Process Chemistry in Antiviral Research
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This article reviews antiviral therapies that have been approved for human use during the last decade, with a focus on the process chemistry that enabled access to these important drugs. In particular, process chemistry highlights from the practical syntheses of the HCV drugs sofosbuvir (Gilead), grazoprevir (Merck), and elbasvir (Merck), the HIV therapy darunavir (Tibotec) and the influenza treatment peramivir (BioCryst) are presented.
KeywordsProcess chemistry Antivirals HCV HIV Influenza Practical synthesis Asymmetric synthesis Sofosbuvir Grazoprevir Elbasvir Darunavir Peramivir
Viruses are the most abundant type of biological entity on earth and they can be found in almost every ecosystem on the planet . Although it is likely that there are millions of different types of viruses in existence, only around 5000 individual virus species have been fully characterized to date . Viruses are small infectious agents that replicate themselves by hijacking the normal molecular processes that occur inside the cells of other organisms. Viruses can infect all types of life forms, including animals, plants, and microorganisms such as bacteria and archaea . Many viruses can be seriously harmful to human health, including the human immunodeficiency virus (HIV), the hepatitis B and C viruses (HBV and HCV), the influenza virus, and the human papillomavirus (HPV).
Within the pharmaceutical industry, synthetic organic chemistry encompasses both medicinal chemistry and process chemistry activities. Medicinal chemists focus on the discovery of appropriate small molecules that interact with biological mechanisms in such a way as to provide a beneficial effect on a given disease state. Process chemists aim to design and develop efficient, practical, and economical chemical syntheses for the drug candidates identified by medicinal chemists. The differing goals of medicinal and process chemists have a significant bearing on the nature of the synthetic chemistry strategies employed by either group of chemists. Broadly, medicinal chemists typically design flexible synthetic routes that quickly access common building blocks from which many candidate compounds can be readily prepared, allowing structure–activity relationships to be determined rapidly and the appropriate molecular entity to be identified. Reaction yield, expense, robustness, and green chemistry considerations are of secondary importance relative to speed of compound synthesis and acquisition of data from biological assays. Conversely, the industrial manufacture of drug compounds at commercial scale puts a premium on those synthesis attributes relegated to lesser significance by medicinal chemists. Consequently, process chemists seek to devise an ideal route that has maximum synthetic efficiency and is high yielding. Often this necessitates the invention of new synthetic methods, providing an excellent venue for creative chemists to conceive novel applications of existing transformations or even to develop entirely new reactions. The commercial process must be operationally safe, cost-effective, and sufficiently robust to deliver API in consistent high quality. Intellectual property (IP) aspects must also be considered when designing and developing the synthesis to assure freedom to operate without infringing upon competitor patents . Lastly, given the larger scale of operation relative to medicinal chemistry activities, minimizing the environmental impact of industrial drug production is a priority and drives the application of green-chemistry principles wherever possible.
In addition to the myriad of technical objectives faced by process chemists, attention must also be given to broader, cross-functional facets of development programs including program drivers and timelines. Typically, in the early stages of pre-clinical and clinical drug development, relatively small quantities of drug substance are required to support initial in vitro and in vivo toxicology studies and the speed of API delivery is normally prioritized over definition of an ideal synthesis. This being the case, it is not unusual to have an interim synthesis (“supply route”) used to support early API deliveries with a parallel (or staggered) effort to develop increasingly refined routes that are aligned with the overall program context, considering factors such as drug demand, synthesis complexity/cost and clinical timelines. In an ideal world, the best, most efficient synthesis is established as early as possible in the overall timeline but in practice it is common to observe an evolution in synthetic efficiency over the development cycle, culminating in the manufacturing route being ready as the program moves closer to regulatory filing (often in phase III) [97, 98]. The examples of drug development featured in the subsequent section were selected to relate various aspects of process chemistry alluded to in this introduction. In each case presented, the intent is to focus on key challenges and highlights encountered during process development. The examples presented in this chapter may not be the actual commercial routes, however they reflect the information that is currently available in the literature. The reader is directed to the original publications for further details on the complete syntheses if desired.
2 Medicinal Therapies for the Treatment of HCV
The management of HCV continues to present a global health problem. According to a report by the World Health Organization (WHO), it is estimated that 130–170 million people have chronic HCV infection worldwide. Approximately half a million deaths each year result from HCV-related diseases [99, 100]. For these reasons, the search for effective antiviral agents to combat HCV is an ongoing endeavor within the global medical/pharmaceutical community [101, 102].
2.1 Chemical Synthesis and Process Development of Sofosbuvir: An HCV NS5B Nucleoside Polymerase Inhibitor
Sofosbuvir is a treatment for HCV marketed by Gilead Sciences under the trade name Sovaldi® (Fig. 1). Initially developed at Pharmasset, following acquisition by Gilead in 2011 for approximately $11 billion, further clinical development of sofosbuvir resulted in FDA approval of this drug in December 2013. The mechanism of action of sofosbuvir is via inhibition of the non-structural 5B (NS5B) nucleoside polymerase enzyme [103, 104]. Treatment duration for a course of Sovaldi® is 12 weeks and the cure rate is 90%. Gilead Sciences sold $10.3 billion of Sovaldi® in 2014, making it the second best-selling drug in the world in its first year on the market [105, 106]. The following sections present the medicinal chemistry route to sofosbuvir and highlights key synthetic advances made during process development.
2.1.2 Medicinal Chemistry Route
The chlorophosphoramidate 9 was prepared by the reaction of isopropyl l-alanate hydrogen chloride with phenyl phosphoro-dichloridate in the presence of N-methylimidazole (NMI). With the two key fragments uridine nucleoside 8 and chlorophosphoramidate 9 in hand, the combination of these in the presence of NMI gave a 1:1 mixture of diastereomers (at the phosphorus center) epi-sofosbuvir and sofosbuvir. The desired sofosbuvir was isolated in high purity after crystallization and recrystallization. Thus, the medicinal route for the synthesis of sofosbuvir was carried out in ten linear chemical steps from cytidine in 0.5% overall yield.
2.1.3 Process Development Routes
220.127.116.11 Retrosynthetic Analysis
18.104.22.168 Synthetic Approaches to Lactone 11
22.214.171.124 Elaboration of Lactone 11 to Nucleoside 29
126.96.36.199 Attachment of the Phosphoramidate Pro-drug Functionality
Overall, the process chemistry route to sofosbuvir successfully addressed two of the major issues identified in the medicinal chemistry route via the implementation of improved approaches for the introduction of fluorine and control of the phosphoramidate stereochemistry. By virtue of these improvements, it is possible to synthesize sofosbuvir (without recourse to chromatography) in approximately 5% overall yield, representing an approximate tenfold improvement from 0.5% yield in the discovery route. As described previously, this is a summary of the work published by the Process Chemistry groups of Pharmasset and Gilead to date. It is unclear which aspects of this work comprise the contemporary manufacturing route for sofosbuvir, but it is anticipated that further improvements have been achieved and may be communicated in due course.
2.2 Chemical Synthesis and Process Development of Zepatier: A Combination of HCV NS3/4A (Non-structural Protein 3/4A) (Grazoprevir) and HCV NS5A (Non-structural Protein 5A) (Elbasvir) Inhibitors
Zepatier® is a two-drug combination therapy for the treatment of HCV that was developed at Merck and approved by the FDA in early 2016. Zepatier® is a combination of a HCV NS3/4A protease inhibitor (grazoprevir) and a HCV NS5A protein inhibitor (elbasvir) (Fig. 1). Similar to Sovaldi®, Zepatier® is intended to be prescribed orally and once-daily for 12 weeks. This provides a significant quality-of-life benefit for patients with respect to existing treatment regimens that typically require co-treatment with a small molecule (ribavarin) and an injectable biological agent (interferon); unpleasant flu-like symptoms are typically associated with this mode of therapy, often causing a failure in patient adherence to the treatment plan. Additionally, the ribavarin/interferon combination achieves only moderate cure rates (in the range 40–80%) depending, among other factors, on the genotype of HCV being treated. Notably, Zepatier has achieved a 92% cure rate in clinical trials without reports of the side effects typically reported for the ribavarin/interferon combination [116, 117]. The sections below outline the major synthetic challenges associated with both grazoprevir and elbasvir and how process chemistry innovation enabled development and commercialization of these critical new drugs.
2.2.2 Development of Grazoprevir
188.8.131.52 Medicinal Chemistry Route to Grazoprevir
The synthesis of key intermediate 43 started with the condensation of 4-methoxy-phenylenediamine hydrochloride with diethyl oxalate in the presence of triethylamine, giving 6-methoxyquinoxaline-2,3-diol 39 in 69% yield (Scheme 10). Regioselective chlorination of the diol 39 with thionyl chloride in DMF afforded 3-chloro-7-methoxyquinoxalin-2-ol 40. S N 2 displacement of bromobenzenesulfonate 41, which was prepared in one step from N-Boc-4-cis-hydroxyproline methyl ester with 40 in the presence of Cs2CO3, followed by a chromatographic purification led to product 42 in 70% overall yield from 39. N-Boc-deprotection with 4 M hydrogen chloride in dioxane provided the target intermediate 43 in 99% yield.
184.108.40.206 Process Chemistry Route
Retrosynthetic Analysis As is often the case in pharmaceutical development, the medicinal chemistry route described above was determined inadequate to prepare the typical quantities of API needed for clinical evaluation of grazoprevir. Major issues for process development included the low yield for the key macrocycle-forming RCM reaction, together with uncertainty around freedom to operate using RCM catalysts for commercial production due to the extensive patent coverage in the methathesis space. Hydrolytic instability of TMS enol ether 34 and overall low efficiency/high cost of the synthesis were additional concerns in the context of increasing scale of operation. Although the size and molecular complexity of grazoprevir provides enormous opportunity for synthesis exploration, it was necessary to balance the use of interim chemistry against the search for the ultimate route in order to expedite clinical development. In this regard, the early process routes to grazoprevir had a focus not only on better chemistry but also on speed of access to clinically useful amounts of API. This approach resulted in several synthetic route variations that evolved over time as the compound advanced through clinical development. In this section, the chemistry used for a first-generation interim “supply route” will be discussed. Approaches to address the significant issues associated with the medicinal chemistry synthesis will be highlighted.
Final Stages for the Synthesis of Grazoprevir via the Synthesis of Chloroquinoxaline 42 Chloroquinoxaline 42 is prepared by a S N Ar displacement of dichloroquinoxaline (DCQ) with inexpensive Boc-HP. The reaction proceeds with 95:5 regioselectivity and a 71% isolated yield is observed after crystallization to reject the undesired regioisomer. The Sonogashira cross-coupling between chloroquinoxaline 42 and alkyne acid 49 is achieved with 3 mol% of Pd(OAc)2, 6 mol% of P(t-Bu)3BF4, and 2.5 equiv of K2CO3 (2.5 equiv) to afford product 55 in 98% HPLC assay (solution) yield. Intermediate 55 is not isolated but instead is processed forward into the downstream chemistry, which involves Pd-catalyzed hydrogenation of the triple bond of 55 was conducted with a catalytic amount of Pd/C to provide the saturated product 56 in 89% HPLC assay yield. The N-Boc group was removed by treatment of crude 56 with PhSO3H and the product benzene sulfonate salt was neutralized in situ by addition of Hünig’s base. The resulting mixture was slowly added to a solution of HATU (1.25 equiv) in MeCN to complete a one-pot process that gave a 65% isolated yield of marcolactam 57 in >98% purity after crystallization. Through this slow addition for the cyclization, the macrolactamization was allowed to run in ~0.2 M concentration, which is 10× more efficient than the RCM reaction (0.02 M). Lithium hydroxide promoted saponification of 57 leads to the corresponding acid 47 in 95% yield, which is converted to grazoprevir in 92% yield via amide coupling with commercially available 48 in the presence of EDC, Hünig’s base and catalytic amount of HOBt [120, also see footnote 4].
2.2.3 Development of Elbasvir
220.127.116.11 Medicinal Chemistry Route
18.104.22.168 Process Chemistry Route
Access to the 2-phenol-indole substrate 59 is relatively straightforward so the asymmetric synthesis of benzoxazinoindole 64 starting from this potential precursor was explored extensively. Initial work focused on the PTC approach under basic reaction conditions. As shown in Scheme 18, although this bond formation was viable in a racemic sense for the discovery route, attempts to render this process asymmetric using PTC were not successful. Switching tactics, the direct condensation of 2-phenol-indole 59 with benzaldehyde in presence of chiral Lewis acid or chiral Brønsted acid was also explored but this gave only the indole C-3 cyclization product due to the intrinsic higher reactivity of the indole C-3 position over the indole nitrogen (for chiral hemiaminal synthesis using chiral phosphoric acid catalysts, see [123, 124]. Other examples [125, 126, 127]).
Completion of the elbasvir first-generation process synthesis from this point onwards is similar to the discovery synthesis. The side-chains 63 are installed in stepwise fashion via a bis-borylation/bis-Suzuki–Miyaura cross-coupling sequence of 64 with Boc-protected bromoimidazole 61, followed by N-Boc deprotection final amide coupling with N-methoxycarbonyl-l-valine. During early development, the understanding of factors which influence reaction byproducts was incomplete and only weakly crystalline forms of the API were known. These deficiencies necessitated purity control at several crystalline intermediates prior to API isolation, requiring the stepwise approach to attaching the side chains 63. These issues were ultimately overcome as knowledge of the end-game chemistry improved and extensive work around the API crystal form identified more robust options for isolation of elbasvir. This understanding was put to good use in development of the second-generation synthesis described below. The first-generation process chemistry route delivers the API in 25% overall yield over an 11-step sequence. This is a significant improvement over the discovery synthesis and is largely due to the definition of a practical asymmetric approach to the benzoxazinoindole core.
Second-Generation Process Chemistry Route to Elbasvir In the first-generation process route described above, the key chiral center is established via a 1,3-stereochemical relay from the stereocenter already present in indoline intermediate 66. Although this approach represented a significant step forward from the discovery route, the process chemistry team sought to push the envelope and seek an even more efficient synthesis of elbasvir. It was recognized that two steps in the overall sequence were potentially redundant, namely the imine reduction and downstream indoline (re)-oxidation to the indole. To eliminate this redox chemistry it was desirable to identify an alternate means of controlling the benzoxazinoindole stereocenter that would maintain the appropriate oxidation state throughout the synthesis. To this end, the project team devised an innovative approach based on a novel dynamic kinetic resolution.
As alluded to in the discussion of the first-generation chemistry, completion of the elbasvir synthesis involves attachment of the side-chain(s) 63. The most convergent approach would couple the fully elaborated 63 directly to benzoxazinoindole 74 however complex reaction purity profiles together with limited availability of suitable crystalline phases of the API in early development hampered execution in this regard due to inadequate control of final drug substance purity. Consequently, in the first-generation synthesis it was necessary to use a stepwise approach that offered several crystalline intermediates for control of purity. With a better understanding of the cross-coupling chemistry and API crystallinity in the late development phase this challenge was overcome. Bis-borylation of 74 catalyzed by Pd(OAc)2/XPhos generated bis-boronate and double-Suzuki–Miyaura coupling with fully elaborated side chain 63 directly affords elbasvir in 82% isolated yield. Thus, the second-generation synthesis of elbasvir provides a 42% overall yield over six synthetic steps—a remarkably concise construction of this complex molecule .
To summarize the development of elbasvir, the variety of different routes described here represents an excellent example of how understanding of the chemistry around the synthetic target matures over time with continued study and leads to increasingly efficient access to the molecule of interest. In this case the chemistry evolved through three different versions of the synthetic route: a racemic discovery synthesis that required preparative chromatographic separation to provide the desired stereoisomer, a practical asymmetric stereochemical relay approach that supported clinical development and launch of elbasvir, and a final iteration based on a novel DKR process that represents the most ideal synthesis of this complex molecule identified to-date. The overall yield across these three syntheses increased from 4% to 25% to 42%, respectively, and the total number of steps was reduced from 11 to nine and finally to six steps in the DKR route.
3 Synthesis of a HIV Protease Inhibitor (Darunavir)
The retrovirus designated as human immunodeficiency virus (HIV) is etiologically linked to the immunosuppressive disease known as acquired immunodeficiency syndrome (AIDS) [131, 132, 133]. The HIV virus gradually attacks the immune system and destroys a type of white blood cell known as a T-helper cell, all the while replicating itself in the host cells. It may take around 10–15 years for AIDS to develop in the body if left untreated. The enzymes known as reverse transcriptase (RT), integrase (IN) and protease (PR) are each involved in key steps of the virus replication cycle. Small molecules that inhibit these enzymes can serve as antiviral agents and these are often taken together as “combination therapies”. The first effective medicine to become available for the treatment of HIV was zidovudine (a.k.a “AZT”), which was FDA approved in 1987. Since then we have witnessed tremendous gains in the control of HIV with the development of many different drugs, including the example discussed in this section (darunavir), which is in the protease inhibitor category. Despite these advances, in 2014 it was estimated by the World Health Organization (WHO) and the United Nations Programme on HIV and AIDS (UNAIDS) that 36.9 million people worldwide are living with HIV, of which 1.2 million unfortunately died of HIV-related causes. In view of these sobering facts, the case for continued research to discover new, even more effective therapies is clear, as is the need for elegant process chemistry solutions to the synthetic problems posed by these novel drug compounds.
3.1 Chemical Synthesis and Process Development of HIV Protease Inhibitor (Darunavir)
Darunavir belongs to the class of HIV therapies known as protease inhibitors, which were first introduced in the 1990s. Improved understanding of the C-2 symmetrical active site of the protease enzyme made it possible to rationally design appropriate drug structures and the search for new compounds continues today. Consequently, darunavir is a second-generation protease inhibitor, intended to overcome problems with the older agents in this class. Early protease inhibitors often had severe side effects and drug toxicities, requiring a high therapeutic dose and exhibiting a disturbing susceptibility to drug resistant mutations. Such mutations can develop in as little as 1 year of use, and effectively render the drugs useless. Darunavir (trade name as Prezista®) was discovered and developed by Tibotec and is one of most recently FDA approved protease inhibitors (June 2006). Darunavir was designed to form robust interactions with the protease enzyme from many strains of HIV, including strains from treatment-experienced patients displaying multiple resistance mutations to previously available protease inhibitors. Darunavir is on the World Health Organization’s List of Essential Medicines, which defines the most important medications needed in a basic health system.
3.1.2 Medicinal Chemistry Route
3.1.3 Process Chemistry Route
22.214.171.124 Retrosynthetic Analysis
126.96.36.199 Synthesis of Intermediate 83
(R)-2,2-dimethyl-1,3-dioxolane-4-carbaldehyde is condensed with dimethyl malonate in the presence of 0.5 equiv of pyridine and 3 equiv of acetic anhydride to yield the condensation product 88. Nitromethane is added to 88 in MeOH with 0.1 equiv of DBU at 0 °C, giving the conjugate addition adduct 89 with high diastereoselectivity (9:3 ratio). A Nef reaction converts the nitro-group into a carbonyl and the resulting aldehyde 90 is treated in situ with acidic methanol to simultaneously achieve acetonide removal/double cyclization to bicyclic lactone 91. Aldehyde intermediate 90 is stereochemically labile (via facile epimerization at the α-position) and any epimer generated results in formation of di-ester 93. This decreases the overall yield of the transformation but does not impact the final purity of 91 because trans-compound 93 cannot readily lactonize and is easily rejected to the aqueous layer during workup. Intermediate 91 is subject to basic solvolysis followed by acidification to obtain crystalline lactone acetal 92 as a single isomer. Lactone 92 is reduced to a lactol with LiBH4 and equilibration under acidic conditions provides the key intermediate 83 as the thermodynamic product in 77–80% isolated yield. The overall yield of this four-step synthesis is 14% (demonstrated at multi-kilogram scale), which compares very favorably with the discovery synthesis yield of 20%. This route was reported in a Tibotec patent application starting from >200 kg of the aldehyde .
Using the same glyceraldehyde chiral pool raw material, a Horner–Wadsworth–Emmons olefination provides the (Z)-alkene 94 with excellent selectivity over the (E)-isomer. Similar to the first-generation route, a DBU-promoted conjugate addition of nitromethane into 94 followed by a Nef reaction affords 96, which does not require decarboxylation before conversion to common intermediate 97. Again, during extractive work-up for lactone 97, the trans isomer 98 is easily rejected to the aqueous layer because it cannot lactonize. Acetal 97 is obtained as a mixture of diastereomers and equilibration under acidic conditions funnels the material to the most thermodynamically stable β isomer 92. Taken as a whole, these modifications deliver significant gains in synthetic efficiency, increasing the overall yield to key intermediate 83 from 14 to 33%.
188.8.131.52 Final Stages for the Synthesis of Darunavir
To summarize the chemical development of darunavir, the overall route is highly convergent and utilizes two key building blocks 79 and 80. The approach to fragment 79 leverages the synthesis previously described in another drug (amprenavir). Therefore, the key challenge for the darunavir process chemistry team was the identification and development of an asymmetric approach to the bis-THF fragment 83. This was achieved via utilization of glyderaldehyde acetonide as a chiral pool raw material. As a result of improved access to intermediate 83 (since the bis-THP intermediate 83 is a privileged pharmacophore, Gilead and GSK independently reported Lewis acid-catalyzed synthesis of 83. see: [141, 142]), the overall yield of darunavir was vastly improved, ensuring a robust supply of this important antiviral drug to patients worldwide.
4 Synthesis of a Compound for the Treatment of Influenza (Peramivir)
Influenza is a RNA virus that has long history of causing health problems for the human race. Influenza is highly contagious and can present serious concerns for those with weakened immune systems (very young children, the elderly and people with certain medical conditions are particularly vulnerable). Of the three known types of influenza virus (A, B and C), type A is the most commonly encountered and was the cause of virulent human influenza pandemics such as Spanish flu outbreak in 1918, the Hong-Kong flu outbreak in 1968, through to more recent outbreaks of Bird flu and Swine flu in 2004 and 2009 respectively. Collectively, these pandemics resulted in more than a million deaths worldwide [143, 144].
Until relatively recently, options for treating the influenza virus were very limited, including vaccinations and a small molecule therapy called amantadine. In the United States, the use of amantadine as a flu therapy was discontinued after a study by the Centers for Disease Control and Prevention (CDC) indicated 100% of 2008/2009 flu virus samples were fully resistant to this treatment. The current state of the art treatment option for influenza is a class of small molecules which block the function of neuraminidase enzymes and prevent viral reproduction. Examples of these neuraminidase inhibitors include oseltamivir (Tamiflu®), zanamivir (Relenza®) and also peramivir (Rapivab™), which is the subject of the process development discussion below.
4.1 Synthesis of a Compound for the Treatment of Influenza (Peramivir)
Peramivir was discovered and developed by BioCryst Pharmaceutical as a potent neuraminidase inhibitor for the treatment of influenza virus [145, 146]. BioCryst collaborated with Ortho-McNeil (a subsidiary of Johnson & Johnson) in 1998, but ultimately partnered with both Green Cross Pharmaceuticals in 2006 and Shionogi Pharmaceutical in 2007 to co-develop Peramivir, which is currently approved in Japan (2010), South Korea (2010), China (2013) and United States (2014).
4.1.2 Medicinal Chemistry Route
Starting from commercially available bicyclic lactam 99, solvolysis in acidic MeOH resulted in conversion of the lactam to the ring-opened methyl ester. The amino group was subsequently protected as the Boc carbamate to form intermediate 100. Next, the key [2 + 3] cycloaddition reaction of 100 with nitrile oxide 101 in benzene delivered the cycloadduct as a mixture of four isomers (102a–d). The desired isomer 102a was isolated as a crystalline solid in 61% yield after chromatographic separation. The isoxazoline ring in 102a was stereoselectively cleaved from the less hindered β face under hydrogenation conditions in the presence of PtO2/HCl. The resulting amine was acetylated to provide 103 in 67% isolated yield after chromatography. After N-Boc deprotection, introduction of the guanidine group was performed using 1,3-bis(tert-butoxycarbonyl)-2-methyl-2-thiopseudourea in the presence of mercury(II) chloride. Methyl ester in 106 was solvolyzed with NaOH and the corresponding acid 107 was isolated in 90% yield as a solid after acidification. Finally, the guanidine N-Boc group was removed by TFA in methylene chloride to provide peramivir as TFA salt.
4.1.3 Process Chemistry Route
184.108.40.206 Retrosynthetic Analysis
From a synthetic strategy perspective, the approach used in the discovery synthesis is relatively efficient and therefore the process chemistry approach retained the [3 + 2] cycloaddition as a key bond formation. However, several characteristics of the discovery route were unsatisfactory with respect to commercial manufacture and these were targets for refinement/optimization in the process route [147, 148]. Specifically, the use of phenyl isocyanate with a nitroalkane to generate the key nitrile oxide intermediate combined with the use of benzene (a genotoxic carcinogen) as solvent was not acceptable. Additional problematic aspects were the need for chromatographic separation of isomeric products after the [3 + 2] cyclization, the cost associated with PtO2 for reduction of the isoxazoline ring and the use of highly toxic mercury(II) chloride for the installation of guanidine group.
220.127.116.11 Process Improvement for the [3 + 2] Cycloaddition and Isolation
18.104.22.168 Process Improvement for the Reduction of Isoxazoline
22.214.171.124 Process Improvement for the Final Stages via Mercury-Free Guanidine Formation
The peramivir case study provides an example of process development where the retrosynthetic analysis and bond disconnections from the original medicinal chemistry synthesis were retained and modifications to the reagents/procedures were made to render the process more practical. The process route is chromatography-free, eliminates benzene/mercury reagents and delivers peramivir in 21% overall yield across a 10 linear-step sequences.
Process chemistry plays a pivotal role in the development and manufacture of drugs. The five antiviral process chemistry case studies presented in this chapter were chosen to reflect the varying approaches to synthetic development taken across the pharmaceutical industry. As alluded to in the introduction, cross-functional demands (e.g., expediting clinical supply) often intertwine with the broad goal of designing and developing ideal syntheses of APIs, sometimes influencing the direction of process development. It is clear however, that the search for the best synthetic routes to drug targets provides an excellent venue for scientific discovery and allows expansion of the current knowledge of organic chemistry as a whole. The development of elegant and practical routes to complex pharmaceutical targets sometimes necessitates the discovery of fundamentally new chemistry, which often has broader applications. Consequently, a mindset of continuous innovation is extremely valuable within process chemistry and paves the way for discovery and development of important new medicines for humankind.
The situation may be actually slightly more complex because the phosphoramidate hydrolyzes to leave the 5′-phosphate, which avoids the need for biological mono-phosphorylation of the 5′-alcohol, on the pathway towards triphosphate formation.
Employed a similar processes developed by Gilead Sciences, lactone 11-OBn was converted to 29 in 25% overall yield.
The chlorides in the chlorophosphates 30a and 30b are replaced by pentafluorophenol due to the instability of the chlorophosphate towards isolation/upgrade.
Intermediate 48 can be sourced.
Alternate, stereoselective approaches were ultimately developed by Merck Research Laboratories and will be the subject of future publications.
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