rac-4-(4-Chlorophenyl)-2-methylamino-3-nitro-5,6,7,8-tetrahydro-4H-chromen-5-one

The title compound, C16H15ClN2O4, contains a chiral centre and crystallizes as a racemate. The methylene group β-positioned to the carbonyl group is partially (21%) disordered. It flips to the opposite sides of the corresponding six-membered carbocycle by −0.304 (3) and 0.197 (11) Å, producing alternative envelope conformations. The planes of the pyran and chlorophenyl rings form a dihedral angle of 86.25 (9)°. The molecular structure is characterized by an intramolecular N—H⋯O interaction, which generates an S(6) ring motif. The corresponding amino N atom deviates from the plane of the pyran ring by 0.1634 (19) Å. In the crystal, molecules are linked via C—H⋯O hydrogen bonds, forming C(8) chains running parallel to the b-axis direction. The crystal structure also features C—H⋯π interactions.

The pyrane ring is almost coplanar with the least-square planes of the nitro and methylene groups, making dihedral angles of 5.19 (14) and 5.01 (16)°, with them, respectively.

Experimental
A solution of 4-chlorobenzaldehyde (0.14 g, 1.0 mmol), cyclic 1,3-dicarbonyl compound (1.0 mmol), NMSM (0.15 g, 1.0 mmol) and piperidine (0.2 equivalents) in EtOH (2 ml) was stirred for 3.5 h. After the reaction was complete as indicated by TLC, the product was filtered and washed with EtOH (2 ml) to remove the excess base and other impurities. Single crystals suitable for X-ray diffraction were prepared by slow evaporation of a solution of the title compound in ethanol at room temperature.

Refinement
Positions of the H atoms were localized from the difference electron-density maps and their distances were geometrically constrained. The H atoms of the amine group were constrained to distances of N-H = 0.901 (10) Å with U iso (H) = 1.2U eq (N). The H atoms bound to the C atoms were treated as riding atoms, with C-H = 0.93 Å and U iso (H) = 1.2U eq (C) for aromatic, C-H = 0.97 Å and U iso (H) = 1.2U eq (C) for methelene, C-H = 0.98 Å and U iso (H) = 1.2U eq (C) for methiene, and C-H = 0.96 Å and U iso (H) = 1.5 U eq (C) for methyl groups. The rotation angles for methyl groups were optimized by least squares. The bond distances of the disordered components were restrained using standard similarity restraint SADI (SHELXL97; Sheldrick, 2008) with an s.u. of 0.01 Å. The atomic displacement parameters of the major and minor components were made similar using the constraint EADP.  The molecular structure of the title compound, with the atom-numbering scheme. The intramolecular hydrogen bond is shown. The displacement ellipsoids are drawn at 30% probability level. H atoms are shown as spheres of arbitary radius.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s 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 > σ(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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (