Application of temperature control to asymmetric protonation

The low-temperature method is effective not only in the kinetic resolution of alcohols but also in the enantioface-selective asymmetric protonation of enol acetate of 2-methylcyclohexanone (15) giving (f )-2-methylcyclohexanone (16). The reaction in H2O at 30°C gave 28% ee (98% conv.), which was improved up to 77% ee (82% conv.) by the reaction using hpase PS-C 11 in /-Pt20 and ethanol at 0°C. Acceleration of the reaction with lipase PS-C 11 made this reaction possible because this reaction required a long reaction time. The temperature effect is shown in Fig. 14. The regular temperature effect was not observed. The protons may be supplied from H2O, methanol, or ethanol, whose bulkiness is important. 

Lipase Amano PS-C II was also found to be useful for a high-temperature reaction in the resolution of a bulky substrate, l,l-diphenyl-2-propanol, which showed no reactivity under usual conditions with lipase PS. An enantiopure product was obtained at 40-120°C, and the highest conversion (39%) was obtained at 80-90°C. It is very interesting that a single enzyme is usable in the reaction at a very wide range of temperatures from —80°C to 120°C. 

---