A New Method of Synthesis of Epalrestat

Abstract

A new synthetic route of Epalrestat was proposed in this study. The new route abandons the raw material carbon disulfide, which is highly harmful to the environment, and optimizes the key steps in the typical synthesis strategy. Epalrestat was prepared through a three-step process, and the reaction products were characterized. The optimum conditions for the synthesis of the substituted rhodanine intermediate are as follows: under the catalysis of 2.0 equivalents of 25%KOH, ethanol was used as the solvent, and the reaction was carried out at 40 °C for 1 h. The optimal conditions for the synthesis of Epalrestat are as follows: under the catalysis of 2.0 equivalents of 50%KOH, ethanol was used as the solvent, and the reaction was carried out at 40 °C for 5 h.

1. Introduction

Diabetes is a globally prevalent disease that seriously endangers human health [1,2]. In recent years, the number of diabetes patients has increased rapidly, with a noticeable trend towards younger individuals. What makes diabetes particularly concerning is its tendency to cause a variety of chronic complications in different parts of the body, such as peripheral neuropathy, kidney dysfunction, and atherosclerosis [3,4]. The abnormally activated polyol pathways are widely considered to be one of the most important causes of morbidity [5]. Excessive glucose is catalyzed by aldose reductase to produce sorbitol and fructose, which are deposited in the peripheral nerves, causing edema, demyelination, and necrosis, ultimately leading to the occurrence of diabetic neuropathy [6,7,8].

Epalrestat (see Figure 1), chemically named 5-[(1Z, 2E)-2-methyl-3-phenylpropenylidene]-2-thio-2,4-thiazolidine-3-acetic acid, is a reversible and non-competitive inhibitor of aldose reductase. Numerous clinical studies have shown that it can effectively improve diabetic neuropathy by inhibiting the activity of aldose reductase and reducing the accumulation of sorbitol and fructose in peripheral nerve tissues [9,10,11]. Epalrestat has a low incidence of adverse reactions and is considered an effective and safe drug for treating chronic complications of diabetes, especially diabetic neuropathy [12,13].

The synthesis methods of Epalrestat have been widely reported in the literature. The main strategy involves a Knoevenagel condensation reaction between α-methylcinnamaldehyde and 3-carboxymethyl rhodanine (see Scheme 1), with the primary differences lying in the specific reaction conditions employed. Early research on the synthesis of Epalrestat used acetic acid as the solvent and sodium acetate as the catalyst for the condensation reaction [14,15,16,17,18]. An improved study used ethanol as the solvent and ammonia water as the catalyst for the condensation reaction [19]. Another improved method employed 3-dimethylamino propylamine as the catalyst for the condensation reaction, water as the solvent, and polyethylene glycol as the phase transfer catalyst to obtain Epalrestat [20].

In terms of raw materials, α-methylcinnamaldehyde can be conveniently obtained through a condensation reaction between benzaldehyde and propionaldehyde under alkaline conditions. In contrast, 3-carboxymethyl rhodanine is relatively expensive and has limited synthetic methods. It is typically synthesized using glycine as the starting material. First, glycine reacts with carbon disulfide and concentrated ammonia to form ammonium N-carboxymethyl dithiocarbamate, which then undergoes alkylation and cyclization with chloroacetic acid to yield the target compound (see Scheme 2).

At present, the preparation of 3-carboxymethyl rhodanine still mainly relies on the aforementioned method. Some studies have proposed an improved approach, in which glycine directly reacts with dicarboxymethyl trithiocarbonate to produce 3-carboxymethylrhodanine (see Scheme 3) [21]. However, the starting material used in this method is expensive and still needs to be prepared with carbon disulfide. Therefore, nearly all studies continue to use the previously described synthetic route (see Scheme 2) to prepare 3-carboxymethyl rhodanine.

In summary, the current production of Epalrestat requires the use of highly toxic carbon disulfide, which is very harmful to the environment. Therefore, finding a new and improved synthetic method for Epalrestat has become an urgent and important scientific challenge. Based on the research of our group [22], a novel optimized synthetic route was proposed (see Scheme 4). This method uses thioglycolic acid to replace carbon disulfide as the sulfur source. Thioglycolic acid first reacts with thiourea to form rhodanine (1), which then reacts with α-methylcinnamaldehyde to yield a substituted rhodanine intermediate (2). Finally, the intermediate reacts with 2-chloroacetic acid to produce Epalrestat (3).

2. Methods and Discussion

Rhodanine was prepared according to ref. [22].

The optimization process for the synthesis of compound 2 is detailed in

Table 1. Optimization of the Knoevenagel condensation for compound 2.

No.	Solvent	Temp. (°C)	Time (h)	Base	Yield (%)
1	CH2Cl2	50	1	none	13.6
2	EtOAc	50	1	none	-- a
3	DMF	50	1	none	12.8
4	EtOH	50	1	none	14.5
5	EtOH	50	5	Na2CO3 (2.0 eq)	35.2
6	EtOH	50	5	K2CO3 (2.0 eq)	45.2
7	EtOH	50	5	SrCO3 (2.0 eq)	15.1
8	EtOH	50	5	K3PO4 (2.0 eq)	56.3
9	EtOH	50	5	25%KOH (2.0 eq)	77.7
10	EtOH	50	5	25%NaOH (2.0 eq)	59.0
11	EtOH	50	5	25%NH4OH (2.0 eq)	65.0
12	EtOH	50	5	NaOAc (2.0 eq)	42.4
13	EtOH	50	5	Et3N (2.0 eq)	--
14	EtOH	50	5	DBU (2.0 eq)	60.0
15	EtOH	50	5	DMAP (2.0 eq)	63.2
16	EtOH	50	5	8%KOH (2.0 eq)	56.3
17	EtOH	50	5	10%KOH (2.0 eq)	59.4
18	EtOH	50	5	15%KOH (2.0 eq)	64.2
19	EtOH	50	5	20%KOH (2.0 eq)	71.3
20	EtOH	50	5	30%KOH (2.0 eq)	77.9
21	EtOH	50	5	40%KOH (2.0 eq)	77.9
22	EtOH	50	5	50%KOH (2.0 eq)	78.0
23	EtOH	25	2	25%KOH (2.0 eq)	56.9
24	EtOH	40	2	25%KOH (2.0 eq)	77.6
25	EtOH	60	2	25%KOH (2.0 eq)	77.9
26	EtOH	70	2	25%KOH (2.0 eq)	77.9
27	EtOH	40	1	25%KOH (2.0 eq)	77.3
28	EtOH	40	3	25%KOH (2.0 eq)	77.7
29	EtOH	40	5	25%KOH (2.0 eq)	77.9
30	EtOH	40	1	25%KOH (1.0 eq)	46.7
31	EtOH	40	1	25%KOH (1.5 eq)	59.8
32	EtOH	40	1	25%KOH (2.5 eq)	77.4
33	CH2Cl2	40	1	25%KOH (2.0 eq)	51.3
34	EtOAc	40	1	25%KOH (2.0 eq)	58.7
35	DMF	40	1	25%KOH (2.0 eq)	56.3

^a—signifies no reaction or low production without separation.

. The effects of solvents such as CH₂Cl₂, EtOAc, DMF, and EtOH on the reaction were tested, and ethanol was found to be the most favorable solvent. Different bases had some influence on the reaction, with a 25% aqueous potassium hydroxide solution giving the most satisfactory result. From the data in the table, it can be seen that although increasing the temperature and prolonging the reaction time can improve the yield, the improvement is not significant. Additionally, at 40 °C for 1 h, the effect of different amounts of base—1.0, 1.5, 2.0, and 2.5 equivalents—was investigated, resulting in yields of 46.7%, 59.8%, 77.3%, and 77.4%, respectively. It was found that beyond a certain amount, further increasing the base had little effect on the reaction outcome. Therefore, the optimal reaction conditions were determined to be catalysis by 2.0 equivalents of 25% potassium hydroxide, using ethanol as the solvent, at 40 °C for 1 h.

The optimization process for the synthesis of Epalrestat is detailed in

Table 2. Optimization of the synthesis of Epalrestat.

No.	Solvent	Temp. (°C)	Time (h)	Base	Yield (%)
1	CH2Cl2	50	1	none	-- a
2	EtOAc	50	1	none	--
3	DMF	50	1	none	2.9
4	EtOH	50	1	none	7.5
5	DMSO	50	1	none	5.6
6	EtOH	50	5	50%KOH (2.0 eq)	52.8
7	EtOH	50	5	50%NaOH (2.0 eq)	39.5
8	EtOH	50	5	SrCO3 (2.0 eq)	--
9	EtOH	50	5	K2CO3 (2.0 eq)	--
10	EtOH	50	5	Et3N (2.0 eq)	--
11	EtOH	50	5	NaOAc (2.0 eq)	26.3
12	EtOH	50	5	DBU (2.0 eq)	34.9
13	EtOH	50	5	DMAP (2.0 eq)	39.8
14	EtOH	50	5	10%KOH (2.0 eq)	10.3
15	EtOH	50	5	20%KOH (2.0 eq)	19.4
16	EtOH	50	5	30%KOH (2.0 eq)	28.2
17	EtOH	50	5	40%KOH (2.0 eq)	35.3
18	EtOH	50	5	60%KOH (2.0 eq)	52.9
19	EtOH	25	2	50%KOH (2.0 eq)	30.3
20	EtOH	40	2	50%KOH (2.0 eq)	41.6
21	EtOH	60	2	50%KOH (2.0 eq)	36.8
22	EtOH	70	2	50%KOH (2.0 eq)	30.8
23	EtOH	40	3	50%KOH (2.0 eq)	45.8
24	EtOH	40	5	50%KOH (2.0 eq)	52.7
25	EtOH	40	7	50%KOH (2.0 eq)	52.6
26	EtOH	40	5	50%KOH (1.0 eq)	31.5
27	EtOH	40	5	50%KOH (1.5 eq)	40.0
28	EtOH	40	5	50%KOH (2.5 eq)	52.7
29	CH2Cl2	40	5	50%KOH (2.0 eq)	27.3
30	EtOAc	40	5	50%KOH (2.0 eq)	32.7
31	DMF	40	5	50%KOH (2.0 eq)	45.6
32	DMSO	40	5	50%KOH (2.0 eq)	41.8

^a—signifies no reaction or low production without separation.

. First, the effects of different solvents on the reaction were examined. The results showed that ethanol was the best solvent for the reaction. Then, the influence of different bases and their dosages on the reaction was investigated. It was found that some weaker bases, such as triethylamine, did not facilitate the smooth progress of the reaction. It can be seen from the data that, similar to the previous step, increasing the reaction temperature slightly improves the yield. Taking all factors into account, the optimal reaction conditions are to use ethanol as the solvent and react at 40 °C for 5 h under the catalysis of 2.0 equivalents of 50% potassium hydroxide.

The one-pot boiling method was also attempted, but due to the low yield, no further research was conducted.

3. Materials and Results

3.1. The Comprehensive Experimental Section

All reagents used in the experiment were commercially available and of analytical grade. FT-IR spectra were recorded using a Nicolet IS 20 instrument. NMR spectral data were obtained on a Varian Mercury Plus 400 MHz NMR spectrometer. Chemical shifts were referenced to the residual non-deuterated solvent and reported in parts per million (ppm).

3.2. Rhodanine Was Prepared According to Ref. [22]

Thiourea (0.76 g, 10 mmol) and thioglycolic acid (0.92 g, 10 mmol) were dissolved in toluene (20 mL), followed by the addition of hydrochloric acid (12 M, 12 mmol). The reaction mixture was heated to 110 °C with stirring and monitored by TLC (ethyl acetate–n-hexane = 1:3). Upon completion, the reaction was quenched with water and extracted with toluene. The organic phase was separated, dried over anhydrous sodium sulfate, filtered, and concentrated under reduced pressure. The crude product was purified by column chromatography on silica gel using ethyl acetate–n-hexane as the eluent, affording a light yellow solid (1.02 g) with a yield of 77.0%.

Rhodanine. Light yellow solid, m.p. 169–170 °C. ¹H NMR (DMSO-d6, 400 MHz), δ: 13.13 (s, 1H), 4.26 (s, 2H). (see Figure S1 in Supplementary Materials).

3.3. The Synthesis of 5-((1Z, 2E)-2-Methyl-3-phenylallylidene)-2-thioxothiazolidin-4-one (Compound 2)

Rhodanine (2.66 g, 20 mmol) and α-methylcinnamaldehyde (2.92 g, 20 mmol) were dissolved in 30 mL of ethanol, followed by the addition of 8 mL of 25% potassium hydroxide aqueous solution (40 mmol). The mixture was stirred and heated at 40 °C for 1 h, during which a yellow solid precipitated. An appropriate amount of dilute hydrochloric acid was added to adjust the pH value of the solution to neutral so that the product could be easily precipitated. After standing and filtration, a yellow solid (4.04 g) was obtained with a yield of 77.3%.

5-((1Z, 2E)-2-methyl-3-phenylallylidene)-2-thioxothiazolidin-4-one (compound 2). Yellow solid, m.p. 215–217 °C, FT-IR 3143 cm⁻¹, 3022 cm⁻¹, 1693 cm⁻¹, 1593 cm⁻¹, 1560 cm⁻¹, 1192 cm⁻¹, 1071 cm⁻¹, 696 cm⁻¹. ¹H NMR (DMSO-d6, 400 MHz), δ: 13.74 (s, 1H), 7.63–7.20 (m, 7H), 2.23 (s, 3H). ¹³C NMR (DMSO-d6, 100 MHz), δ: 196.13, 169.10, 143.78, 138.31, 136.43, 133.39, 129.97, 129.05(2C), 129.00(2C), 124.55, 16.21. HRMS (ESI): m/z calcd for M + H⁺: 262.0355, found: 262.0352. (see Figures S2–S5 in Supplementary Materials).

3.4. The Synthesis of Epalrestat

The above product (2.62 g, 10 mmol) and 2-chloroacetic acid (0.94 g, 10 mmol) were dissolved in 15 mL of ethanol, followed by the addition of 5 mL of 50% potassium hydroxide aqueous solution (20 mmol). The mixture was stirred and reacted at 40 °C for 5 h. After the reaction, the mixture was cooled to room temperature, and an appropriate amount of dilute hydrochloric acid was added to adjust the pH value of the solution to a weakly acidic state so that the product could be easily precipitated. The solution was left to stand for 1 h and then filtered to obtain a red solid (1.68 g) with a yield of 52.7%.

Epalrestat. Red solid, m.p. 210–213 °C. ¹H NMR (DMSO-d6, 400 MHz), δ: 13.21 (s, 1H), 7.62–7.34 (m, 7H), 4.29 (s, 2H), 2.22 (s, 3H). ref. [20] (Food and Drug, 2022, 24(3): 215–217.). (see Figure S6 in Supplementary Materials).

4. Conclusions

A new synthetic route for Epalrestat was proposed in this study. Using thiourea, thioglycolic acid, α-methylcinnamaldehyde, and chloroacetic acid as starting materials, the Epalrestat molecule was synthesized through three steps: cyclization, condensation, and substitution. This synthetic route opens up a new approach for the industrial production of Epalrestat. Further research on this topic is in progress.