Preparation of ursodeoxycholic acid from 7-ketone lithocholic acid by stereoselective electroreduction
© Huang and Cao. 2015
Received: 13 April 2015
Accepted: 10 June 2015
Published: 26 June 2015
Ursodeoxycholic acid (UDCA) is an important clinical drug in the treatment of liver disease. In previous work, ursodeoxycholic acid was prepared by traditional organic synthesis. The preparation of ursodeoxycholic acid through an electrochemical method with higher stereoselectivity and environmental friendliness is described herein.
Dimethyl sulfoxide (DMSO), dimethylformamide (DMF), and N-methyl-2-pyrrolidone (NMP) were used as stereoselectivity additives during electroreduction. With 107.5 mM DMSO in methanol containing potassium bromide and a continuous current of 20 mA, 936 Coulombs was passed into the electrolysis system, achieving 88.5 % conversion of 7-ketone lithocholic acid (7K-LCA), while the yield of UDCA reached 72.8 %. Cyclic voltammetry (CV) was used to explore the electrochemical behavior of the reaction, and the electrolysis results were consistent with the cyclic voltammograms.
Ursodeoxycholic acid can be prepared by electroreduction with high stereoselectivity. The method developed here offers a potential application for large-scale production of ursodeoxycholic acid and an interesting reference to asymmetric electrochemical reduction of the keto group.
Keywords7-Ketolithocholic acid Ursodeoxycholic acid Stereoselective Electroreduction
Ursodeoxycholic acid (3α, 7β-2-hydroxy-5β-bile acid, UDCA) was first found by Shoda  in the bile of a black bear. It is used as a clinical drug in the treatment of gallstones, cholecystitis, PBC, and PSC and has broad market prospects and important scientific research value [2–9]. Kanazawa et al.  first synthesized ursodeoxycholic acid in 1954 by reducing 7-ketolithocholic acid (3α-hydroxy-7-oxo-5β-cholanic acid, 7K-LCA) in n-propyl alcohol by using metallic sodium with a 14 % yield. At present, the industrial production of ursodeoxycholic acid is based on the synthesis process outlined by Kanazawa. Bharucha and Slemon  demonstrated that 7K-LCA could be reduced to UDCA in an electrolyte containing short-chain alcohol, preferably with weakly acidic dipolar additives such as hexamethylphosphoric triamide (HMPA) with ruthenized titanium or mercury electrodes, and a 91 % yield of UDCA could be obtained. Magni et al.  stated that a 97 % yield of UDCA could be obtained by catalytic hydrogenation of 3α-hydroxy-7-oxo-5β-cholanate using Raney nickel as the catalyst and beta-branched alcohols as the solvent in the presence of a base at 40 °C and atmospheric pressure. 3α-Hydroxy-7-oxo-5β-cholanate dissolved in alcohols could also be reduced to UDCA by hydrogen at a pressure of 0.5 MPa and a temperature of 80 °C catalyzed by Raney nickel, giving a 92.5 % yield of UDCA . In addition, some biological methods are developed in recent years and showed high potential for industrial application [14, 15]. Herein, 3α-hydroxy-7-oxo-5β-cholanate was formed by adding hydrate to 7K-LCA. Currently, there have been very few reports on the preparation of ursodeoxycholic acid by an electrochemical method, despite the fact that it is a relatively simple and safe process that does not require harsh reaction conditions or complicated reaction steps.
The purpose of this paper was to electroreduce 7-ketolithocholic acid stereoselectively into ursodeoxycholic acid in a solution comprising a highly polar and aprotic solvent [dimethyl sulfoxide (DMSO), dimethylformamide (DMF), and N-methyl-2-pyrrolidone (NMP)]. With the correct amount of aprotic solvent, the C-7 carbonyl group of 7K-LCA can be selectively converted into a 7β-hydroxy group on the cathode surface. Experiments show that the aprotic solvent can significantly increase the conversion of 7-ketolithocholic acid and the yield of ursodeoxycholic acid, perhaps indicating the asymmetric reduction of the keto group.
UDCA (99 %) was purchased from Aladdin Chemistry Co. Ltd. 7K-LCA was prepared according to our previous work, via indirect electrooxidation of CDCA using a Br−/Br2 cycle. Methanol and potassium bromide were from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Acetonitrile [high-performance liquid chromatography (HPLC) grade] was from Shanghai Xingke Biochemistry Co. Ltd. (Shanghai, China). All other reagents were of analytical grade, from the Shanghai Ling Feng Chemical Reagent Co. Ltd. (Shanghai, China). WY-30.5 DC power with a single regulator constant flow was from Shanghai Starfish Instrument Co. Ltd. (Shanghai, China). A H-divided electrolytic cell was manufactured by the Glass Instrument Factory of the East China University of Science and Technology (ECUST, Shanghai, China). Cathode material consisting of a high-purity lead plate electrode was obtained from the Engineering Training Center of ECUST. The size of the lead plate was 145 × 30 mm. The anode material consisted of a titanium-ruthenium mesh electrode from Henan Xinxiang Future Water Chemical Co. Ltd. (Xingxiang, Henan). The size of the titanium-ruthenium mesh electrode was 145 × 30 mm. A HF-101 fluoroplastic cation exchange membrane was purchased from the Shanghai Hua Kai Technology Company (Shanghai, China). A cyclic voltameter with a PARSTAT 2273 electrochemical workstation (Princeton Applied Research, Princeton, NJ, USA) was used to measure cyclic voltammograms.
The electroanalytical experiments were carried out in an electrochemical workstation with a co-solvent (MeOH/H2O = 9.83/1, volume ratio) containing 0.08 M potassium bromide as the supporting electrolyte in a divided glass cell, with a lead plate as the working electrode, a titanium-ruthenium mesh as the counter electrode, and Ag|AgCl|KCl as the reference electrode. All experiments were performed at 65 °C under atmospheric pressure. In addition, oxygen was removed from the solution by continuous bubbling with nitrogen before the experiment.
Selective electroreduction of 7K-LCA
The galvanostatic electrolysis was carried out in a H-divided electrolytic cell with the HF-101 cation exchange membrane, and the reaction systems consisted of 0.04 M substrate 7-ketone lithocholic acid, 0.08 M supporting electrolyte, and 0.12 M aprotic solvent in a 131-ml co-solvent (MeOH/H2O = 9.83/1, volume ratio) bubbled with N2 to remove oxygen. A high-purity lead plate was used as the cathode and a titanium-ruthenium mesh as the anode. A continuous current of 20 mA was maintained during the reaction. After 936 Coulombs had passed through, the current was switched off. All experiments were carried out at 65 °C.
When the reaction was complete, the organic solvent from the catholyte was removed using a rotary evaporator to obtain a solid sample. It was then dissolved in NaOH solution, and the pH was adjusted to 2.0 using hydrochloric acid. The precipitate was collected by filtration and then dried in an oven. The final product was detected by HPLC.
Scale-up of electrolysis
During the scale-up of electrolysis, 6 kg of 7K-LCA was dissolved in 280 kg of methanol and 2.7 kg of KBr was dissolved in 24 kg of water; then, the two solutions were mixed and added to a hold-up vessel containing 6.5 kg of DMF, which was used as the catholyte. An equal volume of 5 % H2SO4 was used as the anolyte. A bipolar diaphragm electrolyzer comprised the reaction vessel separated by the HF-101 cation exchange membrane. A high-purity lead plate and titanium-ruthenium mesh were used as the cathode and anode, respectively. The effective overlap area of the electrodes was 70 × 35 cm. The electrolyte was pumped into the electrolyzer with a circulating pump before the reaction started. A continuous current of 20 A was used for electrolysis over 20 h at 55 °C. When the reaction was complete, the organic solvent from the catholyte was removed, and the final product was collected for detection.
UDCA was analyzed by reversed phase HPLC, using a C-18 column with Welchrom-C18 5-μm resin in a 4.6 × 150 mm column (Welch Material Inc., Shanghai, China). The HPLC instrument (LC-20A) was from Shimadzu Corporation, Kyoto, Japan, and was used for quantitative analysis of the reaction products at 208 nm. The mobile phase was a mixture of acetonitrile and phosphate acid buffer (pH 3.0) with a volume ratio of 50:50 at a flow rate of 1.0 ml/min at 25 °C. Fifty percent methanol was used to clean the syringe in the SIL-20A autosampler (Shimadzu Corporation, Kyoto, Japan). The conversion of 7K-LCA was calculated as C = M t/M 0 × 100 %, where M t is the amount of 7K-LCA consumed and M 0 is the total amount of 7K-LCA used during the electrolysis. The yield of UDCA was calculated as Y = A r/A t × 100 %, where A r is the actual productivity of UDCA and A t is the theoretical productivity of UDCA. The electrolysis product was purified by silylation crystallization .
Results and discussion
Stereoselective electrochemical reduction is important both in research and applications. In this electrochemical reaction, ursodeoxycholic acid was synthesized by electroreduction of 7-ketolithocholic acid under certain conditions.
To study the CVs of 7K-LCA in MeOH with 0.08 M KBr, a potential-controlled electrolysis experiment was carried out at −1.2 V, and a current-controlled electrolysis experiment was carried out at 60 mA, because the electrolysis was an irreversible electrochemical reaction.
Preparative electrolysis was carried out in a reaction system consisting of co-solvent (MeOH/H2O = 9.83/1), 0.08 M KBr, 0.04 M 7K-LCA, and 0.12 M DMSO in a divided cell equipped with a lead plate cathode and a titanium-ruthenium mesh anode at 65 °C. A continuous current of 20 mA was applied, and after 936 Coulombs had passed through, the current was switched off. The organic solvent from the catholyte was removed using a rotary evaporator to obtain a solid sample. It was then dissolved in NaOH and the pH was adjusted to 2.0 using hydrochloric acid. The precipitate was collected by filtration and dried in an oven.
The influence of DMSO on the yield of UDCA and conversion ratio of 7K-LCA
Influence of DMSO on the stereoselective electroreduction of 7K-LCA
DMSO content (mM)
Conversion of 7K-LCAa (%)
Yield of UDCAa (%)
Influence of aprotic polar solvent type on the yield of UDCA and conversion ratio of 7K-LCA
These results indicate that the addition of aprotic solvent was favorable to the electroreduction of 7-ketolithocholic acid, and the mechanism is similar to the situation when DMSO is added. However, different types of aprotic solvent have different influences on the reduction, and the addition of aprotic solvents changed the resistance, dielectric constant, and polarity of the electrolyte, which may lower the current efficiency.
Influence of aprotic polar solvent type on the stereoselective electroreduction of 7K-LCA
Conversion of 7K-LCAa (%)
Yield of UDCAa (%)
Scale-up of electrolysis
In prior experiments, DMSO shows better results compared with DMF obviously. However, considering the solvent in separation by silylation crystallization and the price of raw materials, we chose DMF as the additive in the scale-up electrolysis.
The main condition of the scale-up electrolysis
Characterization of the product
The product was determined by HPLC after purification and drying. As shown in Fig. 7, the retention times of UDCA and 7K-LCA were 6.0 and 8.2 min, respectively. Many impurity peaks can be seen between 2.0 and 6.0 min, and some of the corresponding materials were likely contained in the raw material and others were produced by the electroreduction process. The yield of UDCA was calculated to reach 72.8 %.
In summary, the electrochemical stereoselective reduction of 7-ketolithocholic acid into UDCA was conducted using aprotic polar solvents. A higher conversion (88.5 %) of 7-ketolithocholic acid was observed with a 72.8 % yield of ursodeoxycholic acid upon addition of 107.5 mM DMSO during the electrolysis process. In the preliminary scale-up, the conversion of 7K-LCA reached 64.2 % and the UDCA yield was 44.5 %. Compared with chemical and biological preparative processes, the electrochemical process offers the advantages of low cost, high stereoselectivity, and production safety.
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