Crystal structure of rac-3,9-bis(2,6-difluorophenyl)-2,4,8,10-tetraoxaspiro[5.5]undecane

The title compound, C19H16F4O4, was prepared by the condensation reaction of 2,6-difluorobenzaldehyde and pentaerythritol. The whole molecule is generated by twofold rotational symmetry. The two six-membered O-heterocycles adopt chair conformations through a shared spiro-carbon atom that is located on the crystallographic twofold rotation axis. In this conformation, the two aromatic rings are located at the equatorial positions of the O-heterocycles. The conformation of this doubly substituted tetraoxaspiro system is chiral. In the crystal, molecules are linked by C—H⋯O hydrogen bonds, forming layers parallel to (100). These layers are linked by C—H⋯F hydrogen bonds into a three-dimensional structure.

Institutions and the Qing-Lan Project of Jiangsu Province for financial support.
Supporting information for this paper is available from the IUCr electronic archives (Reference: SU5070).
In the title compound, (I), the molecule is consisted of the identical two components though a shared spiro-C9 atom with a crystallographic two-fold symmetry axis (Fig. 1). The two six-membered heterocycles both adopt chair conformation. And the two aromatic residues both are located in the equatorial positions (atom C7) of the tetra-oxaspiro skeleton, through the staggered structure with the six-membered heterocycles, with the torsion angle O1-C7-C6-C1 of -112.5 (2) °.
The phenyl rings, as groups are much larger than hydrogen atoms, are located at the equatorial positions (atom C7) of six-membered O-heterocycles, while the H7 atoms are placed at the axial positions. In the oxo-spirocyclic skeleton, the distance between atoms O1 and O1A is longer than the distance between atoms O2 and O2A, with difference in value of ca. 1.115 Å. The molecule looks like a two-bladed propeller with the dihedral angle between the mean planes of (C1-C6) and (C1A-C6A) equal to 82.9 (5) °. Two opposite enantiomers, with equal numbers, are present in the crystal ( Fig. 2) with a centrosymmetric space group (C2/c).
The packing of molecules of (I), is determined by the presence of numerous weak intermolecular interactions (Table 1).
The C10-H10B···O1 interactions link the same molecules with the identical configurations R or S, respectively, into parallel chains along the b axis, furthermore, the C8-H8A···O2 interactions link the R and S chains alternatively along the c axis into a two-dimensional layered reticulate structure ( Fig. 3) in bc plane. The donors and acceptors of the C-H···O interactions are barely constituted by the C and O atoms from the oxo-spirocyclic skeleton.
In crystalline states, those achiral layered structures are stacked into a three-dimensional structure by weak C-H···F intermolecular interactions between the same enantiomers ( Fig. 4 and Table 1). It is worth mentioning that the C-H···F interactions constitute left/right-handed homochiral helical chains along the b axis (Fig. 4), with left-handed helical chains of the molecules of R configuration (Fig. 4a), and right-handed helical chains of S conformation molecules (Fig.   4c). And, we can clearly see that each helix in the homochiral chains consists of two molecules, with a pitch of 5.5627 (11) A (Fig. 4a). The two kinds of helical chains are arranged parallel to each other in the vertical direction of bc plan, with the same-handed chains along the a axis, and the different-handed chains along the c axis alternatively (Fig. 4b).
supporting information

S3. Refinement
All the H atoms were placed in calculated positions and allowed to ride on their parent atoms: C-H = 0.93 -0.98 Å with U iso (H) = 1.2U eq (C).

Figure 1
The molecular structure of the title compound, with atom labelling. Displacement ellipsoids are drawn at the 30% probability level.

Figure 2
The crystal structure of enantiomers of the title compound, showing the two opposite enantiomers.   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.