Meerwein–Ponndorf–Verley–Oppenauer Non-catalytic Reaction of Monoterpenoids in Supercritical Fluids

—The Meerwein–Ponndorf–Verley–Oppenauer non-catalytic reaction of cyclic monoterpenic ketones and alcohols (menthone, camphor, menthol, and borneol) was studied under supercritical conditions. It has been showed that the ketones can be reduced in supercritical isopropanol with high selectivity (89–98%) without any catalyst or base. At the same time, the conversion of menthone was 63% at 350°C, and that of camphor was 11% at 300°C for 5 h reaction. The conversions of menthol and borneol in the reverse dehydrogenation reaction in supercritical acetone were 34 and 22%, respectively, and the selectivities were 95 and 88%. Thermal dehydration is a main side reaction for all the reactions.