Ethyl 3-(9-chloro-10-oxo-9,10-di-hydro-anthracen-9-yl)-5-methyl-isoxazole-4-carboxyl-ate

The asymmetric unit of the title compound, C21H16ClNO4, contains two independent mol-ecules (A and B), each adopting a conformation wherein the isoxazole ring is roughly orthogonal to the anthrone ring. The dihedral angle between the mean plane of the isoxazole (all atoms) and the mean plane of the anthrone (all atoms) is 88.48 (3)° in one mol-ecule and 89.92 (4)° in the other. The ester is almost coplanar with the isoxazole ring, with mean-plane dihedral angles of 2.48 (15) and 8.62 (5)°. In both mol-ecules, the distance between the ester carbonyl O atom and the anthrone ketone C atom is about 3.3 Å. The anthrone ring is virtually planar (r.m.s. deviations of 0.070 and 0.065 Å) and adopts a shallow boat conformation in each mol-ecule, as evidenced by the sum of the six intra-B-ring torsion angles [41.43 (15) and 34.38 (15)° for molecules A and B, respectively]. The closest separation between the benzene moieties of anthrones A and B is 5.1162 (7) Å, with an angle of 57.98 (5)°, consistent with an edge-to-face π-stacking inter-action. In the crystal, weak C-H⋯O and C-H⋯N inter-actions link the mol-ecules, forming a three-dimensional network.