To the same reaction apparatus as shown in Example 1 was added 3,5-dimethyl-1-adamantanol (purity: 99%, 30.0 g), followed by 96% (mass fraction) concentrated sulfuric acid (208 g). After stirring the mixture at room temperature until the ingredients were completely dissolved, the reaction system was cooled to a temperature range of 10 °C to 20 °C, and formic acid (7.76 g) was added dropwise at this temperature, the dropwise addition process lasted for 30 min. After the dropwise addition, the reaction mixture was warmed up to 35 °C and kept at this temperature for 3 hours. The reaction process was monitored by gas chromatography (GC), which confirmed 100% conversion of 3,5-dimethyl-1-adamantanol and 92.6% yield of 3,5-dimethyl-1-adamantanecarboxylic acid. Subsequently, the reaction system was again cooled to a temperature range of 10 °C to 20 °C and 70% (mass fraction) nitric acid (19.6 g) was added dropwise over 30 minutes. After completion of the dropwise addition, the reaction mixture was warmed up to 50 °C and kept at this temperature for 5 h of reaction. The reaction progress was monitored by GC and it was confirmed that the conversion of 3,5-dimethyl-1-adamantane carboxylic acid was 100% and the yield of 3-hydroxy-5,7-dimethyladamantane-1-carboxylic acid was 98.0%. Next, a mixed solution was prepared by adding sodium hydroxide (90.2 g), sodium sulfite (35.5 g), and deionized water (812 g) to the same purification apparatus as shown in Example 1, and the solution was cooled. A reaction mixture containing 3-hydroxy-5,7-dimethyladamantane-1-carboxylic acid was added to the above mixed solution under the condition that the solution temperature was maintained in the range of 10 °C to 40 °C. Subsequently, the precipitated crystals were collected by filtration and washed with deionized water. Finally, the crystals were dried under reduced pressure at 40 °C for 8 h to obtain white crystals of 3-hydroxy-5,7-dimethyladamantane-1-carboxylic acid (32.6 g, yield: 87.5%).
