Crystal structure and Hirshfeld surface analysis of dimethyl (1R*,3aS*,3a1 R*,6aS*,9R*,9aS*)-3a1,5,6,9a-tetrahydro-1H,4H,9H-1,3a:6a,9-diepoxyphenalene-2,3-dicarboxylate

In the title diepoxyphenalene derivative, two dihydrofuran and two tetrahydrofuran rings, as well as one cyclohexane ring, are fused together with two methyl carboxylate groups in positions 2- and 3-. In the crystal, two pairs of C—H⋯O hydrogen bonds link the molecules to form inversion dimers, enclosing two (6) ring motifs.


Chemical context
Reactions totally depending on thermodynamic and kinetic control are infrequently found in the field of organic synthesis, at the same time such transformations are very perspective and attractive from a practical point of view since they allow the direction of the reaction to be changed radically by varying only one of the reaction parameters (usually the catalyst or temperature).
The first example of kinetic/thermodynamic control in the course of the Diels-Alder reaction was reported in 1948 (Woodward & Baer, 1948). Since then, the reversibility of the [4 + 2] cycloaddition was observed many times for examples of a broad range of dienes and dienophiles, including alkynes and furans (Boutelle & Northrop, 2011;Taffin et al., 2010;White et al., 2000;Marchand et al., 1998;Manoharan & Venuvanalingam, 1997;Bott et al., 1996;Bartlett & Wu, 1985). From this diversity of diene/dienophile combinations, tandem and domino reactions of the [4 + 2] cycloaddition based on acetylenic dienophiles are more interesting for the total synthesis of natural or bioactive products (Sears & Boger, 2016;Parvatkar et al., 2014;Winkler, 1996). However, the range of bis-dienes suitable for such tandem transformations is very limited and, currently, there are only a few published examples of full kinetic/thermodynamic control in the course of the tandem intramolecular [4 + 2] cycloaddition (reactions leading to either kinetically or thermodynamically controlled products, depending on temperature; Marchionni et al., 1996;Oh et al., 2010;Criado et al., 2010;Paquette et al., 1978;Visnick & Battiste, 1985).
The present paper describes the uncommon thermal rearrangement of the 'pincer-adduct' (1) into the 'domino-adduct' (2) [the terminology and the mechanism of the reaction are given in references  and Borisova, Kvyatkovskaya et al. (2018); for references of works related to the present paper, see also Lautens & Fillion (1998), Lautens & Fillion (1997) and Domingo et al. (2000)]. The transformation proceeds through the reversible retro-Diels-Alder reaction of the kinetically controlled 'pincer-adduct' (1), followed by the repeated intramolecular [4 + 2] cycloaddition in an intermediate, leading to the formation of the thermodynamically controlled 'domino-adduct' (2) in an almost quantitative yield.

Supramolecular features and Hirshfeld surface analysis
In the crystal, two pairs of C-HÁ Á ÁO hydrogen bonds link the molecules forming inversion dimers, enclosing two R 2 2 (6) ring motifs. The dimers stack along the a-axis direction and are arranged in layers parallel to the bc plane (Table 1 and Fig. 2). C-HÁ Á Á andinteractions are not observed, but HÁ Á ÁH contacts (Tables 2 and 3) dominate in the packing, as detailed in the next section.

Hirshfeld surface analysis and two-dimensional fingerprint plots
Hirshfeld surface and fingerprint plots were generated using CrystalExplorer (McKinnon et al., 2007). Hirshfeld surfaces enable the visualization of intermolecular interactions by different colours and colour intensity, representing short or long contacts and indicating the relative strength of the interactions. Fig. 3 shows the Hirshfeld surface of the title compound mapped over d norm , where it is evident from the bright-red spots appearing near the O atoms that these atoms play a significant role in the molecular packing. The red spots represent closer contacts and negative d norm values on the surface, corresponding to the C-HÁ Á ÁO interactions.
The bright-red spots indicate their roles as the respective donors and/or acceptors; they also appear as blue and red regions corresponding to positive and negative potentials on the Hirshfeld surface mapped over electrostatic potential (Fig. 4;Spackman et al., 2008;Jayatilaka et al., 2005). The blue regions indicate the positive electrostatic potential (hydrogenbond donors), while the red regions indicate the negative electrostatic potential (hydrogen-bond acceptors). The shape index of the Hirshfeld surface is a tool to visualize thestacking by the presence of adjacent red and blue triangles; if there are no adjacent red and/or blue triangles, then there are nointeractions. Fig. 5 clearly suggest that nointeractions are present in the title compound.
The percentage contributions of various contacts to the total Hirshfeld surface are given in Table 3 and are also shown as two-dimensional (2D) fingerprint plots in Fig Hirshfeld surface of compound (2) mapped over d norm .

Figure 4
View of the three-dimensional Hirshfeld surface of compound (2) plotted over electrostatic potential energy in the range À0.0500 to 0.0500 a.u. using the STO-3 G basis set at the Hartree-Fock level of theory. Hydrogen-bond donors and acceptors are shown as blue and red regions around the atoms corresponding to positive and negative potentials, respectively.

Figure 6
The 2D fingerprint plots of compound (2) involving the O atoms of the carbonyl groups, the oxygen bridgehead atoms and the methoxy O atoms, as well as C-HÁ Á ÁF hydrogen bonds, define the crystal packing. These packing features lead to the formation of a supramolecular three-dimensional structure. C-HÁ Á Á andinteractions are not observed, but HÁ Á ÁH interactions dominate in the packing. This situation is similar to that in the crystal of the title compound.

Synthesis and crystallization
The synthesis of the title compound (2)

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. All H atoms were fixed and allowed to ride on the parent atoms, with C-H = 0.95-1.00 Å , and with U iso (H) = 1.5U eq (C) for methyl H atoms and 1.2U eq (C) for other H atoms.

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.