**6. Antimalarial drugs**

Malaria, existing in over 100 countries, is one of the deadliest infectious diseases and major health problem worldwide. Antimalarial drugs are designed to cure malaria, many of which are in market.[28] From the 17th century onward quinine had been the drug of choice for the treatment of malaria. Later on, medication therapies heavily relied on chloroquine, prima‐ quine, mefloquine, etc.<sup>4</sup> These drugs especially chloroquine have saved more lives than any other drugs in history. Recently, artemisinin and its derivatives have emerged as a new generation of antimalarials (Figure 2).

There is a critical need to develop newer synthetic and more effective drugs that could address the issues associated with the existing and traditional drug therapies. The availability of artemisinin also causes supply constraints because artemisinin and its derivatives constitute an active ingredient of many combination therapy dugs. For example, Coartem contains a fixed combination of artemether and lumefantrine. In 2012, Ranbaxy also launched a new synthetic peroxide antimalarial drug SynriamTM in the market in line with the recommendations of the WHO. It is a fixed dose combination of arterolane maleate and piperaquine phosphate. The chemical structures are shown below.

Recent Advances in Antimalarial Drug Discovery — Challenges and Opportunities http://dx.doi.org/10.5772/61191 47

**Figure 2.** Antimalarial drugs.

**Parameters Remarks**

populations

**Immunogenicity** ⋅ DNA alone is poorly immunogenic

malarial antigen

in sexual stage

**6. Antimalarial drugs**

quine, mefloquine, etc.<sup>4</sup>

generation of antimalarials (Figure 2).

chemical structures are shown below.

**Table 2.** Lessons from two decades of malaria vaccine development research.

⋅ It is often lower in semi-immune populations living in endemic areas than in naïve

⋅ Reactogenicity in young children has not been worse than in adult populations

**Efficacy** ⋅ Only RTS,S-based vaccines proved to be effective to reduce morbidity in endemic areas

Malaria, existing in over 100 countries, is one of the deadliest infectious diseases and major health problem worldwide. Antimalarial drugs are designed to cure malaria, many of which are in market.[28] From the 17th century onward quinine had been the drug of choice for the treatment of malaria. Later on, medication therapies heavily relied on chloroquine, prima‐

other drugs in history. Recently, artemisinin and its derivatives have emerged as a new

There is a critical need to develop newer synthetic and more effective drugs that could address the issues associated with the existing and traditional drug therapies. The availability of artemisinin also causes supply constraints because artemisinin and its derivatives constitute an active ingredient of many combination therapy dugs. For example, Coartem contains a fixed combination of artemether and lumefantrine. In 2012, Ranbaxy also launched a new synthetic peroxide antimalarial drug SynriamTM in the market in line with the recommendations of the WHO. It is a fixed dose combination of arterolane maleate and piperaquine phosphate. The

poor efficacy against the population of circulating strains

**Methodology** ⋅ In vitro studies and animal studies does not correlate well

⋅ Little clinically significant interference is observed between vaccine antigens and the

⋅ For testing of new malaria vaccines, ethical and methodological issues may arise

⋅ There is a need to make formal trial design for phase trials and sample size calculations. Epidemiological studies are required to assess the effectiveness of mosquito antigen vaccines

These drugs especially chloroquine have saved more lives than any

⋅ Highly polymorphic blood-stage antigens have tended to lead to allele-specific efficacy, but

**Safety**

46 An Overview of Tropical Diseases

Research groups across the world are united in the efforts to discover new chemicals for the treatment of malaria. Attempts to modify the established drugs are also ongoing. Long-term hopes are resting on the modification of the synthetic artemisinin-based drugs containing endoperoxide rings. The following sections will mainly focus on the development of peroxidic antimalarial agents.

#### **6.1. Natural products**

Natural products continue to make an immense contribution to malaria chemotherapy. The discovery of quinine and artemisinin proves that nature is a rich source of lead compounds that can provide cure and medicine for malaria. Nature has been extremely generous when it comes to search of new molecular scaffolds for good malarial activity. These scaffolds later serve as template for the development of structurally diverse analogues with more potent activity.[29] For example, quinine a bitter-tasting alkaloid, is one of the earliest natural compounds that helped man in the fight against malaria. It was isolated from the *Cinchona* bark. Later, it also served as a template for the synthesis of more potent and structurally simpler analogues such as chloroquine, primaquine, mepacrine, and mefloquine (Figure 2). Artemisi‐ nin extracted from *Artemesia annua* is another example whose diverse pharmacological potential has attracted the researchers worldwide. Artemisinin also gave rise to the develop‐ ment of dihydroxyartemisinin, artemether, arteether and artesunate. Thus, natural products such as quinine and artemisinin have demonstrated the enormous potential of nature in providing lead compounds, which can be further manipulated structurally for the develop‐ ment of more effective antimalarial agents. Many more natural products possessing various chemical structures, such as alkaloids, steroids, chalcones, terpenes, flavonoids, peptides, quinones, xanthones, coumarines, naphthopyrones, polyketides, phenols, lignans, chromenes, etc., have been tested as antimalarial drugs.[30, 33]

#### **6.2. Semisynthetic drugs**

The success of the most potent antimalarial drugs, quinine and artemisinin, has brought some optimism. Due to the widespread emergence of drug-resistant chloroquine, primaquine, mepacrine, and mefloquine (Figure 2) were developed. Despite the remarkable antimalarial activity, artemisinin suffers from limited availability, low solubility, high cost, metabolic stability, short half-life, poor bioavailability, and chemical stability. Thus, there is a need for new compounds more active than the parent artemisinin. To circumvent some of these problems, semisynthetic analogs were prepared. The reduction of artemisinin yields dihy‐ droartemisinin, and the lactol group can be further converted to its ether (artemether, arteether, and artelinic acid) and ester (sodium artesunate) derivatives.[34]

#### **6.3. Synthetic drugs**

Artemisinin, a sesquiterpene endoperoxide, has established the role of peroxide ring for potential antimalarial activity. However, the naturally isolated artemisinin is available in short supply and expensive to synthesize. As a consequence, extensive research directed towards the discovery of peroxidic antimalarials inspired researchers to explore structurally simple peroxides. Trioxanes, tetraoxanes, and their hybrids were consequently identified as promis‐ ing candidates for the development of next generation antimalarial drugs.

#### *6.3.1. Various synthetic procedures for the synthesis of trioxanes*

Trioxanes can be synthesized from inexpensive starting materials, and their scale-up prepa‐ rations are feasible. Most methods reported for the synthesis of trioxanes starts with the reaction of singlet oxygen with carbonyls in the presence of Lewis acids. Then acid-catalyzed cyclization of hydroxyperoxides with olefins and reaction of α-peroxy aldehydes with carbonyl compound yields trioxanes in good yields. Many synthetic strategies were developed for the synthesis 1,2,4-trioxanes, which are described below.

#### *6.3.1.1. Photooxygenation method*

Research groups across the world are united in the efforts to discover new chemicals for the treatment of malaria. Attempts to modify the established drugs are also ongoing. Long-term hopes are resting on the modification of the synthetic artemisinin-based drugs containing endoperoxide rings. The following sections will mainly focus on the development of peroxidic

Natural products continue to make an immense contribution to malaria chemotherapy. The discovery of quinine and artemisinin proves that nature is a rich source of lead compounds that can provide cure and medicine for malaria. Nature has been extremely generous when it comes to search of new molecular scaffolds for good malarial activity. These scaffolds later serve as template for the development of structurally diverse analogues with more potent activity.[29] For example, quinine a bitter-tasting alkaloid, is one of the earliest natural compounds that helped man in the fight against malaria. It was isolated from the *Cinchona* bark. Later, it also served as a template for the synthesis of more potent and structurally simpler analogues such as chloroquine, primaquine, mepacrine, and mefloquine (Figure 2). Artemisi‐ nin extracted from *Artemesia annua* is another example whose diverse pharmacological potential has attracted the researchers worldwide. Artemisinin also gave rise to the develop‐ ment of dihydroxyartemisinin, artemether, arteether and artesunate. Thus, natural products such as quinine and artemisinin have demonstrated the enormous potential of nature in providing lead compounds, which can be further manipulated structurally for the develop‐ ment of more effective antimalarial agents. Many more natural products possessing various chemical structures, such as alkaloids, steroids, chalcones, terpenes, flavonoids, peptides, quinones, xanthones, coumarines, naphthopyrones, polyketides, phenols, lignans, chromenes,

The success of the most potent antimalarial drugs, quinine and artemisinin, has brought some optimism. Due to the widespread emergence of drug-resistant chloroquine, primaquine, mepacrine, and mefloquine (Figure 2) were developed. Despite the remarkable antimalarial activity, artemisinin suffers from limited availability, low solubility, high cost, metabolic stability, short half-life, poor bioavailability, and chemical stability. Thus, there is a need for new compounds more active than the parent artemisinin. To circumvent some of these problems, semisynthetic analogs were prepared. The reduction of artemisinin yields dihy‐ droartemisinin, and the lactol group can be further converted to its ether (artemether, arteether,

Artemisinin, a sesquiterpene endoperoxide, has established the role of peroxide ring for potential antimalarial activity. However, the naturally isolated artemisinin is available in short supply and expensive to synthesize. As a consequence, extensive research directed towards the discovery of peroxidic antimalarials inspired researchers to explore structurally simple

antimalarial agents.

48 An Overview of Tropical Diseases

**6.1. Natural products**

etc., have been tested as antimalarial drugs.[30, 33]

and artelinic acid) and ester (sodium artesunate) derivatives.[34]

**6.2. Semisynthetic drugs**

**6.3. Synthetic drugs**

Starting from commercially available cyclohexanediones, tricyclic 1,2,4-trioxanes can be synthesized by following simple method. Briefly, photooxygenation of the electron-rich allylic alcohols **1** using singlet oxygen gives β-hydroxyperoxide **2.** Further, β-hydroxyperoxide **2** was condensed with 1,4-cyclohexadiene followed by Lewis acid-mediated cyclization to give ketotrioxane **3.** Amino functionalized trioxanes **4** were also synthesized on reductive amination with various amines in the presence of sodium triacetoxy borohydride (Scheme 1).[35]

**Sheme 1.** Photooxygenation method for trioxane synthesis.

In another synthetic procedure, the geranyl acetate was transformed into aldehyde acetate **5**, which is converted into allylic alcohol **6**. Photooxygenation of **6** followed by subsequent acid catalyzed condensation of β-hydroxyhydroperoxides **7** with various ketones resulted in the formation of new 1,2,4-trioxanes **8** (Scheme 2).[36] The hydroxyl functionalized side chains can be further manipulated for the synthesize of a diverse library of compounds.

**Sheme 2.** Synthesis of Geraniol derived 1,2,4-trioxanes.

#### *6.3.1.2. Epoxidation method*

The epoxidation of N-Boc piperidone **9** gives N-Boc spirooxirane **10.** Dispiro N-Boc-protected 1,2,4-trioxane can then be synthesized by MoO2(acac)2 catalyzed perhydrolysis of N-Boc spirooxirane **10**, as shown in Scheme 3.[37] Subsequent condensation of the resulting βhydroperoxy alcohol **11** with 2-adamantanone gives N-Boc 1,2,4-trioxane **12**, which can be converted into the amine 1,2,4-trioxane hydrochloride salt **13**. Further, alkylation may result in a diversified sulfonamide trioxane derivatives **14**.

**Sheme 3.** Trioxane synthesis using epoxidation method.

#### *6.3.1.3. Catalytic enantioselective synthesis*

Trioxanes can also be synthesized by catalytic enantioselective synthesis. Para-cresol **15** is converted into *p*-peroxyquinols **16.** The desymmetrization of *p*-peroxyquinols **16** occurs via an acetalization/oxa-Michael cascade reaction (Scheme 4).[38] The reaction proceeds via a dynamic kinetic resolution of a peroxyhemiacetal intermediate. Various derivatized trioxanes **17** can be easily obtained by this method. The use of chiral Brønsted acid catalyst TRIP **18** gave a single diastereomer trioxane **17** in 86% ee, while using bis-(2,4,6-triisopropylphenyl)spiro‐ biindane phosphoric acid **19** gave 96% ee. The use of thiourea **20** as cocatalyst helped to restore the reactivity even at lower catalyst loading.

**Sheme 4.** Enantioselective synthesis of trioxanes.

#### *6.3.1.4. Solid phase synthesis*

**Sheme 2.** Synthesis of Geraniol derived 1,2,4-trioxanes.

in a diversified sulfonamide trioxane derivatives **14**.

**Sheme 3.** Trioxane synthesis using epoxidation method.

The epoxidation of N-Boc piperidone **9** gives N-Boc spirooxirane **10.** Dispiro N-Boc-protected 1,2,4-trioxane can then be synthesized by MoO2(acac)2 catalyzed perhydrolysis of N-Boc spirooxirane **10**, as shown in Scheme 3.[37] Subsequent condensation of the resulting βhydroperoxy alcohol **11** with 2-adamantanone gives N-Boc 1,2,4-trioxane **12**, which can be converted into the amine 1,2,4-trioxane hydrochloride salt **13**. Further, alkylation may result

*6.3.1.2. Epoxidation method*

50 An Overview of Tropical Diseases

The solid support synthesis of 1,2,4-trioxanes also needs light mediated oxygenation on polystyrene polymer support. Wang and Rink amide resins can be used as linkers. The reaction of resin-bound *p*-carboxybenzaldehydes **21** with excess of ionone derivatives **22** gave immo‐ bilized dienones **23** in the presence of LiOH in DME (Scheme 5).[39] Resin-bound trioxane **24** was obtained upon irradiation of compound **23** with UV light (354 nm) in toluene yielded. After cleavage from the solid support, the formation of **25** was confirmed by 13C NMR. Peaks at 82.4 and 94.4 ppm corresponded to the peroxy-bearing carbon and peroxyketal carbon of the trioxane ring system.

#### *6.3.2. Various synthetic procedures for the synthesis of tetraoxanes*

The chemical modification of artemisinin retaining the crucial endoperoxide ring has resulted in yet another simplified structure known as 1,2,4,5-tetraoxane. Tetraoxanes show significantly higher stability and exhibit even higher activity than natural peroxidic drugs for curing malaria

**Sheme 5.** Solid phase trioxane synthesis.

infections. In 1899, Baeyer and Villiger reported the synthesis of the first dimeric acetone peroxide upon treatment of acetone and Caro's acid in ether. Since then, the field has moved ahead significantly and newer synthetic routes and efficient methodologies were developed. The synthesis can be carried out by several methods as described below.

#### *6.3.2.1. Peroxidation method*

The most commonly and widely used method for tetraoxane synthesis is known as peroxida‐ tion method. In this method, acid-catalyzed cyclocondensation of ketones or aldehydes gives the *gem-*dihydroperoxide as an important active intermediate. Generally, the acid-catalyzed addition of hydrogen peroxide to carbonyl compound **26** produce *gem-*dihydroperoxide **27**, which on subsequent cyclocondensation in the presence of strong acid such as sulfuric acid, perchloric acid, or methanesulfuric acid yield more stable symmetrical tetraoxane **28** along with side product hexaoxane **29,** as shown in Scheme 6. It is also known that the trimeric cyclic peroxide by-product hexaoxonane is formed in the presence of excess hydrogen peroxide. Dimethyl sulfide and potassium iodide can be used for the removal of hydroperoxide-related impurities. Hexaoxonanes could be removed by washing the reaction mixture with cold methanol. [40]

In our lab, we also attempted the synthesis of a new series of tetraoxane by incorporating nitrogen within the cyclohexyl ring. [41] Methyl 2-(4-oxopiperidin-1-yl)acetate **30** on reaction with *gem-*dihydroperoxide **27** may give very small amount of tetraoxane **31** and trimer **29**, as shown in Scheme 7. We characterized hexaoxonane **29** as a main side product by spectroscopy and x-ray crystallography.

Recent Advances in Antimalarial Drug Discovery — Challenges and Opportunities http://dx.doi.org/10.5772/61191 53

**Sheme 6.** Acid catalyzed synthesis of tetraoxanes and hexaoxonanes.

**Sheme 7.** Synthesis of piperidinetetraoxane

#### *6.3.2.2. One pot synthesis*

infections. In 1899, Baeyer and Villiger reported the synthesis of the first dimeric acetone peroxide upon treatment of acetone and Caro's acid in ether. Since then, the field has moved ahead significantly and newer synthetic routes and efficient methodologies were developed.

The most commonly and widely used method for tetraoxane synthesis is known as peroxida‐ tion method. In this method, acid-catalyzed cyclocondensation of ketones or aldehydes gives the *gem-*dihydroperoxide as an important active intermediate. Generally, the acid-catalyzed addition of hydrogen peroxide to carbonyl compound **26** produce *gem-*dihydroperoxide **27**, which on subsequent cyclocondensation in the presence of strong acid such as sulfuric acid, perchloric acid, or methanesulfuric acid yield more stable symmetrical tetraoxane **28** along with side product hexaoxane **29,** as shown in Scheme 6. It is also known that the trimeric cyclic peroxide by-product hexaoxonane is formed in the presence of excess hydrogen peroxide. Dimethyl sulfide and potassium iodide can be used for the removal of hydroperoxide-related impurities. Hexaoxonanes could be removed by washing the reaction mixture with cold

In our lab, we also attempted the synthesis of a new series of tetraoxane by incorporating nitrogen within the cyclohexyl ring. [41] Methyl 2-(4-oxopiperidin-1-yl)acetate **30** on reaction with *gem-*dihydroperoxide **27** may give very small amount of tetraoxane **31** and trimer **29**, as shown in Scheme 7. We characterized hexaoxonane **29** as a main side product by spectroscopy

The synthesis can be carried out by several methods as described below.

*6.3.2.1. Peroxidation method*

**Sheme 5.** Solid phase trioxane synthesis.

52 An Overview of Tropical Diseases

methanol. [40]

and x-ray crystallography.

Iskara et al.[42] developed the first one-pot synthesis of tetraoxane. Simple carbonyl com‐ pounds **32** in the presence of 30% H2O2, 0.1% MTO, and fluorous alcohols (TFE and HFIP) selectively gives tetraoxanes **33** (Scheme 8). Fluorous solvents TFE and HFIP activate both H2O2 and MTO for oxidation reactions. The one-pot synthesis of mixed tetraoxanes begins with the oxidation of the most reactive carbonyl compound, and then less oxidizable carbonyl compound is added in the presence of acid. In this reaction, no trimeric product is formed.

**Sheme 8.** One pot tetraoxane synthesis.

#### *6.3.2.3. Ozonolysis method*

The most prolific strategy for the synthesis of tetraoxanes is the ozonolysis of suitable olefins and oximes. This method has dual advantage over others: (1) the absence of hexaoxonane (a usual by-product), which is very common in acid catalyzed reactions, and (2) it is useful for the synthesis of aromatic tetraoxanes, which could not be obtained by other methods. In the 1970s, Keul et al. reported the synthesis of dimeric adamantine peroxide **35** by ozonization of methyleneadamantane **34** in pentane at –78°C. The ozonolysis of valerophenone oxime *o*methyl ether **36** produces carbonyl oxide **38** via an intermediate ozonoid **37** to give the crystalline dimeric valerophenone peroxides **39** in the absence of carbonyl compounds or protic solvents.[43]

**Sheme 9.** Tetraoxane synthesis by ozonolysis method.

#### **7. Prodrug and combination therapies**

The search of newer drugs and the enhancement of antimalarial activity of the existing ones have led to the development of prodrug and combination therapy approaches. It presents a good platform for the usage of readily available drugs in combination with other effective drugs. The potential of drug hybrids, prodrugs, and combination therapy as new approaches are immense.[44]

Tetraoxaquine **40** contain two covalently linked pharmacophores, i.e., a tetraoxane (a radical donor) and an aminoquinoline (interferes with hematin polymerization).[45] Moreover, trioxaquine **41** contains covalently attached trioxane to a 4-aminoquinoline moiety.[46] The chimeric drug penetrates (enabled by aminoquinoline) into infected erythrocyte and targets the free heme. The hemoglobin digestion of the schizonts within infected red blood cells liberates free heme, which is alkylated by the peroxidic part. Trioxaferroquine **42** consists of a trioxane, a substituted quinoline, and an iron (II) species within a single structure.[47]

These new chimeric molecules containing two covalently attached moieties can be expected to possess synergistic therapeutic value, reduce resistance, and toxicity. These strategies offer a rational drug design approach for the development of next generation drug candidates. Notwithstanding few selected examples, which are discussed in this section, it explains the concept and potential applications.

## **8. Conclusion and future prospect**

*6.3.2.3. Ozonolysis method*

54 An Overview of Tropical Diseases

protic solvents.[43]

are immense.[44]

**Sheme 9.** Tetraoxane synthesis by ozonolysis method.

**7. Prodrug and combination therapies**

The most prolific strategy for the synthesis of tetraoxanes is the ozonolysis of suitable olefins and oximes. This method has dual advantage over others: (1) the absence of hexaoxonane (a usual by-product), which is very common in acid catalyzed reactions, and (2) it is useful for the synthesis of aromatic tetraoxanes, which could not be obtained by other methods. In the 1970s, Keul et al. reported the synthesis of dimeric adamantine peroxide **35** by ozonization of methyleneadamantane **34** in pentane at –78°C. The ozonolysis of valerophenone oxime *o*methyl ether **36** produces carbonyl oxide **38** via an intermediate ozonoid **37** to give the crystalline dimeric valerophenone peroxides **39** in the absence of carbonyl compounds or

The search of newer drugs and the enhancement of antimalarial activity of the existing ones have led to the development of prodrug and combination therapy approaches. It presents a good platform for the usage of readily available drugs in combination with other effective drugs. The potential of drug hybrids, prodrugs, and combination therapy as new approaches

Tetraoxaquine **40** contain two covalently linked pharmacophores, i.e., a tetraoxane (a radical donor) and an aminoquinoline (interferes with hematin polymerization).[45] Moreover, trioxaquine **41** contains covalently attached trioxane to a 4-aminoquinoline moiety.[46] The chimeric drug penetrates (enabled by aminoquinoline) into infected erythrocyte and targets the free heme. The hemoglobin digestion of the schizonts within infected red blood cells liberates free heme, which is alkylated by the peroxidic part. Trioxaferroquine **42** consists of a trioxane, a substituted quinoline, and an iron (II) species within a single structure.[47]

The development of new drugs for malaria presents a challenging situation. Lack of alterna‐ tives and increasing ineffectiveness of the existing drugs are the main reasons for increased mortality. Traditional medicines have provided few drugs, but to combat malaria, new drugs are urgently needed. These new drugs must ideally possess minimal toxicity, rapid efficacy, and low cost. However, there is consensus among scientific community that drug combina‐ tions may create optimal control of malaria because the combination therapies are believed to be additive in potency, provide synergistic activity, and is more advantageous than mono‐ therapies. Unfortunately, these requirements are not met by any combination at the current window of time. Besides all the challenges, failures, and setbacks, the global importance of fighting malaria is recognized. Dedicated efforts and academic engagement to discover, develop, and deliver new, effective, and affordable antimalarials have thus increased dramat‐ ically. Natural products, semisynthetic drugs, and synthetic compounds offer vast opportunity for the drug development process. Further, assessment and clinical evaluation of RTS,S/AS01E for malaria vaccination offers hope that we may soon expect some good news. Malaria drug discovery is undoubtedly challenging, but scientists are optimistic as they also have got various opportunities too. The *status quo* seems balanced. However, we believe that we have to provoke the *status quo* to gain the upper hand in the battle against this tropical scourge.
