Reactions In The Synthetic Routes Of Menthol

1As one of the most popular spice, menthol is applied in many products due to unique sense of cool and a mint flavor. Menthol mainly comes from natural peppermint oil. In recent years, the market demand for menthol is also growing up with Chinese economic development. As an abundant renewable resources, turpentine not only can provide the molecular skeleton, but also can be applied in many reactions as a popular reagant. Turpentine is also good natural raw materials for the synthesis of menthol. In this paper, our research work were focused on some reactions of synthesis routes of menthol.For Route I,α-pinene as starting material was oxidated to verbenone. After pyrolyzation reaction of verbenone to thymol, menthol finally was obtained through hydrogenation of thymol. About the reaction ofα-pinene oxidated to verbenone, the research result was shown, as follow: Using 300 mol% tert-butyl peroxide, 10 mol% cuprous chloride and 10 mol% tetramethylethylenediamine as catalyst system,α-pinene was oxidated to verbenone under air, at room temperature. The conversion rate ofα-pinene was 99.74%, the selectivity of verbenone was 36%.For Route II, pinene was hydrogenated to pinane with turpentine oil as starting material. After pyrolyzation reaction of pinane to dihydromyrcene, citronellol was obtained through oxidization reaction of dihydromyrcene. Following oxidization of citronellol to citronellal, isopulegol was gained by cyclizatation reaction of citronellal. Menthol was finally achieved through hydrogenation of isopulegol. In this route, our main research work is about searching for the optimum conditions of three reactions, such as pyrolyzation of pinane, oxidization of dihydromyrcene and oxidatation of citronellol. (1) For preparation of dihydromyrcene from pinane, the quartz tube temperature and nitrogen flow rate was 440℃and 100 mL/min, respectively. The conversion rate of pinane was 83.32% with the selectivity of dihydromyrcene of 63.47%. (2) For preparation of citronellol through hydroboration -oxidation of dihydromyrcene, the conversion rate of dihydromyrcene was 94.5% with selectivity of citronellol of 34.8% at nitrogen atmosphere, THF and 60℃. The amount of sodium borohydride and malonate is 120 mol% and 120 mol% respectively. (3) For preparation of citronellal from citronellol, citronellol was oxidated to citronellal with the conversion and selectivity rate of 100% and 84.7%, respectively. The reaction was carried out at room temperature, air and acetonitrile as solvent, in catalyst system of 10 mol% TEMPO/5 mol%[Cu (OH)/TMEDA]₂Cl₂ and 5 mol% NaOH.