基于羟基保护的百里酚催化加氢合成薄荷醇

Synthesis of Racemic Menthols via Catalyzed Hydrogenation of Thymol Based on Hydroxyl Protection

以三甲基氯硅烷(TMS)羟基保护的百里酚为原料,研究了羟基保护的百里酚催化加氢,同时比较了原料是否羟基保护的2种反应条件。结果表明,在8MPa,180℃条件下,没有羟基保护的百里酚直接催化加氢,获得4对消旋体薄荷醇,其中(±)-薄荷醇的选择性为51.6%,且伴有薄荷醇羟基被破坏或重排的副产物,副产物的产率约为10%～15%;而具有羟基保护的催化加氢,产物为100%消旋体薄荷酮,反应条件比未羟基保护的加氢更温和为5MPa,150℃。采用Cu/SiO2催化剂对消旋体薄荷酮加氢的研究结果表明,在2MPa,150℃以及通过酒石酸调节pH=4的条件下,薄荷酮加氢合成了4对消旋体薄荷醇,其中(±)-薄荷醇的选择性为72%。分析了反应路径与反应机理,提出了利用百里酚合成薄荷醇的新途径。

The racemic mixture of menthone was synthesized by catalytic hydrogenation of thymol based on hydroxyl protection using TMS （trimethylchorosilane） with excellent selectivity and good yield （both of 100% ） under mild and neutral reaction conditions. The racemic menthone was hydrogenated to racemic menthol over 8% Cu/SiO2 in a medium with pH = 4 by tartaric acid modifying. A selectivity of 72% for the desired product, （ ＋ ）-menthol,was achieved under a pressure of 2 MPa, at a temperature of 150 ℃. For this novel process, the mechanism of reaction was proposed. Menthenol is firstly accomplished by hydrogenation of thymol with hydroxyl protection, secondly the menthenol which is unstable in air was transformed completely to the corresponding racemic menthones, and finally, racemic menthones were hydrogenated to racemic menthols. This proposed approach is promising for independent manufacture of flavors with the optical purity of （ - ） -menthol in the future.