Synthesis of Optically Active l ‐Menthol Part II. Isolation of dl ‐Menthol from the Mixture of dl ‐Menthol Isomers

The mixture of dl ‐menthyl chloroaeetate isomers and the mixture of dl ‐menthyl cyanoacetate isomers were prepared by the esterification of the mixture of dl ‐menthol with chloroacetic acid and cyanoacetic acid, respectively. In these mixtures the dl ‐menthyl chloroacetate m. p. 23–24° and the dl ‐menthyl cyanoacetate m. p. 63–63.5° are crystal form, the other isomers, such as dl ‐isomenthyl cyanoacetate b.p. 136–138°/3 mmHg are liquid form. The crystal form ester was isolatded from liquid form ester by cooling crystallization, and then dl ‐menthol m. p. 32–33° was obtained by hydrolysis. The isolation effect has intimately relation to the dl ‐menthol content in raw material, the more dl ‐menthol content, the more yield and purity of crystal form ester, therefore, after hydrolysis, the obtained dl ‐menthol was excellent in purity.