Synthesis
In a round bottom flask, 40.00 g (0.105 mol) of (3aS,6R,7aR)-1-[(2S)-2-allyl octanoyl]-8,8-dimethylhexahydro-3a,6-methanol-2,1-benzisothiazole-2,2-dioxide and 160 ml of diethylene glycol dimethyl ether were added, and the mixture was cooled to 17 °C while nitrogen was passed through the mixture to maintain the oxygen concentration below 5% until the end of the reaction. Subsequently, 0.51 g (0.005 mol) of 35% hydrogen peroxide solution was added, followed by the simultaneous dropwise addition of 15.79 g (0.162 mol) of 35% aqueous hydrogen peroxide solution and 19.61 g (0.168 mol) of 48% aqueous potassium hydroxide solution at 17°C. After the reaction lasted for 1 h, stirring was continued for 2 h at 17 °C and the completion of the reaction was monitored by HPLC. Subsequently, 21.14 g (0.168 mol) of sodium sulfite was dissolved in 110 ml of water and added to the reaction mixture, and 58.97 g (0.566 mol) of 35% hydrochloric acid was added dropwise at room temperature, with completion of the dropwise addition within 1 hour at a controlled temperature ranging from 5 to 12 °C. After heating the mixture to 21 °C and stirring for 35 min, 200 ml of methyl tert-butyl ether was added and stirred for 10 min, the aqueous layer was separated and discarded. The organic layer was washed with brine solution (1.60 g NaCl dissolved in 200 ml tap water), then 2.40 g n-heptane and brine solution of the same formulation were added, stirred for 15 min and the aqueous layer was separated and discarded, and the completion of washing was confirmed by GC. The organic layer was concentrated under atmospheric pressure, 240 ml of n-heptane was added and continued to be concentrated under vacuum to about 160 ml. The residue was cooled to 0-10 °C and stirred for 30 min, the crystals were collected by filtration, washed with 40 ml of n-heptane and dried under reduced pressure to give 6.48 g of camphorsulfonylactam (yield 28.7%, GC purity 98.23%). The filtrate was concentrated under reduced pressure and n-heptane was added to give 72.17 g (0.105 mol) of (2S)-2-(2-propenyl)octanoic acid/heptane solution. Further treatment gave 20.29 g of cyclohexylamine salt of (2S)-2-(2-propenyl)octanoic acid (82.1% yield, 100% optical purity).
