2-(9H-Fluoren-9-yl)-4-(4-fluoroanilino)-4-oxobutanoic acid

In the title compound, C23H18FNO3, the tricyclic 9-fluorenyl system is approximately planar (r.m.s. deviation = 0.0279 Å). The N—C(=O) bond length is comparatively short [1.359 (3) Å], which is typical for such conjugated systems. The N atom has a planar configuration [sum of bond angles= 359.8°] due to conjugation of its lone pair with the π-system of the carbonyl group. In the crystal, a three-dimensional network is formed through N—H⋯O and O—H⋯O hydrogen bonds between the amide and carboxylic acid groups and carbonyl O-atom acceptors.

In the structure of (II) (Fig. 1) the tricyclic 9-fluorenyl system C1-C13 is planar with an r.m.s. deviation of 0.0279 Å, wich is typical for this class of compounds. The N1-C17 bond distance is comparatively short (1.359 (3) Å) which is typical for such conjugated systems The N1 atom has a planar configuration, as the sum of bond angles on the N1 atom is 359.5 (17)°, due to conjugation of the lone pair of N1 atom with π-system of the carbonyl group. Molecules of compound (II) (Fig. 1) in the crystal are connected across a center of inversion by O1-H1···O2a hydrogen bonds forming dimers which are then connected into chains parallel to c by N1-H1N···O3b bonds (Table 1).

Experimental
The synthesis of the cyclic anhydride (I) (Fig. 2) was carried out according to the literature method (Clar, 1942).

Refinement
Carboxylic acid and amide H-atoms were located in a difference-Fourier synthesis and both positional and displacement parameters were allowed to refine. Other hydrogen atoms were positioned geometrically, with C-H = 0.96-0.98 Å and were allowed to ride on their parent atoms, with U iso (H) = 1.2U eq (methine or methylene C) or 1.5U eq (methyl C). In the absence of a suitable heavy atom, the absolute configuration of the title compound could not be determined.

Figure 1
The molecular structure and atom numbering scheme for the title compound, showing 50% probability displacement ellipsoids.

Figure 2
The synthetic route to the title compound (II).

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 C18 0.0042 (