(1) Isomerization reaction: 0.5 mol (100.1 g) of dimethyl 1,4-cyclohexanedicarboxylate (Ib) with a trans/cis isomer ratio of 3/7 to 5/5 was dissolved in 200 g of methanol at 40°C. The reaction was carried out by adding 15 g of pyridine as a catalyst. 15 g of pyridine was added as a catalyst, and after 2 h of reaction, a methanolic solution of dimethyl 1,4-cyclohexanedicarboxylate (II-b) with a trans/cis isomer ratio of 9/1 was obtained.
(2) Monoester hydrolysis reaction: The methanol solution of dimethyl 1,4-cyclohexanedicarboxylate (II-b) with a trans/cis isomer ratio of 9/1 obtained in step (1) was cooled to room temperature, and 150 g of potassium hydroxide was added. The reaction was carried out at 20°C for 3 h. Monoester hydrolysis was carried out to obtain a water-soluble carboxylic acid ester of trans-1,4-cyclohexanedicarboxylic acid monomethyl ester.
(3) Acidification and post-treatment: 200 g of water and 100 g of toluene were added to the reaction system of step (2), and the organic phase was removed by extraction and stratification. The aqueous phase was adjusted to pH 1-2 with concentrated hydrochloric acid and the mixture was cooled to 0 °C. After filtration, the mixture was dried under reduced pressure at 50 °C to afford 66.4 g of white solid powder of monomethyl trans-1,4-cyclohexanedicarboxylate (III). The yield was 71.32% and the purity of compound (III) was 98.9% as determined by gas chromatography.
