rac-(2aS,2a1 R,3aR,3a1 S,5aS,6aR)-2a-Allyl-2,4-dichloro-2a,2a1,3a1,5a,6,6a-hexahydro-3aH-3-oxadicyclopenta[cd,gh]pentalen-3a-ol

The title dichloro-substituted hemiketal derivative of a cis–syn–cis triquinane is described.

The title racemic triquinane, C 14 H 14 Cl 2 O 2 , is composed of four five-membered rings, one of which is a tetrahydrofuran ring to which an allyl group on one side and a hydroxyl group on the other side are attached. The core of the triquinane unit has a cis-syn-cis configuration. In the crystal, the molecules are linked by pairwise O-HÁ Á ÁO hydrogen bonds, generating inversion dimers featuring R 2 2 (8) loops.

Structure description
Compounds with three fused five-membered rings, known as triquinanes, have gained considerable importance because this core is found in several biologically active compounds (Qiu et al., 2018;Kotha et al., 2020). Therefore, convenient methods to prepare and functionalize triquinanes and the study of their stereochemistry are useful exercises (Mehta & Rao, 1985). Our group has prepared triquinanes from cage compounds in a simplified manner using microwave irradiation (Kotha et al., 2019). Thereafter, we attempted to functionalize the triquinanes and observed a transannular attack at the keto centre (O1-C1-O2) leading to the formation of the title compound, 1.

data reports
The triquinane ring system consists of a cis-syn-cis configuration, i.e., the hydrogen atoms at the ring junction are all above the plane and the first and the third rings are below the plane (Fig. 1b). The chlorine atoms are attached to the unsaturated bonds C2-C3 and C9-C10 in anti-manner with respect to the H atoms of the ring junction. The middle cyclopentyl ring adopts an envelope conformation and the side rings are almost planar.
In the crystal, the molecules are linked by O-HÁ Á ÁO hydrogen bonds, generating inversion dimers featuring R 2 2 (8) loops (Table 1, Fig. 2) but no intramolecular hydrogen bonds are present.

Synthesis and crystallization
The synthesis scheme is shown in Fig. 3. Indium ingots (51 mg, 2.7 eq) were cut into small pieces and transferred to a twoneck round-bottomed flask. Tetrahydrofuran (3 ml) was transferred to the flask under nitrogen at room temperature. Allyl iodide (0.5 ml) was added to this solution via a syringe. After one h, the starting material 2 (40 mg) and trimethylchlorosilane (3 drops) was added to the reaction mixture. On completion of the reaction (TLC monitoring) after 1 h, water was added to the reaction mixture. The aqueous layer was extracted with diethyl ether (Lee et al. 2001). The compound was purified with column chromatography and silica gel (100-200 mesh) was used. Ethyl acetate:petroleum ether (8% of ethyl acetate in total in 100 ml of solution) was used an eluent. Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
The crystal packing of 1, viewed along the b-axis direction. The hydrogen bonding is shown using dotted lines.

Figure 3
Synthesis scheme for 1.