Crystal structure and Hirshfeld surface analysis of ethyl (4R,4aS)-2-methyl-5,8-dioxo-6-phenyl-4a,5,6,7,7a,8-hexahydro-4H-furo[2,3-f]isoindole-4-carboxylate

The central six-membered ring of the title compound has a slightly distorted half-chair conformation while the conformation of the fused pyrrolidine ring is that of an envelope. Molecules are connected by intermolecular C—H⋯O hydrogen bonds, C—H⋯π interactions and π–π stacking interactions, forming a three dimensional network.


Chemical context
This work is a continuation of Diels-Alder reaction studies on vinylarene systems, previously carried out for the tandem acylation/[4 + 2] cycloaddition between 3-(aryl)allylamines and maleic anhydrides or acryloyl chlorides as an example of an IMDAV (the acronym for Intra Molecular Diels-Alder Vinylarene) reaction. An IMDAV reaction is a useful tool for the one-step synthesis of benzofurans, indoles and benzothiophenes annulated with other carbo-and heterocycles (Krishna et al., 2020). Previously, our group carried out a domino-sequence reaction involving acylation/IMDAV/ aromatization steps, which led to the target furo-and thieno[2,3-f]isoindoles (Zubkov et al., 2016;Horak et al., 2015Horak et al., , 2017Nadirova et al., 2020).
The present communication is devoted to another IMDAV reaction involving an oxidation step. We report here the first case of a three-component IMDAV/oxidation reaction between 3-(furyl)allylamine (1), ethylfumaroyl chloride (2) and oxygen. Unlike many other reactions, this process does not stop at the furo[2,3-f]isoindole (4) formation but is continued by an oxidation step yielding the 8-oxofuro[2,3f]isoindole (5) (Fig. 1). The intramolecular [4 + 2] cycloaddition/oxidation sequence occurs under reflux conditions of the reaction mixture in benzene as a solvent and in ambient atmosphere; after standard purification procedures the title compound (5) was isolated in low yield.

Supramolecular features
In the crystal structure of 5, molecules are linked by two kinds of C-HÁ Á Á interactions (Table 1). The first one is between an aromatic H atom (H14) of the phenyl group (C11-C16) and the centroid of the O1/C1-C4 furan ring (Cg1) of an adjacent molecule, and the second one is between the methine H atom (H6) of the fused pyrrolidine ring (N1/C6-C9) and the centroid of the C11-C16 phenyl ring (Cg4) of another adjacent molecule (Fig. 3).

Hirshfeld surface analysis
Hirshfeld surfaces and their associated two-dimensional fingerprint plots (McKinnon et al., 2007) were used to quantify the various intermolecular interactions, and were generated using CrystalExplorer17 (Turner et al., 2017). The shorter and longer contacts are indicated as red and blue spots on the Hirshfeld surfaces, and contacts with distances equal to the sum of the van der Waals radii are represented as white spots. Hirshfeld surfaces of the title compound 5 mapped over the normalized distance, d norm , using a standard surface resolution with a fixed colour scale of À0.2980 (red) to 1.4527 a.u. (blue) are illustrated in Fig. 5a. The shape-index of the Hirshfeld surface is a tool for visualizing thestacking by the presence of adjacent red and blue triangles. The plot of the Hirshfeld surface mapped over shape-index shown in Fig. 5b clearly suggests thatinteractions in (5)   (a) A view of the three-dimensional Hirshfeld surface for 5, plotted over d norm in the range À0.2980 to 1.4527 a.u.; (b) Hirshfeld surface of the title compound 5 plotted over shape-index. Table 1 Hydrogen-bond geometry (Å , ).

Database survey
In the crystal structure of JOGYIP (space group P1), the packing is stabilized by C-HÁ Á ÁO intermolecular contacts, C-HÁ Á Á interactions andstacking interactions, forming a three-dimensional network.
In the crystal structure of LESXIS (Pbca), the asymmetric unit contains two molecules with similar bond lengths and angles. In both molecules, the conformation of the cyclohexene ring is that of a half-chair, while the pyrrolidinone ring adopts an envelope conformation with the -carbon atom of the -pyrrolidinone ring as the flap. In the crystal, O-HÁ Á ÁO hydrogen bonds between the carboxylic and carbonyl groups link alternate independent molecules into chains propagating parallel to the b-axis direction. The crystal packing also features weak C-HÁ Á Á interactions.
The asymmetric unit of QUKPAP (P2 1 /c) comprises two similar molecules, A and B, of the same chirality. The only considerable difference concerns the conformation of the allyl group. The five-membered isoindole rings adopt envelope conformations, whereas the six-membered rings are half-chairpuckered. The carboxyl hydrogen atoms are involved in strong hydrogen-bond formation with the carbonyl atoms of neighboring molecules, giving rise to (AÁ Á ÁBÁ Á Á) n chains.
In the five structures, the different groups bonded to the central twelve-membered ring systems account for the distinct intermolecular interactions in the crystals.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. H atoms bound to C atoms were placed in geometrically idealized positions and constrained to ride on their parent atoms, with C-H = 0.95-1.00 Å and U iso (H) = 1.2 or 1.5U eq (C).   Table 2 Summary of short interatomic contacts (Å ) in the title compound 5.