Crystal structure and Hirshfeld surface analysis of 2-(2-oxo-3-phenyl-1,2,3,8a-tetrahydroquinoxalin-1-yl)ethyl acetate

The dihydroquinoxaline moiety, with the exception of the N atom, is essentially planar with the attached phenyl ring inclined to it by 11.64 (6)° and the inner part of the methylpropanoate group nearly perpendicular to it. In the crystal, inversion dimers formed by C—H⋯O hydrogen bonds are connected into oblique stacks by π-stacking and C—H⋯π(ring) interactions.


Chemical context
Quinoxaline are a class of nitrogen containing heterocyclic compounds, found in many biologically active drugs (Ramli & Essassi, 2015;Ramli et al., 2014). In addition, this heterocyclic scaffold possess anticorrosion characteristics (El Ouali et al., 2010;Zarrok et al., 2012;Tazouti et al., 2016;El Aoufir et al., 2016;Laabaissi et al., 2019). In a continuation of our recent work focused on the synthesis and biological evaluation of novel heterocyclic compounds (Guerrab et al. 2019(Guerrab et al. , 2020Abad et al., 2021a,b;Missioui et al. 2021) we report here the crystal structure of the title compound ( Fig. 1). As with many biologically active molecules, the molecular conformation adopted may have a significant effect on its activity.
In the majority of the hits, the dihydroquinoxaline ring is essentially planar with the dihedral angle between the constituent rings being less than 1 or having the nitrogen bearing the exocyclic substituent less than 0.03 Å from the mean plane of the remaining nine atoms. Two notable exceptions are DEZJAW, where the dihedral angle between the two rings is 3.32 , and RIRBOM, where the nitrogen bearing the exocyclic substituent deviates by 0.062 Å from the plane defined by the other nine atoms.  Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
Perspective view of the packing. Intermolecular C-HÁ Á ÁO hydrogen bonds are shown by black dashed lines while -stacking and C-HÁ Á Á(ring) interactions are shown, respectively, by orange and green dashed lines.

Figure 1
The title molecule with labelling scheme and 50% probability ellipsoids. The intramolecular C-HÁ Á ÁO hydrogen bonds are shown by dashed lines.
& Jayatilaka, 2009), which can be conveniently carried out with Crystal Explorer 17 (Turner et al., 2017). A detailed description of the use of Crystal Explorer 17 and the plots obtained has been published (Tan et al., 2019) and will not be given here. Fig. 3a presents front (top) and side (bottom) views of the Hirshfeld surface plotted over d norm in the range À0.1367 to 1.2965 a.u. One of the intramolecular C-HÁ Á ÁO hydrogen bonds is indicated by the arrow at the left in the front view while those leading to the formation of the inversion dimers are shown by the arrows on the right of the front view. The C-HÁ Á Á(ring) interaction and the -stacking interactions are represented by the red spots designated by arrows in the side view. Fig. 3b presents the same two views of the surface plotted over the shape-index. In the front view, the -stacking interaction is evident at the center as an orange triangle surrounded by blue triangles. Fig. 3c has the same two views of the surface plotted over the curvature index, with the flat area in the center indicating the locus of the -stacking interaction. Fig. 4 presents fingerprint plots for all intermolecular interactions (a) and those delineated into HÁ Á ÁH contacts (b, 49.4%), HÁ Á ÁO/OÁ Á ÁH contacts (c, 18.2%), HÁ Á ÁC/ CÁ Á ÁH contacts (d, 17.8%) and CÁ Á ÁC contacts (e, 7.2%).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All hydrogen atoms were located from a difference electron-density map and freely refined.     program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018/1 (Sheldrick, 2015b); molecular graphics: DIAMOND (Brandenburg & Putz, 2012); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Special details
Experimental. The diffraction data were obtained from 3 sets of 400 frames, each of width 0.5° in ω, colllected at φ = 0.00, 90.00 and 180.00° and 2 sets of 800 frames, each of width 0.45° in φ, collected at ω = -30.00 and 210.00°. The scan time was 30 sec/frame. 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.