(2R*,4R*,7S*,10R*,12R*)-3,11,13,15-Tetraoxapentacyclo[5.5.3.01,7.02,4.010,12]pentadeca-5,8-dien-14-one

The title compound, C11H8O5, features a ‘skipped’ diene, an anti-bis(epoxide) and a cyclic carbonate, all embedded in a densely functionalized [4.4.3]propellane scaffold. The crystal packing of this diepoxide is effected primarily by C—H⋯O hydrogen bonds, which link the molecules into tapes along the b axis. Inter-tape connectivity is brought about by centrosymmetrically disposed pairs of C⋯O contacts [3.183 (4) Å] between the Cδ+=Oδ- dipoles of neighbouring carbonate moieties.

The title compound, C 11 H 8 O 5 , features a 'skipped' diene, an anti-bis(epoxide) and a cyclic carbonate, all embedded in a densely functionalized [4.4.3]propellane scaffold. The crystal packing of this diepoxide is effected primarily by C-HÁ Á ÁO hydrogen bonds, which link the molecules into tapes along the b axis. Inter-tape connectivity is brought about by centrosymmetrically disposed pairs of CÁ Á ÁO contacts [3.183 (4) Å ] between the C + Odipoles of neighbouring carbonate moieties.
During the course of this study, we recognized that the tricyclic carbonate 1 features a novel structural attribute -namely, two abutting 1,3-cyclohexadiene (CHD) units embedded in a rigid 11,13-dioxa[4.4.3]propellane framework (Ashkenazi et al., 1978;Paquette et al., 1990). In view of our on-going activity in delineating the patterns of self-assembly in oxygenated CHDs (Mehta & Sen, 2010), it was of considerable interest to investigate the diversely oxy-functionalized CHD moieties that might be accessed from the tetraene 1.
m-Chloroperbenzoic acid (mCPBA) mediated epoxidation of 1 was carried out and though sluggish, led to the formation of the title compound 2 in presence of excess peracid (Fig. 1). Routine characterization of 2 by NMR spectroscopy revealed it to be the product of an unsymmetrical bis-epoxidation of the C 2v -symmetric 1. This was decidedly an uncanny result that was quite unpredictable and could not be readily reconciled with stereo-electronic preferences. In order to settle its stereo-structure by single-crystal XRD analysis, the diepoxide 2 was crystallized by slow evaporation of its saturated solution in 1:2 EtOAc-hexanes.
The crystal structure of 2 was solved and refined in the centrosymmetric monoclinic space group P2 1 /c (Z = 4).
Molecular packing in 2 was brought about primarily via the agency of C-H···O hydrogen bonds which linked the molecules into zigzag tapes essentially along the b axis (Fig. 3). Within each of these tapes, a pair of hydrogen bonds (C2 -H2···O4 and C10-H10···O5), defining an R 2 2 (11) motif (Etter et al., 1990;Bernstein et al., 1995) connected the diepoxide molecules related by the 2 1 symmetry, while another H-bond (C3-H3···O5) consolidated the architecture by linking the molecules, translated along the b axis. While no inter-tape hydrogen bonds were observed, a closer analysis of the crystal packing in 2 revealed C···O short contacts [C11···O5, d = 3.183 (4) Å, symmetry code: -x + 1, -y, -z + 1] between the molecular tapes along the longest c axis. These C···O contacts involved centrosymmetrically disposed pairs of neighbouring C δ+ ═O δdipoles (Fig. 3) Allen, 2002;Bruno et al., 2002] with such a C···O interaction quadrilateral involving the carbonyl group of carbonate moieties (Fig. 4), generated only 10 hits (CSD codes: ARUJEC, CHPCBO, JOKSUX, MHIQXI, QENJUP, SECPUL, SEGCAI, VECQUP, WIZQOL and XEXVEB). Even within this coterie, the observed C···O interaction motif was, in most cases, adventitious/supportive in nature and resulted merely on account of centrosymmetrically related pairs of hydrogen bonds forcing the carbonyls to approach closer to one another. It is worth mentioning at this point that the centrosymmetric C···O interaction motif in 2, though unsupported by hydrogen bonds, might owe its existence to a congenial synergy between the shape and charge distribution in the molecule. As illustrated in Figure 5, each of the two carbonyl groups, involved in the C···O short contacts, fits in a complementary lock-and-key fashion within the single accessible 'groove′ (defined by the central five-membered ring and the cyclohexene, bearing the endo epoxy moiety) that the diepoxide 2 presents. Coincidentally, this groove also bears that 'face′ of the carbonyl functionality in 2 which features a well defined C δ+ ═O δdipolar charge separation (Fig. 6).
To summarize, we have provided herein the first report of an attempted oxyfunctionalization of the tetraene 1 and the complete structural elucidation of the diepoxide 2, obtained in the endeavour. The supramolecular structure of 2, as obtained from the analysis of its single-crystal XRD data, was found to be quite noteworthy, particularly because the carbonyl group of the carbonate moiety in 2 not only functioned as the donor in two C-H···O hydrogen bonds, but participated in a scarcely encountered cyclic dipolar interaction motif as well.

Experimental
As delineated in Figure 1, the title compound was obtained by mCPBA mediated epoxidation of the tetraene 1. Thus, 1 (0.188 g, 1.00 mmol) was dissolved in dichloromethane (8 ml) and solid m-chloroperbenzoic acid (70% purity, 0.740 g, 3.00 mmol) was added portion wise to the stirred solution, cooled on an ice-bath. Thereafter, the reaction mixture was allowed to stir while gradually warming to room temperature on its own. The progress of the reaction was monitored by thin layer chromatography. Even after allowing the reaction to proceed for three days at room temperature, the TLC profile showed no apparent change beyond the disappearance of the starting material and the formation of 2 as the predominant product. The excess peracid was therefore decomposed with saturated Na 2 SO 3 solution and the reaction mixture extracted thrice with dichloromethane. The combined extracts were washed with saturated NaHCO 3 solution and then dried over anhydrous Na 2 SO 4 . Evaporation of the solvent and purification of the residue by column chromatography over silica gel with 40% EtOAc-hexanes furnished the pure diepoxide 2 (0.170 g, 77%) as a colourless solid. M.p. 158 -159 °C.
Single crystals of the diepoxide 2, suitable for X-ray diffraction, were obtained by slow evaporation of its saturated solution in 1:2 EtOAc-hexanes.

Refinement
H atoms were placed in geometrically idealized positions with C-H distances 0.93 or 0.98 Å and allowed to ride on their parent atoms with U iso (H) = 1.2U eq (C).

Figure 4
The centrosymmetric C···O interaction quadrilateral, employed in the CSD search.