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ACS Omega
ACS Omega
ao
acsodf
ACS Omega
2470-1343
American Chemical Society

10.1021/acsomega.4c04507
Article
Scalable Synthesis of the Key Fexuprazan Intermediate and Investigation of a Reaction Mechanism
Wang Ting †‡
Hua Yueting †‡
Deng Fangbo †‡
Wen Rui †‡
Zhang Haoyu †‡
Li Chunshi †‡
https://orcid.org/0000-0002-9284-1944
Liu Yang *†‡
https://orcid.org/0000-0001-9073-4806
Cheng Maosheng *†‡
† Key Laboratory of Structure-Based Drug Design & Discovery of Ministry of Education, Shenyang Pharmaceutical University, Shenyang 110016, China
‡ Key Laboratory of Intelligent Drug Design and New Drug Discovery of Liaoning Province, Shenyang Pharmaceutical University, Shenyang 110016, China
* Email: y.liu@syphu.edu.cn.
* Email: mscheng@syphu.edu.cn.
23 08 2024
10 09 2024
9 36 3794237952
12 05 2024
18 07 2024
17 07 2024
© 2024 The Authors. Published by American Chemical Society
2024
The Authors
https://creativecommons.org/licenses/by-nc-nd/4.0/ Permits non-commercial access and re-use, provided that author attribution and integrity are maintained; but does not permit creation of adaptations or other derivative works (https://creativecommons.org/licenses/by-nc-nd/4.0/).

A three-step method for the synthesis of methyl 5-(2,4-difluorophenyl)-4-methoxy-1H-pyrrole-3-carboxylate 1, a key intermediate of fexuprazan, is reported in this paper. Using low-priced sodium p-tolylsulfinate as the starting material, compound 28 was obtained with a 96.8% yield in the first step by optimizing the experimental parameters with a Box–Behnken experimental design. Subsequently, isonitrile compound 29 was obtained by dehydration. Finally, impurities 31 and 32 in the last step of the reaction were identified and converted into target product 1 by a one-pot method, which significantly improves the utilization of atoms. Key intermediate 1 was obtained via a three-step process with a yield of 68–70% and purity that exceeded 99%. In addition, the mechanism of the cyclization reaction was proposed. This method has potential for industrial production because the raw materials are inexpensive, the procedure is simple, and a high yield is obtained.

National Natural Science Foundation of China 10.13039/501100001809 22177079 document-id-old-9ao4c04507
document-id-new-14ao4c04507
ccc-price
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pmc1 Introduction

Fexuprazan (DWP14012), a novel potassium-competitive acid blocker (P-CAB) developed by Daewoong Pharmaceutical Co., Ltd. (Seoul, Korea), was approved in Korea in 2022 for the treatment of gastroesophageal reflux disease (GERD). The clinical management of GERD primarily involves treatments that neutralize or reduce gastric acid, and proton pump inhibitors (PPIs) are the most commonly prescribed medications. In contrast to irreversible PPIs, fexuprazan is acid-stable and inhibits H+, K+-ATPase in a competitive and reversible manner, which results in a faster onset and a longer duration of gastric acid suppression. According to the results of a phase III, randomized, and double-blind trial, the healing rate of erosive esophagitis in patients who received fexuprazan 40 mg once daily was 99.1% at week 8, which was equal to the rate observed for patients receiving esomeprazole 40 mg once daily (99.1%).1−3 Fexuprazan was well tolerated and much more effective than esomeprazole at treating heartburn and other general symptoms in patients with moderate to severe symptoms, and the relief effect lasted overnight.1−3 Additionally, unlike PPIs, the metabolism of fexuprazan is independent of CYP2C19; thus, therapeutic applications can be achieved without the need to consider genetic polymorphism.1,4

Fexuprazan is commonly prepared from compound 1 (Figure 1) through sulfonamidation, reduction, oxidation, and reduced-amination reactions. Considering that methyl 5-(2,4-difluorophenyl)-4-methoxy-1H-pyrrole-3-carboxylate 1 is the key intermediate for the synthesis of fexuprazan, its synthesis is well worth studying. Currently, there are four reported synthesis routes of intermediate 1, three of which are shown in Scheme 1.

Figure 1 Fexuprazan and its key intermediate 1.

Scheme 1 Synthesis Routes of the Key Intermediate 1

The first route for the synthesis of intermediate 1 utilized 2-amino-2-(2,4-difluorophenyl)acetic acid 2 and dimethyl 2-(methoxymethylene)malonate 3 as the raw materials and involved nucleophilic substitution, cyclization reaction, and hydrolysis to obtain methyl 5-(2,4-difluorophenyl)-4-hydroxy-1H-pyrrole-3-carboxylate 6; then, methylation with (diazomethyl)tri-methylsilane 7 afforded target intermediate 1 (Scheme 1A).5 The total yield of intermediate 1 in this route was only 9.0%, and the raw materials were expensive and scarce, making the procedure impractical on a large scale. Although the cheaper 2,4-difluorobenzaldehyde 8 was used in the second route (Scheme 1B) to improve the yield, it was not suitable for industrial production due to the lengthy reaction steps and usage of highly toxic reagents, such as sodium cyanide and dimethyl sulfate.6 In the third method, (2,4-difluorophenyl)methanamine 16 and dimethyl malonate 15 were condensed to dimethyl 2-(((2,4-difluorobenzyl)amino) methylene) malonate 18 (Scheme 1C), followed by amino protection, cyclization reaction, methylation reaction, and finally deprotection to obtain intermediate 1.7 Although the method was inexpensive, the complicated purification processes and toxic dimethyl sulfate restricted its industrial production. In the fourth route, m-difluorobenzene reacted with methyl oxalyl chloride to yield compound 24. After undergoing dehydration and intramolecular ring formation reactions, it led to compound 26. Finally, it reacted with dimethyl sulfate to produce intermediate 1.8 The methods used to synthesize intermediate 1 involve several disadvantages, including the utilization of expensive raw materials and hazardous chemicals, lengthy reaction routes, and low overall yields.

Herein, we used the fifth method9 (Scheme 2E) with a shorter route for optimization. The raw materials of the original route were expensive and commercially unavailable at bulk scale, and the purification method was complicated, resulting in a total yield of just 24.3%. The improved, three-step procedure (Scheme 2) for the synthesis of intermediate 1 by starting with commercially available sodium p-tolylsulfinate 30 was reported, which afforded target compound 1 with 99.8% purity and 68.1–69.6% overall yield. Notably, we proposed a mechanism for the last step (Scheme 3) in which the unreported compounds 31 and 32 were converted into key intermediate 1 by a one-pot reaction and purified by reslurry, resulting in a significantly improved yield.10,11

Scheme 2 Original Route and Optimized Route for the Synthesis of the Key Intermediate

30: stability: stable at room temperature and pressure, avoid contact with strong oxidizers. Slightly soluble in water. Storage conditions: keep the container sealed and store in a cool, dry place.

Scheme 3 Probable Mechanism of the Cyclization Reaction

2 Results and Discussion

A new method was adopted to synthesize key intermediate 1 of fexuprazan, sodium p-tolylsulfinate and 2,4-difluorobenzaldehyde as starting materials reacted with formamide under acidic conditions to obtain compound 28 with a 94.9% isolated yield. The isocyanide compound 29 was then obtained by dehydrating compound 28 with phosphorus oxychloride, with a yield of 84.8%. Finally, under strong base conditions, compound 29 reacted with dimethyl 2-(methoxymethylene) malonate 3 to yield key intermediate 1 in 85.8% yield. In contrast to other reported synthesis routes, the three-step workup method used to remove impurities comprised washing and reslurry rather than column chromatographic purification. This method produced a total yield of more than 68% and was notable for its novel approach, low cost, mild reaction conditions, and simple purification method.

2.1 Preparation of N-((2,4-Difluorophenyl)(tosyl)methyl)formamide 28

Because 4-methylbenzenesulfinic acid is expensive, sodium p-tolylsulfinate was selected as the starting material. The sodium salt was converted to free acid by adding excess acid, and compound 28 was obtained by a one-pot method. The effects of various acids on the yield were investigated, including organic acid (formic acid), mineral acid (hydrochloric acid), and camphorsulfonic acid as catalysts, as reported in the original literature.9 As shown in Table 1, the highest yield of only 67.3% was obtained using low-cost formic acid; therefore, the reaction conditions were further examined.

Table 1 Effect of Acid on the Synthesis of Compound 28a

entry	acid	time (h)	yieldc (%)	
1	hydrochloric acidb	10	46.5	
2	formic acid	10	67.3	
3	camphorsulfonic acid	10	47.9	
a Reaction conditions: sodium p-tolylsulfinate (0.030 mol), 2,4-difluorobenzaldehyde (0.030 mol), formamide (0.300 mol), and acid (0.060 mol), 65 °C.

b Hydrochloric acid with quality percent of 36.0–38.0%.

c Yield of the isolated product.

The reaction parameters were optimized using the response surface method (RSM), a statistical and mathematical technique for process improvement.12−14 The most commonly used experimental design method in RSM is Box–Behnken design (BBD), which involves fewer experiments than the central composite design and does not arrange all test factors into a high-level test combination at the same time, especially for certain tests with safety requirements or special needs.15−19 In this experiment, BBD was used to construct the experimental design and analysis. There were three factors with three levels each (Table 2), and a total of 15 experiments, including three central point experiments, were needed to achieve the best process conditions (Table 3). As shown in Table 3, these 15 experimental schemes were designed by Design-Expert 10 software, and the results were obtained through the actual experiment, from which the model was established to obtain the predicted value.

Table 2 Codes and Levels of Factors for BBDa

factors	coded	actual levels of coded factors	
–1	0	1	
2,4-difluorobenzaldehyde (eq)	A	1	1.5	2	
formamide (eq)	B	5	10	15	
formic acid (eq)	C	2	3.5	5	
a Reaction conditions: sodium p-tolylsulfinate (5 g, 1.0 equiv). The time was determined based on observation during the reaction (after 10 h of reaction, a large amount of solid precipitated in the reaction mixture and stirring became difficult).

Table 3 Results of the Designed Experiments Based on Box–Behnken

entry	A	B	C	yielda (%)	
actual value	predicted value	
1	0	–1	1	88.08	87.53	
2	0	0	0	91.58	90.37	
3	0	1	–1	85.16	85.71	
4	0	0	0	90.37	90.37	
5	–1	0	1	71.42	71.83	
6	1	0	–1	88.73	88.32	
7	–1	0	–1	66.60	66.44	
8	–1	–1	0	69.23	69.38	
9	1	1	0	92.89	92.75	
10	1	–1	0	89.12	89.52	
11	0	1	1	88.40	88.39	
12	1	0	1	91.47	91.63	
13	0	–1	–1	81.50	81.51	
14	–1	1	0	71.60	71.21	
15	0	0	0	89.17	90.37	
a Isolated yield of 28. The properties of the products obtained in 15 experiments are shown in Figure S4.

The precision, coefficient of determination R2, coefficient of variation (CV), and residual analysis showed that the quadratic polynomial regression model we created was accurate. From Table 4, it could be inferred that the model accounted for 99.64% of the total variation, and the difference between the value of R2 (0.9964) and RAdj2 (0.9899) is significantly less than 0.2. The prediction coefficient R2 of 0.9964 is preferably close to 1, indicating a strong correlation between the experimental results and the quadratic model. The P value and F value are the criteria used for testing the model’s significance; the higher the F value, the lower the P value, suggesting the greater the model’s significance.20 The F value and P value for the model were 154.09 and 0.0001, respectively, as shown in Table 4. There was only a 0.01% probability that noise would result in a very large F value. In this analysis, the model was deemed significant if the P value was less than 5%, and very significant if the P value was less than 1%,21 indicating that the model we established was highly significant. In this case, A, B, C, A2, B2, and C2 were significant model terms, while AB, AC, and BC were insignificant (P > 0.05). Precision is determined by the signal-to-noise ratio, which should be greater than 4. The signal-to-noise ratio (34.938) indicated that this model could be used to explore the system design. The CV, which is frequently employed to express the accuracy and reproducibility of an assay, reflects the proportion of the entire experimental error as a percentage of the total mean.17,22 The CV% score (1.10%) < 10% in this instance demonstrated that the model exhibited a high level of accuracy and reproducibility.23Figure 2A shows that the actual and predicted values were highly linear, with almost all points distributed along a straight line and no outliers.24 Externally studentized residuals are used to search for outliers or significant values using a normal probability plot of the studentized residuals to test for residual normality. The residual distribution between 2 and −2 clearly matched the normal distribution, as shown in Figure 2B. As a result, this model is reliable and standard and can be used to assess the effectiveness of an experiment.

Table 4 ANOVA on the Response Surface Quadratic Model about Yield

source	sum of squares	df	mean square	F-value	P-value prob > F	
model	1179.00	9	131.00	154.09	0.0001a	
A–A	868.61	1	868.61	1021.72	0.0001	
B–B	12.80	1	12.80	15.06	0.0116	
C–C	37.76	1	37.76	44.41	0.0011	
AB	0.49	1	0.49	0.58	0.4820	
AC	1.08	1	1.08	1.27	0.3105	
BC	2.79	1	2.79	3.28	0.1299	
A2	233.17	1	233.17	274.27	0.0001	
B2	10.88	1	10.88	12.80	0.0159	
C2	30.45	1	30.45	35.82	0.0019	
residual	4.25	5	0.85	 	 	
lack of fit	1.35	3	0.45	0.31	0.8217b	
pure error	2.90	2	1.45	 	 	
cor total	1183.25	14	 	 	 	
std. dev. 0.92	pred R-square 0.9763	
R-squared 0.9964	CV% 1.10	
adj R-squared 0.9899	adeq precision 34.938	
a Significant.

b Not significant.

Figure 2 Diagnostic plots of the quadratic model of yield: (A) predicted against actual, (B) normal % probability against externally studentized residuals.

A 3D surface diagram illustrating the impact of the interaction of the two factors on the product yield was built to better study the interaction of the three variables. Figure 3 displays the 3D response surface plots of a quadratic model in which two variables varied along the experimental range and one variable remained at the central level. Additionally, the degree of interaction between the two parameters varies, and the nonlinear relationship between the two affects the value of the product performance response.25 The interaction between the two factors can be more easily interpreted from the interaction diagram in Figure 4. The degree of overlap of the two curves increases as the degree of the factor interactions decreases.17Figure 4A illustrates that under the same feed ratio of formic acid, the red curve represents the yield at B = 15, and the black curve represents the yield at B = 5. The red curve was higher than the black curve; thus, with increasing doses of A and B, the yield of compound 28 was higher. With an increase in A, the two curves gradually separated, which also indicated that A and B had obvious interactions. Similarly, Figure 4B,C shows that there was a clear interaction between A and C or B and C.

Figure 3 3D surface plots of effects of binary interactions among factors on response value yield of compound 28: (A) formamide and 2,4-difluorobenzaldehyde; (B) formic acid and 2,4-difluorobenzaldehyde; (C) formic acid and formamide.

Figure 4 Interaction plot between different factors: (A) interaction plot of formamide and 2,4-difluorobenzaldehyde, under the same formic acid, the red curve represents the yield at the B = 15 and the black curve represents the yield at B = 5; (B) interaction plot of formic acid and 2,4-difluorobenzaldehyde, under the same formamide, the red curve represents the yield at the C = 5 and the black curve represents the yield at C = 2; (C) interaction plot of formic acid and formamide, under the same 2,4-difluorobenzaldehyde, the red curve represents the yield at the A = 2 and the black curve represents the yield at A = 1.

Through the aforementioned analysis, the three variables’ values can be appropriately increased to obtain the ideal yield value. The ideal values of the input factors were determined using Design-Expert 10 software for numerical optimization as A = 1.83, B = 11.84, and C = 3.90, and the highest yield value was expected to be 94.3%. The optimal point predicted by the model was then used for three sets of simultaneous verification trials, and the average yield was 94.2% with an error of 0.1%, which was consistent with the value predicted, confirming the accuracy of the model. The total cost of route D in the first step is 6.7 times higher than that of our optimized method (Tables S4 and S6). The model was constructed using 2,4-difluorobenzaldehyde as unit 1, and the corresponding data can be found in the Supporting Information. Table S5 reveals that the cost of producing 1 kg of the product is comparable to the cost of using sodium p-tolylsulfinate as unit 1. Therefore, considering the actual raw material price at the time of production, we can opt for a more cost-effective method. The one-pot method is employed to streamline the synthesis process and simplify postprocessing. Finally, using the optimal conditions, we scaled up the reaction to 500 g and produced 884 g of compound 28 as a white solid with a purity of 97.1% in a 96.8% isolated yield.

2.2 Preparation of 2,4-Difluoro-1-(isocyano(tosyl)methyl)benzene 29

Compound 29 was synthesized through the dehydration of compound 28 using phosphorus oxychloride under alkaline conditions. Initially, phosphorus oxychloride and compound 28 were combined, followed by the gradual addition of triethylamine to the reaction mixture.26 To achieve the maximum reaction yield, we investigated several factors including the reaction solvent, the feed ratio of phosphorus oxychloride, and the reaction temperature. We examined the impact of the reaction solvent on the yield by comparing entries 1–3 in Table 5. The results indicated that using THF as the solvent resulted in a higher yield compared to DCM and CH3CN. Among the different quantities of POCl3 tested (entries 3–6 in Table 5), 3.5 equiv yielded the highest result. Analyzing entries 5 and 5, 7–9 in Table 5, we observed that as the reaction temperature increased beyond 0 °C, the yield decreased due to the presence of more impurities with all variables being the same. Further investigation revealed that the optimal reaction temperature was 0 °C. Finally, employing the aforementioned reaction conditions, we obtained compound 29 with a purity of 99.4% and a yield of 84.7% on a 500 g scale.

Table 5 Optimization for Synthesizing Compound 29a

entry	solvent	POCl3 (equiv)	T (°C)	HPLCb (%)	
1	CH2Cl2	2.0	0	tracec	
2	CH3CN	2.0	0	tracec	
3	THF	2.0	0	70.52	
4	THF	3.0	0	86.40	
5	THF	3.5	0	92.36	
6	THF	4.0	0	86.87	
7	THF	3.5	–10	63.51	
8	THF	3.5	10	71.82	
9	THF	3.5	25	16.88	
10d	THF	3.5	0	99.43e	
a Reagents and conditions: 28 (5 g, 1.0 equiv), POCl3, TEA (6.0 equiv), THF (60 mL).

b Numbers listed indicate conversion which were assessed by HPLC analysis.

c By TLC analysis, the product was oil-like, no solids were precipitated.

d 28 (500 g).

e Purity of 29 after reslurry.

2.3 Preparation of Methyl 5-(2,4-Difluorophenyl)-4-methoxy-1H-pyrrole-3-carboxylate 1

According to published research, compound 29 underwent a reaction with dimethyl 2-(methoxymethylene) malonate 3 in the presence of sodium hydride to obtain key intermediate 1.(9) However, the procedure described in the literature yielded poor results, requiring purification through silica gel column chromatography.9 Therefore, the conditions of the cyclization reaction were thoroughly investigated, as presented in Table 6. HPLC analysis of the reaction solution revealed the presence of two impurities in the acetonitrile solvent, with respective contents of 25.12 and 21.30%. The Supporting Information identified these impurities as compound 31 and compound 32. The reaction temperature and time significantly influenced the chemical reaction rate and had varying effects on the yield. Therefore, we investigated the reaction temperature and time, as shown in entries 4–9 of Table 6. Increasing the temperature and time gradually decreased the content of compound 1 while increasing the content of compound 32, indicating that a higher reaction temperature or longer reaction time converted compound 1 into compound 32. It is important to note that the total content of these three compounds (1, 31, and 32) ranged from 86.99 to 90.74%, making them the main components in the reaction solution. Furthermore, we examined the effect of NaH dosage on the reaction, which revealed that increasing the NaH dosage significantly increased the content of compound 1. When NaH was used in 8 equiv, the content of 1 reached its maximum value (88.29%). However, further increasing the NaH dosage resulted in a decrease in the content of 1 and the generation of new impurities (Table 6, entries 9–14). Although a higher yield was achieved with 8 equiv of NaH, it also led to the production of a large amount of H2, posing safety hazards and making it unsuitable for mass production.

Table 6 Optimization for Synthesizing Key Intermediate 1a

entry	base (equiv)	solvent	T (°C)	time (h)	HPLCb (%)	
1	31	32	
1	NaH (1.1)	CH3CN	r.t.	1	43.84	25.12	21.30	
2	NaH (1.1)	DMFc	r.t.	1	24.35	21.62	1.49	
3	NaH (1.1)	THF	r.t.	1	39.72	18.61	7.31	
4	NaH (1.1)	CH3CN	–10	1	45.79	22.71	18.49	
5	NaH (1.1)	CH3CN	40	1	39.94	23.47	27.33	
6	NaH (1.1)	CH3CN	70	1	35.49	22.50	29.42	
7	NaH (1.1)	CH3CN	r.t.	0.5	46.24	23.33	20.01	
8	NaH (1.1)	CH3CN	r.t.	3	43.07	21.96	25.39	
9	NaH (1.1)	CH3CN	r.t.	10	41.73	21.27	25.46	
10	NaH (2.0)	CH3CN	r.t.	1	66.10	13.56	11.87	
11	NaH (4.0)	CH3CN	r.t.	1	72.82	7.28	7.50	
12	NaH (6.0)	CH3CN	r.t.	1	85.43	NDd	1.61	
13	NaH (8.0)	CH3CN	r.t.	1	88.29	NDd	0.55	
14	NaH (10)	CH3CN	r.t.	1	86.48	NDd	NDd	
15	NaH (15)	CH3CN	r.t.	1	75.75	0.34	0.30	
a Reagents and conditions: 29 (0.5 g), 3 (1.0 equiv), solvent (15 mL).

b Numbers listed indicate conversion which were assessed by high-performance liquid chromatography (HPLC) analysis of the organic layer.

c Explosion hazards of sodium hydride in DMF.

d Not detected.

Through the analysis of experimental data, we proposed a plausible reaction mechanism, as illustrated in Scheme 3. Initially, deprotonation of compound 29 using NaH generated a carbanion, which subsequently attacked the double bond of compound 3, leading to the cyclization and formation of compound 33. The tosyl (Ts) group attached to compound 33 underwent rearrangement through a 1,3 shift, resulting in the formation of impurity 31. The nucleophilic attack on impurity 31 caused the expulsion of the tosyl group, yielding compound 1. Compound 1, upon deprotonation by the base reagent, underwent exchange with the tosyl group on impurity 31, generating impurity 32. Finally, impurity 32 was attacked by a nucleophilic reagent, leading to the formation of the desired 2-molecule 1. Ultimately, both impurities 31 and 32 were converted into the target product 1. Based on this mechanism, we hypothesize that an excess amount of the base reagent (greater than 2.0 equiv) is required to facilitate deprotonation of the hydrogen on the carbon and subsequently act as a nucleophilic reagent. When using only 1.1 equiv of NaH, only the first step of deprotonation is completed, resulting in the main components in the reaction solution being compound 1, 31, and 32. However, employing 2.0 equiv of sodium hydride leads to a content of compound 1 of only 66.10%. This discrepancy may be attributed to the relatively poor nucleophilicity of NaH. Under conditions of excess NaH, acetonitrile undergoes deprotonation to form −CH2CN, which acts as a nucleophilic reagent and converts impurities 31 and 32 into compound 1, thereby significantly improving the reaction yield.

To test this hypothesis, we synthesized compounds 31 and 32 and subjected them to reactions with commonly employed and potent nucleophilic alkaline reagents, namely, sodium hydroxide and sodium methoxide. We were delighted to witness the transformation of both 31 and 32 into compound 1. Given that NaH is unsuitable for industrial-scale production, we explored alternative bases, particularly focusing on commonly used inorganic bases that facilitate subsequent treatments. The details of our investigations are presented in Table 7.27−29 From entries 1–8, it was evident that increasing the reaction temperature or duration did not significantly enhance the reaction yield when using DBU, Na2CO3, K2CO3, Cs2CO3, or K3PO4 as alkaline reagents. However, when the stronger alkaline reagent NaOCH3 was employed (Table 7, entries 9–11), the content of compound 1 in the reaction solution noticeably increased, while 17.0% of impurity 31 was not converted. Conversely, the use of the even stronger alkaline reagent t-BuOK (Table 7, entries 12–14) led to the generation of new impurities, resulting in a significant decline in the yield of compound 1. Consequently, sodium hydroxide (NaOH) was selected as the alkaline reagent due to its strong alkalinity and affinity. Entries 15–17 demonstrated a significant conversion of impurities 31 and 32 into desired compound 1, leading to a considerable improvement in the reaction yield.

Table 7 Optimization of the Reaction Conditionsa

entry	base (equiv)	Tc (°C)	time (h)	HPLCb (%)	
1	31	32	
1	DBU (3.0)	r.t.	1	48.00	29.79	NDd	
2	DBU (3.0)	r.t.	10	37.55	38.21	1.31	
3	Na2CO3(3.0)	60	10	NRe	NRe	NRe	
4	K2CO3(3.0)	60	10	NRe	NRe	NRe	
5	Cs2CO3(3.0)	r.t.	1	3.39	28.65	NDd	
6	Cs2CO3(3.0)	80	10	20.13	10.38	18.03	
7	K3PO4(3.0)	r.t.	1	11.17	11.10	NDd	
8	K3PO4(3.0)	60	10	33.29	36.67	NDd	
9	NaOCH3(1.2)	r.t.	1	19.21	10.77	NDd	
10	NaOCH3(3.0)	r.t.	1	72.93	19.19	1.06	
11	NaOCH3(3.0)	r.t.	6	76.22	16.96	1.01	
12	t-BuOK(1.2)	r.t.	1	33.27	11.60	7.59	
13	t-BuOK(3.0)	r.t.	1	38.72	0.19	NDd	
14	t-BuOK(3.0)	r.t.	6	36.20	0.23	NDd	
15	NaOH (1.2)	r.t.	1	57.87	24.38	10.09	
16	NaOH (3.0)	r.t.	1	93.74	0.40	1.55	
17	NaOH (3.0)	r.t.	3	92.53	NDd	1.19	
a Reagents and conditions: 29 (0.5 g), 3 (1.0 equiv), solvent: CH3CN (15 mL).

b Numbers listed indicate conversion which were assessed by HPLC analysis of the organic layer.

c r.t. = 20–25 °C.

d Not detected.

e No reaction.

Subsequently, with NaOH as the alkaline reagent, we explored the reaction solvent and the NaOH feeding ratio, as presented in Table 8. Ultimately, the highest yield was achieved by employing 2.5 times the amount of NaOH as the alkaline reagent and using acetonitrile as the reaction solvent at room temperature for 1 h.

Table 8 Optimization of the Reaction Conditions with Sodium Hydroxide as Basea

entry	base (equiv)	solvent	HPLCb (%)	
1	31	32	
1	NaOH (3.0)	C2H5OH	3.72	0.30	10.09	
2	NaOH (3.0)	CH2Cl2	46.44	22.52	NDc	
3	NaOH (3.0)	THF	88.23	0.30	4.07	
4	NaOH (3.0)	acetone	92.01	1.51	1.64	
5	NaOH (2.0)	CH3CN	89.19	4.60	2.06	
6	NaOH (2.5)	CH3CN	93.11	0.47	1.54	
a Reagents and conditions: 29 (0.5 g), 3 (1.0 equiv), solvent: CH3CN (15 mL), r.t. = 20–25 °C, 1 h.

b Numbers listed indicate conversion which were assessed by HPLC analysis of the organic layer.

c Not detected.

Then, the investigation focused on the solvents used for the slurry of the crude product, as shown in Table 9. Compound 1 purity was below 99% when the ethyl acetate/n-heptane ratio was 3:20 or less. At a 4:15 ethyl acetate/n-heptane ratio, the purity reached 99.73%; however, the yield was also reduced. Consequently, reslurrying with ethyl acetate and n-heptane (3:15) yielded a white solid. Ultimately, key intermediate 1 was obtained with a purity of 99.2% and a yield of 85.8% on a 400 g scale.

Table 9 Screening of Solvent during Post-Treatment of 1

entry	solvent (ratio)	purity (%)	yield (%)	
1	ethyl acetate/n-heptane (2:15)	95.13	88.2	
2	ethyl acetate/n-heptane (3:15)	99.24	85.8	
3	ethyl acetate/n-heptane (4:15)	99.73	84.6	
4	ethyl acetate/n-heptane (3:17)	99.17	86.3	
5	ethyl acetate/n-heptane (3:20)	98.34	87.5	

2.4 Quality Detection and Yield Stability of Intermediate 1

The new technique was carried out for sodium p-tolylsulfinate 30 on a 100 g scale with the optimization condition in hand to confirm the robustness and scalability of this procedure, as indicated in Table 10. The technique has good yield stability and potential for industrial application, as evidenced by the overall yield of 68.1–69.6%, purity of more than 99%, loss on drying of less than 1.0%, and residue on ignition of less than 0.5%.

Table 10 Results of Three Batches of Experimenta

entry	mass of 30	loss on drying	residue on ignition	water	total yield	purity	
1	100 g	0.08%	0.04%	0.24%	69.6%	99.59%	
2	100 g	0.04%	0.02%	0.20%	68.5%	99.13%	
3	100 g	0.06%	0.01%	0.21%	68.1%	99.47%	
a Product was dissolved in ethyl acetate and then filtered to remove insoluble impurities.

3 Conclusions

In conclusion, we have developed a synthetic approach to synthesize the key intermediate of fexuprazan (1) from sodium p-tolylsulfinate with a much lower cost and higher yield than those of previous approaches. The three-step reaction produced the key intermediate 1, the overall yield was greater than 68% and the purity was greater than 99%. The process parameters of the first step were optimized using the Box–Behnken experimental design, and impurities 31 and 32 were converted into the desired product 1 by studying the reaction mechanism. The current process was successfully tested on a 500 g scale using a simple purification method, providing a new synthetic strategy for the synthesis of the key fexuprazan intermediate.

4 Experimental Section

4.1 Materials and Methods

Unless otherwise specified, commercially obtained solvents and reagents were used in the experiments without further purification. Analytical thin-layer chromatography (TLC) was used to monitor the reactions, which were then visualized under ultraviolet light (254 and 365 nm). HPLC analysis on a JASCO pu-1580 system with a JASCO UV-1575 detector was used to determine the chemical purity. Using isocratic elution of water and methanol/acetonitrile at a flow rate of 1.0 mL/min, the analytes were separated using a ZORBAX Eclipse XDB-C18 (4.6 × 250 mm2, 5 μm particle size) column. HPLC analysis on a Waters Alliance 2695 system with W2489 ChA 254 nm detector was used to determine quality detection (chemical purity). Using gradient elution of Mobile phase A (dipotassium hydrogen phosphate 1.74 g into 1000 mL water, then add trifluoroacetic acid and mix well, then adjust to pH 2.80–2.90) and Mobile phase B (methanol:acetonitrile = 45:55 (v/v)) at a flow rate of 1.0 mL/min, the analytes were separated using a Kromasil C8 (4.6 mm × 250 mm, 5 μm) column. Using DMSO-d6 as the test solvent and TMS as an internal reference, NMR spectra were captured using a Bruker Avance 600/400 MHz spectrometer. High-resolution mass spectrometry (HRMS) data were produced by using an Agilent Q-TOF 6530/6550 mass spectrometer. Design-Expert 10 software was used to create these experimental schemes in the first step. The differential scanning calorimetry (DSC) curves and thermogravimetric analysis (TGA) were investigated with Netzsch differential scanning calorimeter (DSC 214) at the heating rate of 5 °C/min in the range of room temperature to 500 °C in a nitrogen atmosphere. Sealed gold-plated high-pressure DSC crucibles were used for the evaluation.

4.2 Synthesis of N-((2,4-Difluorophenyl)(tosyl)methyl)formamide 28

Sodium p-tolylsulfinate 30 (500 g, 2.81 mol), 2,4-difluorobenzaldehyde 8 (730 g, 5.14 mol), formic acid (504 g, 10.9 mol), and formamide (1496 g, 33.2 mol) were added to a 5 L flask, and the mixture was mechanically stirred for 10 h at 65 °C. After cooling to room temperature, water (2.5 L) was added, and the mixture was mechanically stirred for 0.5 h, and a white crude product was obtained by filtration under reduced pressure. The white crude product was filtered and washed with hexane (2 L × 3) and water (2 L × 3) dried in a blast drying oven at 60 °C for 48 h to give 884 g. (96.8% yield.) HPLC purity: 97.1%. 1H NMR (600 MHz, DMSO-d6) δ 9.93 (dd, J = 10.3, 1.5 Hz, 1H), 8.03 (d, J = 1.3 Hz, 1H), 7.74 (td, J = 8.5, 8.4, 6.3 Hz, 1H), 7.69–7.65 (m, 2H), 7.46 (d, J = 8.0 Hz, 2H), 7.35 (td, J = 9.7, 9.7, 2.6 Hz, 1H), 7.28 (td, J = 8.6, 8.5, 2.6 Hz, 1H), 6.52 (d, J = 10.4 Hz, 1H), 2.41 (s, 3H). 13C NMR (151 MHz, DMSO-d6) δ163.1 (dd, J = 249.8, 12.6 Hz), 160.6, 160.4 (dd, J = 250.7, 12.7 Hz), 145.4, 132.8, 131.7 (dd, J = 10.4, 3.6 Hz), 130.0, 129.1, 114.6 (dd, J = 14.3, 3.8 Hz), 112.3 (dd, J = 21.8, 3.4 Hz), 104.2 (t, J = 26.3, 26.3 Hz), 63.3 (d, J = 2.4 Hz), 21.2. HRMS (ESI-TOF) m/z: [M + Na]+ calcd for C15H13NO3NaSF2, 348.0482; found, 348.0481.

4.3 Synthesis of 2,4-Difluoro-1-(isocyano(tosyl)methyl)benzene 29

Tetrahydrofuran (3.5 L) and 28 (500 g, 1.54 mol) were placed in a 10 L three-necked flask equipped with an overhead stirrer, an addition funnel, and a temperature probe. The solution was stirred at 25 °C after adding 825 g (5.38 mol) of phosphorus oxychloride. Triethylamine (933 g, 9.22 mol) was added gradually after the solution had been cooled to 0 °C while keeping the internal reaction temperature 0 °C. Following the completion of the triethylamine addition, the reaction continued for 30 min. The reaction was quenched by slowly adding the reaction solution to aqueous solution of NaOH (10 L, 2 mol/L), then ethyl acetate (10 L) was added and the mixture was mechanically stirred for 0.5 h. After concentrating the organic layer, it was reslurry with n-propanol (0.3 L) to give a yellow solid. The yellow solid was filtered and the filter cake is rinsed with n-propanol (0.1 L × 2) to produce a yellowish solid dried in a blast drying oven at 40 °C (400 g, 84.7% yield). HPLC purity: 99.4%. 1H NMR (600 MHz, DMSO-d6) δ 7.73–7.69 (m, 2H), 7.56–7.52 (m, 2H), 7.45–7.40 (m, 2H), 7.26 (td, J = 8.6, 8.3, 2.2 Hz, 1H), 7.10 (s, 1H), 2.46 (s, 3H). 13C NMR (151 MHz, DMSO-d6) δ 164.3, 163.8 (dd, J = 251.2, 12.2 Hz), 160.4 (dd, J = 255.0, 13.0 Hz), 146.8, 131.5 (dd, J = 10.9, 3.2 Hz), 130.3, 130.2, 129.9, 112.6 (dd, J = 22.2, 3.4 Hz), 111.3 (dd, J = 13.0, 3.8 Hz), 104.9 (t, J = 25.9, 25.9 Hz), 69.4, 21.3. HRMS (ESI-TOF) m/z: [M + Na]+ calcd for C15H11NO2F2NaS, 330.0376; found, 330.0369.

4.4 Synthesis of Dimethyl 5-(2,4-Difluorophenyl)-4-methoxy-2-tosyl-2,4-dihydro-3H-pyrrole-3,3-dicarboxylate 31 and Trimethyl 5,5′-Bis(2,4-difluorophenyl)-4,4′-dimethoxy-2′,4′-dihydro-3′H-[1,2′-bipyrrole]-3,3′,3′-tricarboxylate 32

Compound 29 (10.0 g, 0.03 mol) and dimethyl methoxymethylenemalonate 3 (5.7 g, 0.03 mol) were added to the reaction flask and dissolved with acetonitrile (90 mL). Sodium hydride (0.86 g, 0.04 mol) was added in batches and stirred at room temperature for 1 h. After the reaction was complete, water (200 mL) and ethyl acetate (200 mL) were added for extraction to concentrate the collected organic layer. Then, 31, 32 were isolated by silica gel column chromatography and obtained as one white solid (31, 2.1 g, 13.2% yield) and another white solid (32, 0.9 g, 4.7% yield).

31: HPLC purity: 99.7%. 1H NMR (600 MHz, DMSO-d6) δ 7.72 (td, J = 8.6, 8.5, 6.5 Hz, 1H), 7.70–7.66 (m, 2H), 7.49–7.43 (m, 1H), 7.42 (d, J = 8.2 Hz, 2H), 7.20 (td, J = 8.4, 8.4, 2.5 Hz, 1H), 6.11 (d, J = 1.2 Hz, 1H), 5.79–5.64 (m, 1H), 3.80 (s, 3H), 3.70 (s, 3H), 3.43 (s, 3H), 2.40 (s, 3H). 13C NMR (151 MHz, DMSO-d6) δ 174.0 (d, J = 2.5 Hz), 164.6 (dd, J = 252.9, 12.6 Hz), 161.1 (dd, J = 255.8, 13.0 Hz), 145.3, 134.7, 132.3 (dd, J = 10.6, 4.3 Hz), 129.6, 129.2, 116.4 (dd, J = 12.3, 3.5 Hz), 112.7 (dd, J = 22.0, 3.2 Hz), 105.0 (t, J = 26.2, 26.2 Hz), 91.1, 89.9 (d, J = 3.3 Hz), 66.3, 59.8, 53.3, 53.2, 21.1. HRMS (ESI-TOF) m/z: [M + Na]+ calcd for C22H21NO7F2NaS, 504.0904; found, 504.0901.

32: HPLC purity: 99.7%. 1H NMR (600 MHz, DMSO-d6) δ 8.01 (q, J = 8.0, 8.0, 7.9 Hz, 1H), 7.46 (ddd, J = 18.3, 9.2, 4.6 Hz, 3H), 7.25 (td, J = 8.4, 8.3, 2.5 Hz, 2H), 7.08 (s, 1H), 6.61 (d, J = 102.1 Hz, 1H), 5.87 (d, J = 60.4 Hz, 1H), 3.70 (s, 3H), 3.62 (s, 3H), 3.58 (s, 3H), 3.45 (s, 3H), 3.39 (s, 3H). 13C NMR (151 MHz, DMSO-d6) δ 170.9, 166.8, 166.1, 165.5 (d, J = 12.5 Hz), 163.8 (d, J = 12.7 Hz), 162.5, 162.2 (d, J = 48.7 Hz), 160.7 (d, J = 12.9 Hz), 144.4, 134.6, 133.1, 122.8, 116.8, 116.2–115.8 (m), 113.0 (d, J = 15.5 Hz), 112.4 (d, J = 3.2 Hz), 112.0, 108.3, 105.1 (t, J = 26.3, 26.3 Hz), 104.6 (t, J = 26.3, 26.3 Hz), 90.2, 84.4, 66.8, 62.0, 60.2, 53.2, 53.2, 50.9. HRMS (ESI-TOF) m/z: [M + Na]+ calcd for C28H24N2O8F4Na, 615.1366; found, 615.1360.

4.5 Synthesis of Methyl 5-(2,4-Difluorophenyl)-4-methoxy-1H-pyrrole-3-carboxylate 1

Compound 29 (400 g, 1.30 mol) and dimethyl methoxymethylenemalonate 3 (227 g, 1.30 mol) were added to the 5 L reaction flask and dissolved with acetonitrile (2.4 L). Sodium hydroxide (130 g, 3.25 mol) was added in batches, and the mixture was stirred at room temperature for 1 h. After the reaction was completed, sodium hydroxide inorganic base was removed through decompression filtration,30 ice water (6 L) was added to the mixture and was mechanically stirred for 1 h. Finally, the crude product was obtained by decompression filtration. The intermediate 1 was obtained by reslurry (0.3 L), and rinsing (0.1 L × 2) with ethyl acetate and n-heptane (3:15), which was white solid. The product was dissolved in ethyl acetate and then filtered to remove insoluble impurities (298 g, 85.8% yield). HPLC purity: 99.8%. 1H NMR (600 MHz, DMSO-d6) δ 11.47 (s, 1H), 7.64 (td, J = 8.7, 8.7, 6.6 Hz, 1H), 7.38–7.31 (m, 2H), 7.17 (td, J = 8.7, 8.6, 3.0 Hz, 1H), 3.72 (s, 3H), 3.71 (s, 3H). 13C NMR (151 MHz, DMSO-d6) δ 163.2, 161.1 (dd, J = 246.8, 12.1 Hz), 158.4 (dd, J = 249.0, 12.2 Hz), 144.2, 130.8 (dd, J = 9.6, 5.0 Hz), 123.0, 115.4 (dd, J = 14.3, 3.8 Hz), 114.0, 111.8 (dd, J = 21.2, 3.5 Hz), 106.9, 104.4 (t, J = 26.3, 26.3 Hz), 61.6, 50.6. HRMS (ESI-TOF) m/z: [M + Na]+ calcd for C13H11NO3F2Na, 290.0605; found, 290.0605.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.4c04507.Experimental procedures and characterization data of compounds (NMR, HRMS, DSC-TGA, single-crystal X-ray diffraction, HPLC analysis) (PDF)

Supplementary Material

ao4c04507_si_001.pdf

The authors declare no competing financial interest.

Acknowledgments

We gratefully acknowledge the Program for the National Natural Science Foundation of China (grant number 22177079).
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