11], developed a novel method which involves the condensation of diethyl ketone with aromatic aldehyde along with ammonium acetate in the presence of ethanol medium leading to the formation of the compound t-3, t-5-dimethyl-r-2,c-6-diphenylpiperidin-4-one (1).
Dry, powdered t-3, t-5-dimethyl-r-2,c-6-diphenylpiperidin-4-one hydrochloride(2) [1.58 g, 5 mmol] was added in portions to cold (5°C) conc. H 2 SO 4 (10-15 ml) in an Erlenmeyer flask equipped with a mechanical stirrer. After the dissolution of (2) was complete, the temperature of the solution was allowed to rise 25°C . Then, NaN 3 (0.65 g, 10 mmol) was added in portions of 0.1 g, with vigorous stirring and the addition was continued for 1 h. Nitrogen gas evolution was observed during each addition. The resulting solution was poured into crushed ice and stirred well with a glass rod and cold sodium hydroxide solution (2 M) was added slowly with stirring until the pH was 8. The white solid separated out was kept in the beaker at room temperature overnight and collected, washed free of sodium hydroxide and dried. The dried solid was dissolved in ethanol and filtered through a fluted filter paper and the solution was concentrated in vacuo to 10 ml. The solution was kept aside for crystallization. The crystals of t-3, t-6-dimethyl-r-2,c-7-diphenyl-1,4diazepan-5-one (3) were obtained and then recrystallized again from dichloromethane and petroleum ether (60-80°C). (Yield: 1.10 g (74.83%) of colorless solid, mp 178-180°C) [12] [Scheme 1].

Results and discussion
The ORTEP plots [13] for the molecules DIAZ1 and DIAZ2 are shown in Figures 1 and 2. The crystal data, experimental conditions and structure refinement parameters for DIAZ1 and DIAZ2 are presented in Table 1. There are two crystallographically independent molecules in the asymmetric unit in both the compounds DIAZ1 and DIAZ2.
A study on asymmetry parameters, torsion angles and least-squares planes reveal that the seven membered diazepine rings in DIAZ1 adopt chair conformation whereas the ring in DIAZ2 assume boat conformation (Tables 2 & 3). The altered boat conformation adopted in DIAZ2 is due to the nitroso group substitution at N1A & N1B positions of the molecule(s).
In In DIAZ2, the atom O2A of nitroso group is disordered over two positions with refined occupancies 0.792 (7):0.208 (7). The nitroso group is coplanar with the best plane of the diazepine ring as can be seen from the torsion angle values of [C7A-N1A-N2A-O2A & C7A-N1A- Figure 1 The molecular structure of DIAZ1 with the atomic numbering and displacement ellipsoids drawn at 20% probability level.

Figure 2
The molecular structure of DIAZ2 with the atomic numbering and displacement ellipsoids drawn at 20% probability level. The atom O2A was disordered in the molecule A. The disordered atom O2A' and all H atoms have been omitted for clarity.    Figure 3. The molecules at (x, y, z) and (−x,1-y,-z) are linked through intermolecular N-H…O hydrogen bonds into a cyclic pseudo-centrosymmetric R 2 2 (8) hetero-dimer as shown in Figure 4 [14].
In DIAZ2, the intermolecular N-H…O hydrogen bonds generate the graph set motif of centrosymmetric R 2 2 (8) homo-dimer as shown in Figure 5 [14]. The molecules at (x, y, z) and (−x,1-y,1-z) are linked by C-H…O hydrogen bonds and form a C(7) chain running along baxis ( Figure 6). A number of N-H…O and C-H…O types of intra and intermolecular interactions form a three dimensional network in the crystal lattice. The relevant details of the intermolecular features of DIAZ1 and DIAZ2 are given in Table 4.

X-Ray Crystallography
Single crystals of the compounds DIAZ1 and DIAZ2 were mounted on a Bruker Kappa APEXII area-detector diffractometer equipped with MoKα (0.71073 Å) radiation [15]. X-ray intensity data were collected for both the compounds at room temperature (293 K). The data reduction and the absorption corrections were carried out using SAINT [15] and SADABS [16] programs. The structures were solved by direct methods using the program SHELXS97 [17]. All the non-hydrogen atoms were refined anisotropically by full-matrix least-squares procedure on F 2 taking all the unique reflections using SHELXL97 [17]. The hydrogen atoms attached with carbon atoms were placed in their calculated positions and included in the isotropic refinement using the riding model with C-H = 0.93 Å (−CH) or 0.97 Å (−CH2) Å or    0.96 Å (−CH3) Å with U iso (H) = 1.2U eq (parent C atom). In DIAZ1, molecules of solvent were severely disordered, but suitable disorder models could not be found. In order to obtain a better quality refinement, the SQUEEZE routine in PLATON was used to remove the contribution from the disordered solvent [18]. The leastsquares planes, geometrical and puckering parameters of both the compounds were calculated using PLATON software package [19][20][21].

Molecular docking studies of diazepine derivatives
Hepatitis C virus (HCV) is a positive sense single stranded RNA virus belonging to the flaviviridae family of enveloped viruses. The hepatitis C viral particle consists of a core of genetic material (RNA), surrounded by a protective shell of protein, and further encased in a lipid (fatty) envelope of cellular material. This protein is processed by host and viral proteases into four structural (Core, E1, E2 and p7) and six nonstructural proteins  (NS2, NS3, NS4A, NS4B, NS5A and NS5B) [22]. The objective of the study is to demonstrate that 1,4-diazepines (DAP) bind to the NS5B enzyme and to evaluate whether these DAP molecules can be used as potential drugs for hepatitis C disease. The diazepine derivatives derived in the present study were analyzed for the binding affinity with NS5B polymerase. The co-crystallization of various dibenzodiazepine with NS5B has already been carried out wherein carbonyl O of the inhibitor forms an intermolecular hydrogen bond interacting with residue TYR 448.

Target protein and ligand structure preparation
The X-ray crystal structure of NS5B complex (PDB ID: 3CSO) was obtained from the RCSB Protein Data Bank (PDB). Prior to optimizing the protein, water molecules were removed from the crystal structure and partial atomic charges were also assigned according to the force field. Minimization of protein was performed until the average root mean square (rms) deviation of the nonhydrogen atoms reached 0.3 Å using OPLS-2005 force field to remove the steric hindrance under Protein Preparation Wizard of Schrödinger Suite 2011 [23].  The above said force field was used in minimizing the energy of the ligands. The pictorial representation is done using the program LIGPLOT [24]. Ligplot generates schematic diagrams of protein-ligand interactions from the 3D coordinates in a PDB file.
The results obtained from this study would be useful in both understanding the inhibitory activity of 1,4-diazepine derivatives and accurately predicting the activities of newly designed inhibitors based on docking scores as well. The two ligands DIAZ1 & DIAZ2 are docked with the NS5B RNA polymerase and compared with the co-crystallized ligand, namely dibenzodiazepine [C 30 H 29 ClN 2 O 3 ]. In DIAZ2, the carbonyl O atom interacts with TYR448 as also seen from the dibenzodiazepine (Figure 7). Based on the docking scores and energy values, DIAZ2 has better values when compared with DIAZ1 (Table 5). Although the docking scores for the new inhibitors are slightly inferior, they are achieved with significantly fewer atoms.

Conclusion
In this study, two new crystal structures of 1,4-diazepine and its nitroso derivative(DIAZ1 & DIAZ2) were synthesized and characterized by X-ray crystallographic methods. Both 1,4-diazepine derivatives are crystallized in triclinic space group wherein the diazepine rings take up chair and boat conformations. In both the compounds, N-H…O hydrogen bonds lead to dimer formation. The molecules DIAZ1 and DIAZ2 are docked with the targeted protein NS5B RNA Polymerase and the results are compared with the cocrystallized ligand dibenzodiazepine.

Additional files
Additional file 1: Structural information (CIF) for DIAZ1.