Pseudosymmetry and high Z′ structures: the case of rac-(2R,2′R,5′S)-2-methyl-5′-[(1R,2R,5S,5′S)-1,4,4,5′-tetramethyldihydro-3′H-3,8-dioxaspiro[bicyclo[3.2.1]octane-2,2′-furan]-5′-yl]-3,4,1′,2′,3′,4′-hexahydro-[2,2′-bifuran]-5(2H)-one

The title compound crystallizes in the P space group, with four crystallographically independent molecules.

The title compound, C 22 H 34 O 6 , is one of the products obtained by oxidation of squalene with the catalytic system RuO 4 (cat.)/NaIO 4 . It crystallizes in the P1 space group, with four crystallographically independent molecules related by a pseudo-C 2 symmetry axis. The structural analysis also shows that the title compound is isomeric with two products previously reported in the literature and that are obtained by the same reaction procedure. In particular, out of the seven chiral C atoms present in the molecule, the title compound shows the opposite configuration at, respectively, four and two chiral centres with respect to the isomeric compounds.
In particular, we have recently reported the synthesis of structurally new spiroketal compounds through ruthenium and chromium chemistry (Piccialli et al., 2009). As a continuation of our efforts in this area, we report here the isolation of the title compound from the oxidation of squalene with the catalytic system RuO 4 /NaIO 4 . In particular, the stereoselective polycyclization of squalene with catalytic amounts of RuO 4 ( Fig. 1) (Bifulco et al., 2003) allows penta-THF 1 to be obtained in a straightforward way and high yields (50% for five consecutive cyclization steps; 87% per cyclization step) through a unique oxidative cascade process. In this way, multi-gram amounts of this substance can easily be obtained from a cheap starting material. Compound 1 has been used as the starting material for the synthesis of a number of new poly-THF and spiroketal substances (2-8, Figs. 1 and 2), among which 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).
The title compound is a stereoisomer of two spiroketal compounds previously reported by us (Piccialli et al., 2009(Piccialli et al., , 2017. The determination of the configuration of the numerous stereogenic centres belonging to polycyclic polyether compounds such as the title compound, which contains seven chiral carbons, can be a challenging task. Although NMR data generally provide pivotal information on the stereostructure of such substances, definitive confirmation has very often required total synthesis or X-ray diffraction analysis, as experienced by us and reported by others. Indeed, NMR data alone gave conflicting evidence on the relative configuration of the title compound as well. Therefore, an X-ray diffraction experiment was undertaken in order to assess the differences in the stereochemistry with respect to the previously synthesized compounds and the possible mechanistic implications related to the concomitant formation of such stereoisomers in the same reaction.

Structural commentary
The crystallographically independent unit contains four molecules of identical configuration. The ORTEP diagram of one independent molecule is shown in Fig. 3. The conformation of the four independent molecules is almost the same, with the exception of the lactone ring, whose orientation is slightly different (Fig. 4).

Figure 3
The molecular structure of one of the four crystallographically independent molecules of the title compound (molecule B). Displacement ellipsoids are drawn at the 30% probability level.
The cluster of four independent molecules has approximate local non-crystallographic C 2 symmetry with respect to an axis parallel to a and intersecting the bc plane at (b/4, c/4). This is clearly shown in Fig. 5. We also note that the pseudo-C 2 symmetry, coupled with truly crystallographic inversion centres, would induce a pseudo-P2/n symmetry with unique axis a (Brock & Dunitz, 1994).
The presence of more than one formula unit in the asymmetric unit (Z 0 > 1) can be considered as an 'exception' to the normal crystallization behaviour, because only about 12% of the structures archived in the Cambridge Structural Database have Z 0 > 1 (Brock, 2016). Actually, the understanding of this phenomenon has been tackled from different points of view. So, structures with Z 0 > 1 have been considered as the result of 'molecular association' (Kitaigorodskii, 1961), or as 'frustrated' crystal structures resulting from competing packing requirements (Anderson et al., 2008) or as products obtained under kinetic control, i.e. 'fossil relics' (Steed, 2003) or 'crystals on the way' (Desiraju, 2007). Actually, one of the problems with high Z 0 structures is that the apparently most simple and acceptable explanation for their occurrence, i.e. that the crystallographic independence comes from the fact that the molecules are related by symmetry operations forbidden in crystals, does not stand. In fact, in many cases of high Z 0 structures, including the present one, the independent molecules are related to each other by local symmetry operations fully compatible, in principle, with the translational symmetry of the crystals (i.e. pseudo inversion centers, pseudo binary axes, etc. Overlay of the four independent molecules A, B, C and D of the title compound viewed in two different orientations (a) and (b). For molecule A, only the major occupancy orientation of the disordered rings is shown.

Figure 5
The cluster of the four crystallographic independent molecules of the title compound. (a) Skew view; (b) view down a. For molecule A, only the major occupancy orientation of the disordered rings is shown.  From the analysis of the molecular structure with respect to one previously reported isomeric compound (Piccialli et al., 2017), it can be seen that the title compound has the same configuration at the stereogenic carbons of the spirochetal moiety (C12, C13 and C16), while all of the other four stereogenic carbons (i.e. C4, C5, C8, C9) have the opposite configuration, Fig. 6. This results in a different shape for the two isomers; compared to the previously reported isomer, the title compound has a more horseshoe-type shape. This, in turn, could imply different metal-chelating abilities, that characterize structurally related ionophoric antibiotics. On the other hand, with respect to the other isomeric compound (compound 10 of Scheme 3 in Piccialli et al., 2009), the title compound has the opposite configuration only at the C4 and C5 stereogenic carbons.

Supramolecular features
Molecules are held in the crystal basically through van der Waals contacts between H atoms and weak C-HÁ Á ÁO interactions that are detailed in Table 1. In order to assess possible packing differences involving the four independent molecules, we have examined their Hirshfeld surfaces (Spackman & McKinnon, 2002;Wolff et al., 2012). Fig. 7 shows the Hirshfeld fingerprint plots of the four independent molecules, while the relevant molecular parameters are reported in Table 2. In the plots, the distance d i to the nearest atom inside the surface and the distance d e to the nearest atom outside the surface are reported for each point of the Hirshfeld surface enveloping the molecule in the crystal. The color of each point in the 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. 7 is represented by the central green stripe, roughly along the diagonal, and centered at d i + d e = 3.6 Å . It corresponds to the loose van der Waals contacts present in the packing, and mainly involving H atoms. Another relevant feature is the sting along the diagonal, down to d i = d e = 1.0 Å , which reflects points on the Hirshfeld surface that involve nearly head-to-head close HÁ Á ÁH contacts. This feature is clearly more pronounced in the plots of molecules A, B and C.

Figure 7
Hirshfeld fingerprint plots of the four crystallographically independent molecules of the title compound.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The H atoms were generated stereochemically and were refined by the riding model. For all H atoms U iso = 1.2ÂU eq of the carrier atom was assumed (1.5 in the case of methyl groups). Some C atoms of two tetrahydrofuran rings of the independent molecule A are disordered over two orientations. The two split positions of the two THF rings were refined by applying DFIX restraints on bond lengths and SIMU restraints on thermal parameters. The final refined occupancy factors of the two components of disorder are 0.694 (9) and 0.306 (9) for one ring and 0.764 (13) and 0.236 (13) for the other.

dioxaspiro[bicyclo[3.2.1]octane-2,2′-furan]-5′-yl]-3,4,1′,2′,3′,4′-hexahydro-[2,2′-bifuran]-5(2H)-one
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. Some C atoms of two tetrahydrofuran rings of the independent molecule A are disordered over two orientations. The two split positions were refined by applying DFIX restraints on bond lengths and SIMU restraints on thermal parameters. The final refined occupancy factors of the two components of disorder are 0.694 (9) and 0.306 (9) for one split position and 0.764 (13) and 0.236 (13) for the other.