Crystal structures of two 2,3-diethylnaphtho[2,3-g]quinoxaline-6,11-dione derivatives

The syntheses and crystal structures of two 2,3-diethylnaphthoquinoxaline-6-11-dione derivatives are described. Molecules of C20H16N2O4 (II) are near planar and form stacks down the c axis through π–π ring interactions. In the substituted derivative, C30H34N4O2 (IV), the polycyclic cores have a significant twist and only minor intermolecular C—H⋯O hydrogen-bonding interactions are present.


Chemical context
As part of a program aimed at the identification of new heterocyclic compounds for organic electronic applications, we sought new or uncommon ring systems that could be synthesized conveniently from cheap, readily available starting materials. In this context, we noted that 2,3-diamino-1,4-dihydroxyanthracene-9,10-dione (I) had been prepared from the inexpensive dye quinizarin (1,4-dihydroxyanthra- quinone) (Shchekotikhin et al., 2005). The diamine (I) appeared to us to be a convenient synthetic building block for fusion of diaza-heterocycles onto the anthraquinone core. Our reaction of the diamine (I) with hexane-3,4-dione in dioxane afforded the 2,3-diethyl-5,12-dihydroxynaphtho[2,3-g]quinoxaline-6,11-dione (II). In exploring the chemistry of compound (II), we found that conversion of the hydroxy groups to the corresponding tosylates gave (III) and subsequent reaction with an excess of piperidine afforded 2,3-diethyl-5,12-bis(piperidin-1-yl)naphtho[2,3-g]quinoxaline-6,11-dione (IV). The reaction scheme for the total synthesis is shown in Fig. 1 and the crystal structures of both the intermediate compound (II) and compound (IV) are reported herein.
The molecular structure of compound (IV) contains two independent, but conformationally very similar molecules (molecule 1 and molecule 2) (Fig. 3). In contrast to (II), the naphthoquinoxaline core of (IV) is significantly twisted, as shown by the dihedral angles between the mean planes of the two terminal six-membered rings [29.79 (6) and 29.31 (7) ]. There is a corresponding twisting of the two central sixmembered rings, presumably resulting from repulsion between neighbouring piperidin-1-yl and carbonyl moieties. The C-N bonds form angles of between 32.3 and 44.5 relative to the neighbouring C O bonds.

Figure 2
Molecular conformation and atom-numbering scheme for (II), with displacement ellipsoids shown at the 50% probability level. Intramolecular hydrogen bonds shown as dashed lines.

Figure 3
Molecular conformation and atom-numbering scheme for the two independent molecules [(a) molecule 1 and (b) molecule 2] in the asymmetric unit of (IV), with displacement ellipsoids shown at the 50% probability level.

Figure 4
A view of an off-set vertical stack of molecules of (II), extending along c.

Figure 5
The packing in the unit cell of (II) as viewed along the c axis, with Cbound H atoms omitted.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Hydrogen atoms potentially involved in hydrogen-bonding interactions were located by difference methods and were freely refined. Other H atoms were included in the refinement at calculated positions with C-H = 0.95-0.99 Å and treated as riding with U iso (H) = 1.2U eq (C) or 1.52U eq (O or methyl C). Electron density associated with additional solvent molecules disordered about a fourfold axis was accounted for using the SQUEEZE procedure in PLATON (Spek, 2015).    program(s) used to refine structure: SHELXL2016 (Sheldrick, 2015b); molecular graphics: X-SEED (Barbour, 2001); software used to prepare material for publication: publCIF (Westrip, 2010).  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. Disordered solvent molecules were accounted for using PLATON SQUEEZE (Spek, 2015).

2,3-Diethyl-5,12-bis(piperidin-1-yl)naphtho[2,3-g]quinoxaline-6,11-dione (IV)
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.