12,15-Dimethyl-8-oxatetracyclo[8.8.0.02,7.011,16]octadeca-1(18),2,4,6,11(16),12,14-heptaen-10-ol

In the title compound, he pyran ring is in a half-chair conformation and the fused ring system comprising the benzene and cyclohexene rings is essentially planar and forms a dihedral angle of 27.95 (6)° with the other benzene ring. In the crystal, O—H⋯O hydrogen bonds connect the molecules into chains propagating along [001].


Structure description
The ring-opening reaction of oxabenzonorbornadiene (OBD) has been well studied by many groups including our own (Lautens et al., 2003;Rayabarapu & Cheng, 2007;Boutin et al., 2019;. Building on the work of Cheng (Duan & Cheng, 1995), our group has also demonstrated the palladium-catalysed regioselective ring-opening of C 1 -substituted OBDs using aryl iodides (Raheem et al., 2014). However, to the best of our knowledge, intramolecular modes of this reactivity have been left unexplored. Currently, the only known intramolecular transformation of OBD was reported by the Lautens group (Loh et al., 2016) with a similar transformation recently reported by our group on cyclopropanated OBD . Based on this, we set out to investigate palladium-catalysed intramolecular ring-openings of OBD with C 1tethered aryl halides. The reaction of C 1 -substituted OBD I (see Fig. 1) in the presence of PdCl 2 (PPh 3 ) 2 , Zn, Et 3 N, and MeCN afforded an expected dehydrated product II in 82% yield, as well as an unexpected and yet unreported hydrated product III in 14% yield. The structure of the alcohol-containing fused tetracycle III was confirmed by single-crystal X-ray analysis.

Synthesis and crystallization
To a 2 dram vial was added oxabenzonorbornadiene I ( Fig. 1) (67.8 mg, 0.168 mmol), then purged with argon before importing into a glove box under an inert argon atmosphere. The vial was loaded sequentially with Zn (123.3 mg, 1.89 mmol, 11.2 eq.), MeCN (1.5 ml), Et 3 N (0.09 ml, 0.669 mmol, 0.25 eq.) and PdCl 2 (PPh 3 ) 2 (12.5 mg, 0.0178 mmol, 10.6 mol%), then exported and stirred at 333 K for 1 day. The mixture was cooled to room temperature and stirred in air for 10 minutes before removing the solvent under reduced pressure. The crude mixture was then purified by flash Table 1 Hydrogen-bond geometry (Å , ).

Figure 3
Part of the crystal structure with O-HÁ Á ÁO hydrogen bonds shown as dashed lines.

Figure 1
The reaction scheme

Figure 2
The molecular structure of the title compound with displacement ellipsoids drawn at the 30% probability level.  column chromatography using gradient elution (EtOAc: hexanes 1:9 to EtOAc:hexanes 1:4) to obtain the ring-opened product II (35.8 mg, 82%) as a white solid and III (6.6 mg, 14%) as a white solid. The product III was subsequently crystallized from methylene chloride solution by slow evaporation to give product III as colourless crystals with orange specks.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2.

data-1
IUCrData ( 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.