Crystal structure of a new spiro-polytetrahydrofuran compound with translational pseudosymmetry: rac-(2S,2′S,5′R)-2-methyl-5′-[(1R,2R,5S,5′R)-1,4,4,5′-tetramethyldihydro-3′H-3,8-dioxaspiro[bicyclo[3.2.1]octane-2,2′-furan]-5′-yl]hexahydro[2,2′-bifuran]-5(2H)-one

The title compound crystallizes in the P space group, with two crystallographically independent molecules approximately related by the non-crystallographic translation vector c/2.

The title compound, C 22 H 34 O 6 , is a product of oxidation of squalene with the catalytic system RuO 4 (cat.)/NaIO 4 . The asymmetric unit contains two crystallographically independent molecules of very similar geometry approximately related by the non-crystallographic translation vector c/2. As a consequence, the average diffracted intensity in the hkl layers with odd l is systematically lower than in the layers with even l. In one molecule, the lactone ring and part of the adjacent tetrahydrofuran ring are disordered over two orientations with refined occupancy ratio of 0.831 (10):0.169 (10). The crystal structure is mainly governed by van der Waals forces.

Chemical context
Our group has long been involved in the synthesis of new biologically active heterocyclic compounds (D'Errico et al., 2011(D'Errico et al., , 2012aOliviero et al., 2008Oliviero et al., , 2010aCentore et al., 2013;Iovine et al., 2014). In particular, we have developed a number of new catalytic oxidative processes mediated by transition metal oxo-species (Piccialli et al., 2009 leading to the stereoselective formation of mono-and polytetrahydrofuran (THF) compounds (Piccialli, 2014), as well as spiroketal compounds. THF-containing substances are widely distributed in nature and display a broad range of biological activities such as cation transport, citotoxic, pesticidal, antitumor and immunosuppressive activity. The oxidation of squalene with catalytic amounts of RuO 4 (Bifulco et al., 2003;Piccialli et al., 2007) is particularly impressive since it undergoes a stereoselective cascade process leading to the penta-THF compound 1 (Fig. 1) in a straightforward way and high yields (50% for five consecutive cyclization steps; 87% per cyclization step). In this way, multi-gram amounts of this substance can be easily obtained starting from a cheap parent material. Compound 1, in turn, has been used as the starting material for the synthesis of a number of new poly-THF and spiroketal substances such as, inter alia, compounds 2 and 3 ( Fig. 1) that have shown anti-cancer activity against ovarian (HEY) and breast cancer-derived (BT474) cell lines (Piccialli et al., 2009). ISSN 2056-9890 Based on the known reactivity of RuO 4 (Piccialli et al., , 2010, we anticipated that truncated spirocompounds structurally related to 2 and 3 of Fig. 1 could likely be produced just during the oxidation of squalene with RuO 4 . We report here that a search for this type of products for biological assays and SAR studies resulted in the isolation of the title compound, a substance possessing the same tricyclic spiroketal terminal moiety found in 2 and 3 and strictly related to them. Although extensive NMR studies allowed to determine the structure of this compound, the configuration of some chiral centres could not be unambiguously determined. This prompted us to undertake the X-ray diffraction study of this compound.

Structural commentary
The asymmetric unit contains two molecules of very similar conformation, shown in Fig. 2. The two molecules are approximately related by a translation vector that can be determined by calculating the difference between the homologue coordinates of corresponding atoms in the two molecules A and B. In this way, fairly constant values of the differences are obtained that, averaged over all the couples of (non H) corresponding atoms in the two molecules, give the final values: <Áx>= À0.02 (3), <Áy>= 0.01 (16) and <Áz>= 0.50 (2). This means that the two molecules, on average, are related by the translation vector t = c/2. This pseudosymmetry has consequences on the diffraction pattern. Of course, if the symmetry were truly crystallographic, then all reflections hkl with l odd would have null intensity, because each structure factor F hkl would bear a factor (1 + e il ). The structure could be described in a cell of half the volume and Z 0 = 1. This is not the case, because the translational symmetry is not crystallographic. However, a trace of it can be found in the fact that the average diffracted intensity in the hkl layers with odd l is systematically lower than in the layers with even l. This is shown in the histogram of Fig. 3, in which we have averaged the measured F o 2 over each layer. The modulation of the average diffracted intensity between layers with even and odd l is dramatically evident.
The conformation of the two independent molecules is almost the same, with exception for the lactone ring, whose orientation is slightly different (Fig. 4). In both molecules the five-membered rings O1/C1-C4 and O3/C9-C12 exhibit a twist conformation, while the O2/C5-C8 rings display an envelope conformation with atom C8 at the flap. View of the molecular structures of the title compound. Displacement ellipsoids are drawn at the 30% probability level. Only the major component of the disordered lactone ring of molecule A is shown for clarity.  analysis of the molecular structure, it turns out that the relative configuration of the two chiral carbons C8 and C9 in the title compound is inverted as compared with the isomeric compound already reported in literature (compound 10 of Scheme 3 in Piccialli et al., 2009). Moreover, the title compound shares the relative configuration of all of its seven chiral centres with the corresponding moiety in a meso-bisspiro-compound previously obtained by oxidation of squalene under the same conditions (compound 8 of Scheme 2 in Piccialli et al., 2010).

Supramolecular features
The crystal packing is shown in Fig. 5. Although some intraand intermolecular C-HÁ Á ÁO hydrogen contacts are observed (Table 1), no classical hydrogen bonds are found and molecules in the crystal are held basically through van der Waals contacts between H atoms.
In order to assess possible packing differences involving the two independent molecules we have examined their Hirshfeld surfaces (Spackman & McKinnon, 2002;Wolff et al., 2012). In Fig. 6 are shown Hirshfeld fingerprint plots of the two independent molecules, while Table 2 gives relevant molecular parameters.
In the plots, for each point of the Hirshfeld surface enveloping the molecule in the crystal, the distance d i to the nearest atom inside the surface and the distance d Average squared observed structure factor per reciprocal lattice layer, as a function of the l index.

Figure 4
Overlay of the two independent molecules A and B. For molecule A, only the major component of the disordered lactone ring is shown. Symmetry code: (i) Àx þ 1; Ày þ 2; Àz þ 1.

Table 2
Parameters of the Hirshfeld surface of the two crystallographically independent molecules.
Hirshfeld surface analysis was performed using the program CrystalExplorer (Wolff et al. 2012

Figure 5
The crystal packing viewed down the c axis. For molecule A, only the major component of the disordered lactone ring is shown.
plot is related to the abundance of that interaction, from blue (low) to green (high) to red (very high). A common feature of each plot of Fig. 6 is represented by the central green area around d i + d e = 3.0 Å , that corresponds to the loose van der Waals contacts present in the packing, and mainly involving H atoms. Another common feature is the sting along the diagonal, down to d i = d e = 0.9 Å , which reflects points on the Hirshfeld surface that involve nearly head-tohead close HÁ Á ÁH contacts. This feature is clearly more pronounced in the plot of molecule A.

Database survey
A search of the Cambridge Structural Database (CSD version 5.38, last update February 2017; Groom et al., 2016) gave no match for the title compound. A search for spiro-THF compounds gave six hits (GUHXOX, GUHXUD, MUZTEH, MUZTIL, MUZTOR and MUZTUX) all coming from our research group (Piccialli et al., 2009(Piccialli et al., , 2010. A search for poly-THF compounds in which one terminal THF group, at least, is in the oxidized lactone form gave three hits: DOJSIE (Still & Romero, 1986), FAZJEV (Russell et al., 1987) and GUHXOX (Piccialli et al., 2009). Finally, the maximum number of consecutive THF units in a poly-THF compound deposited in the CSD is five: ACUWIG (Yang et al., 2012) and LOJLUR (Xiong & Corey, 2000).

Synthesis and crystallization
The title compound was obtained by oxidation of squalene with RuO 4 (cat.)/NaIO 4 , as previously described (Piccialli et al., 2010). The crude product was purified by repeated silicagel column chromatography, eluting with increasing amounts of Et 2 O in hexane. The fractions enriched in the title compound were collected and evaporated under reduced pressure. Further separation by reversed-phase HPLC (Hibar RP-18 columns, 250 Â 10 and 250 Â 4 mm, eluent MeOH/H 2 O, 6:4 v/v) gave the pure title compound as an oil. It was dissolved in the minimal amount of MeOH and the solution was left to evaporate slowly overnight at room temperature to give crystals suitable for X-ray diffraction analysis.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The H atoms were generated stereochemically and were refined using the riding model, with C-H = 0.98-1.00 Å , and with U iso = 1.2U eq (C) or 1.5U eq (C) for methyl H atoms. A rotating model was used for the methyl groups. The lactone ring and, in part, the adjacent tetrahydrofuran ring of the independent molecule A are disordered over two orientations. The two split positions were refined by applying SADI restraints on bond lengths and SIMU/EADP restraints on thermal parameters. Constraints were also applied to the C4AA--O1AA [1.40 (2)   Hirshfeld fingerprint plots of the two crystallographically independent molecules of the title compound.  Data collection: COLLECT (Nonius, 1999); cell refinement: DIRAX/LSQ (Duisenberg et al., 2000); data reduction: EVALCCD (Duisenberg et al., 2003); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL2016 (Sheldrick, 2015); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and Mercury (Macrae et al., 2006); software used to prepare material for publication: WinGX (Farrugia, 2012). where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.35 e Å −3 Δρ min = −0.21 e Å −3 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. Reflection 1 1 0 was not considered in the refinement, because its intensity was affected by the beamstop. The lactone ring and, in part, the adjacent tetrahydrofuran ring of the independent molecule A are disordered over two sites. The two split positions were refined by using some restraints on bond lengths and thermal parameters.