A. endo-Tricyclo[6.2.1.0^2,7]undeca-4,9-diene-3,6-dione 1. A 1-L, three-necked, round-bottomed flask, equipped with a mechanical stirrer, 100-mL pressure-equalizing dropping funnel, and a thermometer, is charged with 108.1 g (1.0 mol) of 1,4-benzoquinone (Note 1) and 350 mL of dichloromethane (Note 2). The flask is cooled to 0°C with an ice bath. Via the addition funnel, 41.5 mL of cyclopentadiene (34.2 g, 0.52 mol) (Note 3) is added dropwise over a 45-min period with stirring at such a rate that the internal temperature remains below 8°C (Note 4). A second 41.5-mL portion (34.2 g, 0.52 mol) of cyclopentadiene is added in a similar way (Note 5). The resulting mixture is stirred for 1 hr in the ice bath, for 0.5 hr at room temperature, and then is transferred to a 1-L, pear-shaped flask by rinsing with a small amount of dichloromethane. Most of the solvent is removed by rotary evaporation under reduced pressure, and 200 mL of hexane is added to the residue. The flask is cooled in an ice bath for 0.5 hr. The pale yellow solids are collected by suction filtration, washed with three 30-mL portions of hexane, and air-dried to afford 150–155 g of the Diels-Alder adduct 1 (D-A adduct) of sufficient purity for use in the next step (Note 6). Concentration of the filtrate to about half the volume and cooling affords 12–15 g of a second crop of 1. The combined yield of 1 is 164–169 g (94–97%).

B. endo-Tricyclo[6.2.1.0^2,7]undec-9-ene-3,6-dione 2. A 3-L, three-necked, round-bottomed flask, equipped with a mechanical stirrer, thermometer, and heating mantle, is charged with 166 g (0.95 mol) of the crude D-A adduct 1 and 1.3 L of acetic acid (Note 7). Portionwise, 228.8 g (3.5 mol) of zinc dust (Note 8) is added to the mixture with stirring. The temperature rises to about 70°C over a 10-min period and then drops. The resulting mixture is then heated at 70–80°C for 1 hr, followed by addition of a further 32.7 g (0.5 mol) of zinc powder and continued heating and stirring for an additional 1.5 hr. After the mixture has cooled to room temperature, the gray solids are collected by suction filtration, and washed with 200 mL of acetic acid (Note 9). The dark yellowish filtrate is then concentrated by rotary evaporation under reduced pressure (Note 10), the residue diluted with 600 mL of water, and the resulting mixture extracted with three 300-mL portions of toluene (Note 11). The combined toluene extracts are washed with two 100-mL portions of water, 100 mL of aqueous 10% sodium hydroxide solution, two 100-mL portions of brine, and dried over sodium sulfate, and filtered into a 500-mL round-bottomed flask. Removal of the solvent by rotary evaporation under reduced pressure yields 148–155 g of crude diketone 2, sufficiently pure for further transformation, as a reddish liquid that solidifies on cooling (Note 12).

C. 2-Cyclohexene-1,4-dione 3 (Note 13) and (Note 14). The 500-mL, round-bottomed flask containing ~150 g of crude diketone 2 from Part B is equipped with a magnetic stirring bar and fitted with a Claisen-type short path distillation head leading to a condenser and a 200-mL, round-bottomed receiver. Two cold traps are placed between the receiver and a vacuum pump. The first trap is cooled in a dry ice-ethanol bath and the second trap in liquid nitrogen (Note 15). The system is evacuated and the distillation pot is placed in an oil bath and heated rapidly to 140–180°C with magnetic stirring, whereupon diketone 2 distills fairly rapidly into the receiver to afford a mixture of diketone 2 and 2-cyclohexene-1,4-dione 3, bp 80–140°C (most at 130–140°C) at 4–7 mm (bath temperature 140–180°C), as a pale yellow oil (Note 16), (Note 17) and (Note 18).

The 200-mL, round-bottomed receiver, containing a mixture of diketone 2 and 2-cyclohexene-1,4-dione 3, is then fitted with a long path (25 cm) distillation head packed with Pyrex glass helices or chips, an air condenser (25 cm), and a 200-mL, two-necked, round-bottomed receiver with one neck connected to the vacuum pump via the traps described earlier. The distillation pot is placed in an oil bath, the system is evacuated, and the receiver is immersed in an ice bath. The bath is heated to 120°C and the mixture of diketone 2 and 2-cyclohexene-1,4-dione 3 is redistilled at 4–8 mm while the bath temperature is gradually increased from 120°C to 190°C. Early in the distillation, the enedione 3, which is already present, distills smoothly, then the rate of dissociation of 2 becomes rate-determining. The bath temperature is maintained at 180–190°C so as to keep the boiling point nearly constant (87°C/6 mm). Care is taken to assure that any enedione 3 that crystallizes in the condenser or receiver is quickly melted with an efficient heat gun to avoid occlusion of the distillation path (Note 19). A total of 73–80 g of crude yellow solid enedione 3 is obtained containing a small amount of diketone 2.

The resulting crude solid enedione 3 is melted at 60–70°C in a water bath and 35 mL of carbon tetrachloride is added (Note 20). The mixture is cooled in an ice bath, 18 mL of hexane is added (Note 21), and the resulting mixture is gently stirred with a glass rod. After cooling for 20–30 min, the resulting pale yellow crystals are collected by suction filtration, washed with 20–30 mL of a cold mixture of carbon tetrachloride and hexane (1:1 v/v), and then with 40 mL of hexane. Brief air drying affords 65–71 g of almost pure enedione 3, mp 54–54.5°C (Note 21) and (Note 22). Concentration of the filtrate to about 15 mL, addition of 8 mL of hexane, and cooling the mixture furnishes a further 1.5–2.5 g of 3. In total, 67.0–72.5 g (61–66% from 1,4-benzoquinone) of nearly pure 3 is obtained (Note 23), (Note 24) and (Note 25).