Development of a New Process for Tulobuterol Hydrochloride

Abstract Tulobuterol is a selective β2-adrenoceptor agonist and has been widely utilized as a therapeutic agent for the treatment of asthma. Synthesis of tulobuterol has achieved many important progresses over the past decades and has gradually become one of the research hotspots in organic chemistry. This study aimed to explore a novel synthesis route to synthesize tulobuterol hydrochloride ( 1 ), an active ingredient of Chlobamolie Hydrochloride Tablets. In the study, 1 was obtained from the cost-effective, commercially available 2-chloroacetophenone through the key steps including the reactions of bromination, NaBH 4 reduction, and amination. Process-related impurities were also investigated. 1 was obtained with excellent purity (99.96%) in 53% overall yield without the need for chromatographic purification. The developed method is green, facile, and cost-effective; thus, it is suitable for the industrial-scale production of 1 .


Introduction
Asthma is a common respiratory disease that causes inflammation in the lungs, and it is usually accompanied by coughing, wheezing, and dyspnea. Tulobuterol is a selective β2-adrenoceptor agonist and was approved in Japan in 1981 for the treatment of asthma. 1 Tulobuterol is commercially available as Chlobamolie Hydrochloride Tablets containing tulobuterol hydrochloride (1) as the active ingredient. Chlobamolie Hydrochloride is prescribed to combat lasting dilation of bronchial smooth muscle, 1,2 dyspnea caused by acute or chronic bronchitis, emphysema, and other asthma-related symptoms. 3,4 In addition to the tablet form, tulobuterol can be administered as a topical patch. Amiaid is a patch formulation of tulobuterol, which was developed in 1998. 5 Amiaid is widely used to treat asthma in children because the active ingredient can be easily absorbed through the skin, is fastacting, and provides long-lasting relief. 6,7 Over the past decades, many synthetic routes of tulobuterol (5) have been reported in the literature (►Scheme 1). In 1973, Kato et al disclosed a reductive amination-based approach 5 using 2-chlorophenyl glyoxal as the starting material with an overall yield of 67% yield. 8 Unfortunately, despite the shortness and efficiency of the synthetic route, the starting material is difficult to obtain, thus limiting the widespread application of this strategy. In 1978, Koshinaka et al disclosed a selenium dioxide oxidation and reductive Keywords ► tulobuterol hydrochloride ► 2-chloroacetophenone ► synthesis process

Abstract
Tulobuterol is a selective β2-adrenoceptor agonist and has been widely utilized as a therapeutic agent for the treatment of asthma. Synthesis of tulobuterol has achieved many important progresses over the past decades and has gradually become one of the research hotspots in organic chemistry. This study aimed to explore a novel synthesis route to synthesize tulobuterol hydrochloride (1), an active ingredient of Chlobamolie Hydrochloride Tablets. In the study, 1 was obtained from the cost-effective, commercially available 2-chloroacetophenone through the key steps including the reactions of bromination, NaBH 4 reduction, and amination. Process-related impurities were also investigated. 1 was obtained with excellent purity (99.96%) in 53% overall yield without the need for chromatographic purification. The developed method is green, facile, and cost-effective; thus, it is suitable for the industrial-scale production of 1. amination approach starting from the commercially available 2-chloroacetophenone (2). Their synthesis of 5 was achieved with an overall yield of 71%. 9 While the yield did improve, the catalyst (selenium dioxide) is both expensive and poisonous, again limiting the widespread application of this strategy. In 1984, Marti and Gnehm focused on tulobuterol synthesis starting with common and low-cost materials. Compound 2 was used as the starting material.
The key steps included bromination, NaBH 4 reduction, and tert-butylamine amination. However, the overall yield of compound 5 was only 47%, 10 which was much lower than the reported studies mentioned above. Besides, column chromatography was repeatedly used in the separations, which restricted the application of the methods on an industrial scale. In 2016, Jing et al selected 1-(2-chlorphenyl) ethanol as a raw material to obtain tulobuterol hydrochloride. The key steps included olefination, epoxidation, and amination. 11 The total yield was 54%. However, the enylation reaction required high temperatures for execu-tion and makes the route unattractive for industrial-scale production as suitable high-temperature equipment set-ups are required. In 2020, Glushkova et al disclosed a rapid twostep synthesis of 5 from 2 12 ; unfortunately, the overall yield was only 15%.
Considering the many drawbacks of existing routes to 1, this study sought a practical synthetic route. The selected synthetic route based on the work by Marti and Gnehm is illustrated in ►Scheme 2. Briefly, 2 was reacted with Br 2 to give a mixture of 2-bromo-1-(2-chlorophenyl)-ethan-1-one (3) and 2,2dibromo-1-(2-chlorophenyl)-ethan-1-one (6), which could be further transformed to 3 with Na 2 SO 3 treatment. Compound 3 was obtained with a total yield of 86% and a purity of 97.6% by using high-performance liquid chromatography (HPLC), and was used to generate tulobuterol (5) through reactions of reduction, epoxidation, and amination in one pot. Compound 5 was acidified with hydrogen chloride (HCl) to obtain the target compound (1) with a yield of 62% and a purity of 99.9%. The total yield of this route was 53% and the purification of the target compound was 99.96%. The method was mild, simple, green, economical, and suitable for adoption on an industrial scale.

Results and Discussion
Optimization of the Bromination Reaction When 1.5 equiv. of Br 2 was used, the content of by-product 6 was 15.61%, which was consistent with a reported study 13 (►Table 1, entry 1). Decreasing the equivalent of Br 2 could reduce the formation of 6; however, the conversion of raw material 2 was also decreased (►Table 1, entries 2-4). When other bromination agents, including NBS, DBH, and HBr/H 2 O 2 , were assessed, the content of by-product 6 was increased dramatically (►Table 1, entries 5-7). Despite our efforts, Br 2 (1.3 equiv.) was selected as the best brominating agent.
To facilitate the conversion of 6 to the desired product 3, debromination additives were screened when the bromination reaction was finished. Taking guidance from the literature, 14 we first started with (EtO) 2 POH (►Table 1, entry 8). Our data showed that (EtO) 2 POH decreased the content of 6 to 0.58% and pushed the content of 3 to 97.19%. However, the over-reduced by-product 1 was found at a level of 2.21% after a full course of the reaction. Other agents were screened. Na 2 HPO 3 and (EtO) 2 POH had a similar effect in terms of yield, and both were superior to Na 2 HPO 3 (►Table 1, entries 8-10). To improve sustainability and avoid phosphorus waste disposal in the environment, cheaper and environment-friendly Na 2 SO 3 and its derivatives were selected according to a reported study. 15 Interestingly, Na 2 SO 3 was similar to (EtO) 2-POH and superior to NaHS 2 O 3 , Na 2 S 2 O 5 , Na 2 S 2 O 3 , and Na 2 S 2 O 4 in converting 6 to 3 (►Table 1, entries 11-15). Compared with Na 2 SO 3 , K 2 SO 3 did not perform better (►Table 1, entry 16). Thus, Na 2 SO 3 was selected for the next stage of development. We further investigated the effect of equivalents of Na 2 SO 3 and temperature on the yield of compound 3 (►Table 1, entries 17-22), and found that when the equivalent of Na 2 SO 3 was 1.3 and the temperature was 15 to 20°C, 3 could be generated with the maximum yield (►Table 1, entry 18). With the optimized reaction conditions in hand, 3 was prepared on a 100 g scale in 86.4% yield and 97.6% HPLC purity.

One-Pot Method to Synthesize 1
Arakawa et al reported the synthesis of tulobuterol (5) from 3 through the successive reaction of reduction and amination. 7 Unfortunately, 7, as an epoxide impurity, was generated in approximately 28% yield during the reduction step. It is well known that ring opening of 7 can be achieved with the nucleophilic amine to generate β-amino alcohols. Thus, when the reduction reaction was finished, tert-butylamine was added directly to convert 7 to the target tulobuterol. After refluxing in EtOH for 6 hours, 5 was obtained smoothly with 92% purity, yet, in parallel with 4% regioisomeric by-product 8. Notably, a quick conversion of 4 to 7 was also observed in the amination reaction (►Fig. 1).
Glushkova et al used HCl/i-PrOH to obtain tulobuterol hydrochloride (1) in Et 2 O. Due to the high toxicity and low boiling point of Et 2 O, the application of this approach was restricted in industry. Thus, Et 2 O was replaced with ethyl acetate (EA) in the salt-forming reaction. Finally, a crude product of 1 was obtained with 95% purity, alongside 5% isomer 8. Lastly, the purification of crude 1 through recrystallization was investigated. Unfortunately, due to the high solubility of 1 in MeOH, EtOH, and acetone, we failed to obtain the crystal of 1 from these solvents (►Table 2, entries 1-3). Using 3 volumes (V) of i-PrOH as the recrystallization solvent, 1 was obtained with a purity of 98.12% and a yield of 79.7% (►Table 2, entry 4). Increasing the dosage of i-PrOH led to increased purity while reduced yield (►Table 2, entries 5-7). We next explored ester solvents for a slurry-based purification to remove isomer impurity 8, such as butyl acetate, i-PrOAc, iso-amyl acetate, and EA (►Table 2, entries [8][9][10][11]. Surprisingly, an excellent purity of 99.96% and a good yield of 85.1% of 1 were obtained by using EA (5 V) as a solvent (►Table 2, entry 11). Further adjusting the amount of EA (3 V, or 8 V) decreased the yield or purity of 1 (►Table 2, entries 12 and 13). Thus, EA (5 V) was selected as the optimal condition for the purification of 1 in a 105 g scale purification.

Impurity Identification
The impurities related to our synthesis were also analyzed (►Scheme 3). By-product 6 was generated from the bromination reaction (Step 1). By-product 6 and the remaining starting material (2) resulted in impurities 9 and 10 after the NaHB 4 -catalyzed reduction reaction (Step 2). Impurities of 11 and isomer 8 were generated from the ring-opening reaction of 7. The impurities can be controlled. Compound 6, 9, 10, and 11 could be removed after the synthesis process of crude product 1. The content of isomer 8 can be controlled to less than 0.05% (HPLC) through a slurry operation at Step 3.

Conclusion
In summary, we have developed an efficient and practical process to synthesize tulobuterol hydrochloride (1) using 2-chloroacetophenone (2) as the starting material. As a result, the total yield of the target product was 53% and the purity was 99.96%. The highlight of this creative synthetic route was the use of inexpensive and environmentally friendly Na 2 SO 3

Preparation of 2-Chlorophenacyl Bromide (3)
Br 2 (134.4 g, 0.84 mol) was slowly added to a solution of 2chloroacetophenone (2) (100.0 g, 0.65 mol) in acetic acid (1.0 L) over 1 hour. The solution was stirred at room temperature for an additional 8 hours, and then cooled to 10°C. A mixture of Na 2 SO 3 (82.0 g, 0.65 mol) in water (400 mL) was then added, and the resulting mixture was stirred at room temperature for additional 2 hours. EA (500 mL) was added for extraction. After stirring for 30 minutes, the organic phase was separated, washed with water (300 mL Â 2), dried over Na 2 SO 4 , and concentrated to give 3 (130.1 g, 86.4% yield based on 2, HPLC purity 97.6%) as a pale-yellow oil. 1  Preparation of 1-(2-Chlorophenyl)-2-tertbutylaminoethanol (5) NaBH 4 (24.57 g, 0.65 mol) was slowly added to a solution of 3 (150 g, 0.65 mol) in EtOH (1.5 L) at 0°C and stirred the solution for 1.5 hours. Thereafter, tert-butylamine (94.8 g, 1.30 mol) was added, and the mixture was refluxed for an additional 6 hours. The progress of the reaction was monitored by HPLC. After the reaction was complete, the reaction solution was cooled to room temperature and filtered. The filtrate was concentrated to dryness, and the resulting residue was dissolved in EA (650 mL) and washed with water (200 mL Â 2). The organic phase of 5 was used in the next reaction without further purification.

Preparation of 1-(2-Chlorophenyl)-2-tertbutylaminoethanol Hydrochloride (1)
Concentrated HCl (71 mL, 0.78 mol) was added to a solution of 5 in EA (from the previous step). The resulting slurry was stirred at ambient temperature for 1 hour and filtered to obtain crude 1 (107.5 g, 73.2% yield, 95.6% HPLC purity). Crude 1 (105.0 g) was then suspended in EA (525 mL) and stirred at 70°C for 4 hours. Thereafter, the mixture was cooled to room temperature and filtered. Preparation of 2,2-Dibromo-1-(2-chlorophenyl) Ethenone (6) Br 2 (4.1 g, 25.9 mmol) was added dropwise at 15 to 20°C to a solution of 2 (1.0 g, 6.5 mmol) in acetic acid (5.0 mL) and the resulting mixture was stirred at ambient temperature for an additional 5 hours. Thereafter, EA (20 mL) was added, and the mixture was stirred for 10 minutes. The organic layer was collected, dried over Na 2 SO 4 , and concentrated at 55°C to give a residue, which was purified by column chromatography (EA:petroleum ether (PE) ¼ 20:1) to obtain 6 (1.6 g, 81% yield) as a pale-yellow oil. 1  Preparation of 2-(2-Chlorophenyl) Oxirane (7) NaBH 4 (0.82 g, 21.6 mmol) was carefully added at 0°C to a solution of 3 (5 g, 21.6 mmol) in ethanol (50 mL). The mixture was stirred at 15 to 20°C for an additional 1 hour, and reaction progress was monitored by thin layer chromatography. When the reaction was complete, the mixture was heated to 35 to 40°C and stirred for 1 hour. The solvent was evaporated in vacuo and the obtained residue was dissolved in EA (50 mL), washed twice with water, dried over Na 2 SO 4 , concentrated, and purified by silica gel column chromatography (EA:PE

Preparation of 1-(2-Chlorophenyl) Ethanol (10)
Compound 2 (3.85 g, 25.0 mmol) was dissolved in 10 mL methanol. Then, NaBH 4 (1.42 g, 37.65 mmol) was added at 0°C and the solution was stirred overnight at room temperature. After completion of the reaction, saturated ammonium chloride solution (10 mL) was added to the mixture. The desired compound was extracted with DCM (10 mL Â 3) and the organic layer was evaporated to concentrate. The residue was purified by silica gel column chromatography to obtain 10 (3.10 g, 79% yield). 1  Preparation of 1-(2-Chlorophenyl)-1,2-ethanediol (11) Compound 7 (1.08 g, 7.0 mmol) was added to 1 M NaOH (10.0 mL) and the mixture was heated at 40°C for 18 hours. The aqueous layer was extracted with DCM (15 mL) and DCM (20 mL Â 2) respectively. The combined organic layers were washed with brine (10 mL), dried over MgSO 4, and concentrated under a reduced pressure to give a residue, which was purified by silica gel column chromatography to give 11 (0.46 g, 38% yield). 1  10, and 11, as well as HPLC results for the purities of compounds 3 and 1, are included in the Supporting Information (►Figs. S1-S18 [online only]).

Funding
We are grateful to the China State Institute of Pharmaceutical Industry (CSIPI) and the Engineering Research Center for Improvement and Industrialization of Pharmaceutical Processes for the financial support. The work was also supported by Disha Pharmaceutical Group of China (Grant No. 2022CXGC020504).