Crystal structure of 7,8,9,10-tetrahydrobenzo[b]naphtho[2,1-d]furan

The reaction of 1-naphthol with cyclohexadiene in the presence of catalytic amounts of Lewis acid, which interacts with 1-naphthol with release of protons, does not afford the Diels–Alder adduct but the Friedel–Crafts products followed by aromatization. The crystal structure of the final tetrahydrobenzonaphthofuran product is described.

In the title compound, C 16 H 14 O, the cyclohexene ring has a half-chair conformation. The mean plane, calculated through all non-H atoms of the molecule, except for the central CH 2 atoms of the cyclohexene ring, which deviate by 0.340 (3) and À0.369 (3) Å from this mean plane, has an r.m.s. deviation of 0.012 Å . In the crystal, there are C-HÁ Á Á contacts present, resulting in the formation of zigzag chains propagating along the [010] direction.

Chemical context
The interaction of Lewis acids with 1-naphthol 1 can be expected to induce metal coordination at the hydroxy function with concomitant increase in Brønsted-acidity (2) (Yamamoto & Futatsugi, 2005;Goering, 1995). It is conceivable that the proton, once released from this intermediate 2, adds reversibly to the 4-position with formation of adduct 3, which is the Lewis acid coordinated form of the keto-tautomer of 1. Even if only minute amounts of 3 were to be formed, this intermediate should be a highly reactive dienophile in Diels-Alder reactions with such dienes as cyclohexadiene 4 leading to adduct 5 (see Scheme). Such a transformation implies dearomatization of 1-naphthol 1.
Alternatively, protonation of diene 4 leading to carbocation 6 would set the stage for Friedel-Crafts reaction with formation of the alkylation product 7, which could continue to react acid catalyzed, leading to adduct 8 and possibly to the aromatized furan product 9. In a previous study, Nová k and coworkers reported the reaction of 1 with 4 in the presence of TsOHÁH 2 O in boiling toluene (26 h) or at room temperature (7 d), furan derivative 9 being formed in 58% yield, presumably via the intermediacy of 7 and 8 (Orovecz et al., 2003;Nová k et al., 2000).
In exploratory experiments, we tested Et 2 OÁBF 3 , FeCl 3 , TiCl 4 and ZrCl 4 as Lewis acids in the reaction of 1 and 4 at room temperature in CH 2 Cl 2 . Essentially only products derived from formal Friedel-Crafts alkylation were identified ISSN 2056-9890 following column chromatographic separation. Small amounts of unidentified compounds which could not be separated were also formed. A general protocol is provided. If a 2.5-fold excess of cyclohexadiene 4 is used in these reactions, only small amounts of Friedel-Crafts products are formed (3-4%). Rather, acid-mediated oligomerization of diene 4 occurs.
In contrast to the acidic conditions employed by Nová k and coworkers, using the present protocol we isolated compound 8 and characterized it for the first time. We report herein on the crystal structure of the final product, furan 9.

Structural commentary
In the title compound 9, illustrated in Fig. 1, the cyclohexene ring (C1-C6) has a half-chair conformation. The mean plane, calculated through all non-hydrogen atoms of the molecule (O1/C1/C2/C5-C16), except atoms C3 and C4 of the cyclohexene ring that deviate by 0.340 (3) and À0.369 (3) Å from this mean plane, has an r.m.s. deviation of 0.012 Å . The other C and O atoms lie in this mean plane with a maximum deviation of À0.051 (3) Å for atom C2.

Supramolecular features
In the crystal of 9, there are C-HÁ Á Á contacts present (Table 1 and Fig. 2), but no classical hydrogen bonds and no interactions present. Intermolecular contacts thus appear to be limited to van der Waals interactions. The two rather short intermolecular C-HÁ Á Áring centroid distances are: H5BÁ Á Ácentroid of ring (C10-C15) = 2.69 Å , H8Á Á Ácentroid of ring (C7-C10/C15/C16) = 2.93 Å . These interactions result in the formation of zigzag chains propagating along the b-axis direction.

Database survey
Only one structure of a tetrahydrobenzonaphthofuran (Refcode PEBDAD; Scully & Porco, 2012) is present in the current version 5.36 of the CSD (Groom & Allen, 2014), and the cyclohexene ring also has a half-chair conformation.

Synthesis and crystallization
General Procedure: To a mixture of 1-naphthol (6.48 g, 45 mmol), catalyst (2.25 mmol) in CH 2 Cl 2 (10 ml), 1,3-cyclohexadiene (0.7 ml, 22.5 mmol) in CH 2 Cl 2 (30 ml) was added drop wise, and the resulting solution was stirred at 273 K for 5 h. After completion of the reaction (TLC) at room temperature, a cold aqueous solution of NaHCO 3 (5%, 20 ml) was added and the mixture was extracted with CH 2 Cl 2 (3 Â 10 ml). The organic extracts were washed with water (2 Â10 mL) and dried over anhydrous Na 2 SO 4 , and concentrated in vacuum. The crude product was purified by silica column chromatography (petroleum ether) to give the desired product, which was identified by NMR spectroscopic comparison with authentic samples of 1, 2 and by X-ray diffraction analysis (Fig. 1). The molecular structure of compound 9, showing the atom labelling. Displacement ellipsoids are drawn at the 50% probability level. Table 1 Hydrogen-bond geometry (Å , ).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms were located in difference Fourier maps, but subsequently included in the refinement using a riding model: C-H = 0.95-0.99 Å with U iso (H) = 1.2U eq (C).

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.