A. (−)-(1R,3R,4S,8R)-p-Menthane-3,9-diol. To a dry, 2-L, three-necked flask equipped with a gas outlet, an overhead stirrer, and a 500-mL pressure-equalizing dropping funnel with a gas inlet, is added a solution of 31 g (0.20 mol) of isopulegol (Note 1) in 550 mL of tetrahydrofuran (THF) (Note 2) under nitrogen. From the funnel, 230 mL of a 1.0 M diborane solution (Note 3) is added dropwise with stirring at room temperature at such a rate to insure that gas evolution is not violent. When the addition is complete, the clear solution is stirred for 3 hr at room temperature. The dropping funnel is removed and 100 mL of aqueous 3 N sodium hydroxide is added slowly; the solution is then warmed to 60°C on the steam bath for 2 hr. The gas outlet is replaced by a thermometer and the septum is replaced by another dropping funnel charged with 120 mL of aqueous 30% hydrogen peroxide and the reaction flask is placed in an ice bath. Hydrogen peroxide is then added and the reaction temperature is maintained between 30–50°C by carefully controlling the rate of addition (the reaction is strongly exothermic at the beginning). When the addition is complete, the ice bath is removed and the reaction mixture is stirred at 50°C for 2 hr. The bulk of the THF is removed under reduced pressure and the residue is diluted with 200 mL of ether and washed with brine. The aqueous layer is extracted with two 50-mL portions of ether; the resulting extracts are combined and dried over anhydrous sodium sulfate for several hours and filtered. The filtrate is concentrated using a rotary evaporator. The resulting oil is dissolved in 100 mL of hot hexane, allowed to cool slowly, and stored at 0°C for 2 days. The supernatant mother liquor is decanted, 100 mL of fresh hexane is added, the crystalline mass is swirled for 2–3 min and the product is separated by filtration. After vacuum drying, 17.6 g (51%) of (−)-(1R,3R,4S,8R)-p-menthane-3,9-diol is obtained as colorless crystals, mp 105–107°C (Note 4).

B. (+)-(3aS,7aR)-Hexahydro-(3S,6R)-dimethyl-2(3H)-benzofuranone. To a 1-L round-bottomed flask equipped with an efficient magnetic stirrer is added 10 g (58 mmol) of (−)-(1R,3R,4S,8R)-p-menthane-3,9-diol and 400 mL of dichloromethane. The resulting suspension is stirred until all the solid dissolves at which time a mixture of powdered potassium permanganate and copper sulfate (CuSO₄·5H₂O) (Note 5) is added in one portion. The reaction mixture is then stirred at 25°C between 6 and 8 hr depending upon the quality of the oxidant (Note 6), 400 mL of ether is added, and stirring is continued for an additional 20 min. The suspension is transferred with the aid of a small amount of ether onto a fritted-disk Büchner funnel containing 50 g of silica gel and a little ether. The spent manganese salt is mixed with some of the silica gel, using a spatula, to insure that all the solution is absorbed. The resulting filter-cake is washed with 500 mL of ether (Note 7). Upon removal of the solvent, the crude product is obtained as a clear, colorless oil. More product is recovered by taking the filter-cake from the funnel and placing it in a 2-L beaker containing 100 mL of ether and a solution of 270 g of sodium metabisulfite in 1 L of water that is cooled in an ice bath (Note 8). The cake is broken up with a glass rod and stirred. To the resulting slurry, 150 mL of aqueous 10% hydrochloric acid is added in portions with stirring until the color turns from dark gray to pale yellow and all the solid material, except the silica, is dissolved. The resulting mixture is filtered and the filtrate is extracted with three 150-mL portions of ether. The ether extracts are combined and washed twice with water and once with brine. The ethereal solution is dried by shaking with 30 g of anhydrous sodium sulfate and filtered. The filtrate is concentrated to about 150 mL and stirred with 10 g of Amberlyst-15 for 3 hr (Note 9). After another filtration, the solvent is evaporated and the residual oil is combined with that previously obtained. Purification by vacuum distillation gives 4.8–5.0 g (49–51% yield) of 3aS,7aR-hexahydro-3S,6R-dimethyl-2(3H)-benzofuranone as a colorless oil, bp 110°C (0.05 mm) (Note 10).