Going Green in Process Chemistry: Optimizing an Asymmetric Oxidation Reaction To Synthesize the Antiulcer Drug Esomeprazole

Abstract

Sustainable practices in process chemistry are highlighted by a novel, 9 week team project of 8–12 students, in collaboration with AstraZeneca chemists, in an organic chemistry laboratory. Students synthesize the antiulcer medicine esomeprazole, which involves the asymmetric oxidation of pyrmetazole. To provide insight into the modern process chemistry industry, they propose environmentally friendly modifications to the asymmetric oxidation. Students first synthesize pyrmetazole and then follow a standard oxidation procedure and carry out modified, greener reactions of their choice. They investigate how a change in reaction conditions affects both the yield and enantioselectivity of esomeprazole. Positive student feedback was received and student postlab reports were analyzed over a 4 year period (2015–2018). Results consistently showed that the project provided students with the key tools to develop greener syntheses. This contextual approach not only offers the opportunity to develop valuable communication and team-working skills, but it also gives students creative input into their experimental work. It teaches the important research skills involved in sustainable process chemistry, from reproducing and modifying a literature procedure to identifying green metrics.

Introduction

Process chemistry involves the development and optimization of production processes, particularly in the pharmaceutical sector. It is typically associated with scaling up reactions: process chemists adapt the synthetic process used by medicinal chemists to make small quantities of a medicine (e.g., for biological testing), so that much larger quantities can be produced. To function on a large scale, the operation should be simple, safe, and straightforward, which is often easier said than done! Not only should the target medicine be prepared in high yield, using inexpensive and readily available starting materials, reagents, and solvents, but any impurities need to be easily removed. Today, green technology is a common concept in process chemistry.¹ Because legal restrictions for the handling of chemicals have become more stringent, initiatives have emerged to exploit greener and more sustainable synthetic chemistry. Initiatives are guided by the 12 principles of green chemistry, which involve high atom economy, minimal amount of waste product, and safe and renewable feedstock.² Educating students about how to apply green chemistry principles to organic synthesis is an important theme in the modern undergraduate organic curriculum.³⁻⁵

Introducing Process Chemistry

At the undergraduate level, the role of process chemistry, in comparison with other careers such as discovery chemistry, is often less appreciated by students. Indeed, there is typically no formal training provided at university levels or at most universities,^6,7 despite the fact that many chemistry graduates enter this challenging and exciting career. To give a flavor of practical process chemistry in our undergraduate course, around 10 years ago, in collaboration with AstraZeneca chemists, we developed a new mini-project for third-year students.⁸

Third-year master’s students were required to complete a mini-project, in a research area of their choice, as preparation for their fourth-year extended research project. The projects are designed to move away from the cookbook style recipes used in years 1 and 2, which, although helpful in developing manipulative skills, do little to stimulate cognitive activity or challenge student curiosity. From a pedagogical perspective the projects aimed to improve the ability of students to plan, design, manage, and execute scientific research. Faculty offered mini-projects in the different branches of chemistry, including green and sustainable chemistry, but this was the only project that involved process chemistry.

This process chemistry project was centered around the synthesis of the antiulcer drug esomeprazole (Nexium). Each year, a group of 8–12 students was tasked with making pyrmetazole and then exploring the asymmetric oxidation of this sulfide. The oxidation used cumene hydroperoxide in the presence of (S,S)-diethyl tartrate ((S,S)-DET) to selectively form the S-enantiomer of the sulfoxide, called esomeprazole (Scheme 1).⁹

Scheme 1. Three-Step Synthesis of Esomeprazole, Involving Asymmetric Oxidation of the Sulfide Pyrmetazole.

A number of variations of the oxidation step are possible, such as changing the reaction temperature and time. Students could decide, as a team, which changes they would like to explore in the laboratory and how this affected the yield of esomeprazole, which was monitored by analysis of the crude product using ¹H NMR spectroscopy (by integrating the signals for −CH₂S– and −CH₂SO– in the crude reaction product). The presence of esomeprazole is easily determined because the signal for the diastereotopic CH₂ protons is split into two (roofed) doublets with a characteristic geminal coupling constant (around −13 Hz, see the Supporting Information). The team of 8–12 students was divided into three groups. For up to 2 days (∼12 h) each week, one of the groups would be in the laboratory, and the groups rotated, typically on a weekly basis. In total, each group was in the laboratory for up to 6 days over a 9 week project period. Consequently, students needed to regularly communicate their experimental results across the entire team so that informed decisions could be made on which future experiments to undertake. As part of their end-of-project report, students were asked to contemplate basic considerations of scale-up, such as laboratory-scale and industrial-scale heat transport (e.g., during the oxidation they monitored the reaction temperature to check for exotherms). The opportunity for students to have input into their experimental work, coupled with this being an authentic example, was designed to develop enthusiasm.^10,11 This project was always a popular choice (typically, more students than we could accommodate ranked it as their top choice), and it received very good student feedback. The project also gave students the opportunity to develop a range of valuable interpersonal skills, most notably team-working, which is important for employability in, for example, industrial chemistry.¹²

Going Green: Experimental Overview and Pedagogical Goals

In 2015, we decided to challenge students further by focusing the project on the development of a greener, more sustainable synthesis of esomeprazole, by evaluating variations in the oxidation reaction according to the 12 principles of green chemistry. This provided a practical application of lecture course material, taught from year 2 onward, on the practice of clean and sustainable production by faculty in our Green Chemistry Centre of Excellence. The pedagogic goal of greening the reaction was to increase student engagement in the design of experiments and analysis of the outcome. Key learning objectives were:

• Knowing how modifying a reaction affects its green chemistry metrics

• Applying knowledge of green chemistry to a new situation

Training students in the critical assessment of an authentic chemical process is a pedagogic goal that has been investigated by others using different experiments,¹³⁻¹⁷ thereby helping students develop key employability skills.

The project was introduced by a presentation from an AstraZeneca process chemist. The presentation showed key aspects of large-scale synthesis, from safety considerations to economics and separation technologies. Key green engineering areas were also highlighted, including solvent selection and mass and energy transfer. A detailed handout was given to students that introduced the background to the project and provided experimental procedures, together with hazard data (Supporting Information). This included a literature procedure⁹ for the oxidation of pyrmetazole into esomeprazole. Students started their experimental work by investigating this reaction, which we termed the “medicinal group” reaction. They then investigated variations, of their choice, to the medicinal group procedure (on a 0.25 g scale) with the aim of making the process greener. To investigate the effectiveness of the variations, the yield of esomeprazole was monitored using ¹H NMR spectroscopy. The enantioselectivity was determined using chiral HPLC (Figure 1). In some reactions, overoxidation of the sulfoxide to the sulfone (−CH₂SO₂−) was observed.

Figure 1.

Separation of a crude oxidation reaction using HPLC (Chiralpak AD column). Peaks from left to right: pyrmetazole (sulfide), R-sulfoxide, sulfone, and S-sulfoxide or esomeprazole.

The oxidation of pyrmetazole into esomeprazole involves two distinct stages, and there are a variety of experimental changes that can be probed (Figure 2). This includes changing the equivalents of reagents, the concentration of pyrmetazole in the organic solvent, the choice of base, the reaction times and temperatures for steps 1 and 2, and the solvent. Indicative variations to the experimental procedure were given to students (many more variations were given than the time of the project allowed). Students were encouraged to look at the chemical literature to help inform their decisions. For example, this typically involved using green solvent selection guides.^18,19 Students could investigate changes outside of the indicative variations (e.g., an alternative oxidant) provided they had good reasons for doing this, including sound literature precedent (perhaps based on mechanistic considerations²⁰), and the reagents were commercially available. Any new variations were easily monitored and checked on health and safety grounds and cost, because students could only carry out an experiment when faculty had signed and approved the risk assessment. Students were also required to do regular chemical stock-takes to ensure that reagents and solvents were always available.

Figure 2.

Medicinal group two-stage oxidation procedure and indicative variations.

Over the 9 weeks, students typically met on a weekly basis to review their results and plan future experiments. This included repeating some of the most promising variations to check that the results were reproducible. Toward the end of the project, if the team identified conditions for an efficient, enantioselective and reproducible synthesis, they were given the opportunity to scale up their optimized oxidation conditions from 0.25 g to up to a 10 g scale, purify their crude esomeprazole product by conversion into esomeprazole sodium (which is a crystalline solid with a known optical rotation value), or both.

At the end of the project, each student was required to write an individually prepared report (maximum of 4 pages in length) together with a group poster (one poster per three to four students), both of which contributed to the summative assessment. As part of their report, students were required to calculate the cost of the reagents and solvents for their optimized process on a 1 kg scale. Finally, there was a nonassessed review meeting, which included the AstraZeneca process chemist and gave the students the opportunity to present their posters and discuss their results.

Hazards and Safety Precautions

All of the experiments were conducted in an efficient fumehood with students wearing safety goggles and appropriate protective clothing, including gloves. Thionyl chloride reacts violently with water and should be handled carefully. Cumene hydroperoxide (80%) is a powerful oxidizing agent and should be handled carefully; it is harmful if swallowed and can cause serious eye and skin irritation. Chloroform-d has highly toxic fumes and is a known or suspected carcinogen and teratogen. Dimethyl sulfoxide-d₆ may be harmful if inhaled or absorbed through skin. Esomeprazole sodium may cause skin irritation and is harmful to aquatic organisms. The physical and hazard data for the starting materials, reagents, and solvents outlined in the indicative variations are given in the Supporting Information. All chemicals should be disposed of properly in appropriate waste containers.

Results and Pedagogical Outcomes

At the end of the project, anonymous student responses to paper-based survey questions, using a 4 point Likert scale, were completed to assess the impact of changing to a sustainable synthesis theme. The feedback from 2015 from the first cohort to explore the greener synthesis of esomeprazole was particularly positive (Table 1). The survey included an open comments section where many students commented positively on the project, with comments such as, “overall really enjoyed the project”, “a very interesting and useful project, especially with respect to process chemistry, which I had not experienced”, and “it was a valuable experience into developing alternative syntheses in the laboratory”. Some students noted how useful the project was for those interested in a career in industry, and others commented positively on the opportunity to further develop their team-working skills.

Table 1. Comparative Student Questionnaire Feedback.

Statements for Response	Average 2014, N = 11	Scoresa 2015, N = 10
Overall, I enjoyed doing the project.	3.0b	3.4d
I found the project improved my practical skills.	3.1b	3.9d
I found the project improved my team-working skills.	3.8b	3.8d
I found that the project improved my time-management skills.	3.1c	3.2e
The handout was clear, easy to follow and contained all of the information I needed.	3.6b	3.7d

^aScores out of 4, where 4 is strongly agree, 3 is agree, 2 is disagree, and 1 is strongly disagree.

^bAll 11 students indicated “strongly agree” or “agree”.

^cTen of 11 students indicated “strongly agree” or “agree”.

^dAll 10 students indicated “strongly agree” or “agree”.

^eNine of 10 students indicated “strongly agree” or “agree”.

From 2015 to 2018, we continued to deliver the project. Each year, students were able to make pyrmetazole in excellent purity (as evidenced by the ¹H NMR spectra) and in sufficient quantities, and they were able to successfully oxidize this to form esomeprazole. The yields and enantioselectivity of the esomeprazole obtained were heavily influenced by subtle changes in the reaction conditions (a representative set of results is included in the Supporting Information).

The choices made by students were strongly influenced by the chemical literature. For example, invariably, some of the first experiments involved changing the base, which is known to have a big impact on enantioselectivity.⁹ Students consistently identified solvent selection as a crucial factor in driving a more sustainable process: they investigated a wide range of solvents to see if they could provide the desired solubility and separability without the undesirable chemical properties that cause environmental, health, and safety issues. This included the use of greener solvents, including trifluorotoluene and propylene carbonate. The latter, widely used as a green alternative to traditional aprotic solvents,^21,22 proved a “recognizably green” choice, with students drawing on existing knowledge to identify it as a favored starting point. Unfortunately, because of problems during workup, this solvent was shown to be unsuitable, reinforcing to the students the challenges involved in balancing greener synthesis with reaction efficiency. Typically, students achieved good yields and enantioselectivities using biomass-derived 2-methyltetrahydrofuran as the solvent. Coupled with this was the desire to carry out the oxidation at or close to room temperature and in the shortest possible reaction time. Reaction concentration was also identified as a key variable, with a reduction in solvent waste being seen as an effective way to reduce the environmental (E) factor. It was very interesting to see (through informal meetings with students throughout the period of the project) how the teams came to grips with making decisions on whether to prioritize variations that gave the highest yield or enantioselectivity of esomeprazole and whether to compromise this to some extent with a process that had greener credentials. Linked to this was the ability of students to correctly identify which of their reaction variants was the greenest synthesis.²³

Student engagement was excellent. This was monitored through attendance in laboratories, contributions in periodic informal faculty–student update meetings, and analysis of the quantity and quality of results achieved. Each year, the student teams investigated a minimum of 20 different oxidation reactions, which allowed a clear understanding of the key factors that affect the yield and enantioselectivity. Through repetition and systematic refining, the yields of esomeprazole improved, and the experimental results were satisfying. Toward the end of the project, each team achieved reproducible yields of esomeprazole of over 70%, in an ee of at least 70%. (Students were not given data from previous years, and there was no evidence of their obtaining the data.) They were also fully engaged in the final review meeting with the AstraZeneca chemist, where they discussed their posters, which allowed them to focus on aspects of personal interest and reflect on their learning. Students invariably recognized that there is no single correct green solution to this synthesis but that the various options need to be evaluated according to certain criteria.

Generally, the individual reports showed significant student learning. Two members of the faculty compiled grade data and provided written feedback for students on different components of the project using a common rubric (Supporting Information). For example, student team members were given the same grade for “volume and quality of results” (based on laboratory work), and individual grades for “quality of presentation” (based on individual reports). A key learning objective involved understanding the greening of the oxidation reaction, which was assessed through the scientific content of the individual reports (Table 2). The marks reflect that most students included accurate discussions of the green credentials of their optimized syntheses. They successfully applied green metrics (such as atom economy, process mass intensity, and E-factor) to a new context, typically with inclusion of literature references to help support their decision making. For example, they justified their decision to use 2-methyltetrahydrofuran as the reaction solvent.²⁴ The group posters further indicated that the majority were able to evaluate the use of different green metrics in developing a sustainable synthesis of esomeprazole.

Table 2. Distribution of Postlab Report Grades for Scientific Content.

	2015,aN = 12	2016,aN = 12	2017,bN = 9	2018,bN = 8
Mean Scores, %	70	71	66	65
SD	2.30	2.93	0.46	0.60

^aIn the 2015 and 2016 course offerings, the maximum score for the postlab report was 30 points.

^bIn the 2017 and 2018 course offerings, the maximum score for the postlab report was 10 points. (This change was enforced across all year 3 mini-projects.).

Conclusions

This green chemistry project gives students the opportunity to prepare a blockbuster medicine in an authentic context. It has motivated and enthused students and is further enhanced through collaboration with AstraZeneca chemists.²⁵ The project has helped develop student appreciation for reaction optimization and assessing the “greenness” of chemical reactions of importance to modern organic synthesis, including scale-up chemistry. Informed by the chemical literature, it mirrors the type of planning integral to research, in which primary and secondary literature is used to establish experimental procedures. The project has allowed students to make environmentally conscious decisions on what experiments they would like to carry out. This includes using the environmental impact to make decisions on the choice of solvent and then, through laboratory testing, determining how this affects the efficiency and selectivity of an asymmetric oxidation reaction. It reinforces topics such as green chemistry, organic synthesis, and analytical chemistry, and a successful project requires students to demonstrate good communication and team-working skills, which align with the needs of employers. This type of experiment helps enable students to participate in productive science work, including work involving environmental impact.

Supporting Information Available

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.9b00350.

• Course handout (PDF, DOCX)

• Table of results obtained by students from some representative variations of the oxidation reaction, HPLC conditions, and CAS numbers for all chemicals (PDF, DOCX)

• Grading rubric (PDF, DOC)

• ¹H NMR spectra of pyrmetazole and the crude product from a representative oxidation reaction (PDF)

The authors declare no competing financial interest.