Synthesis and antimalarial activities of base-catalyzed adducts of 11-azaartemisinin

Inverse α-Effect as the Ariadne's Thread on the Way to Tricyclic Aminoperoxides: Avoiding Thermodynamic Traps in the Labyrinth of Possibilities

Stable tricyclic aminoperoxides can be selectively assembled via a catalyst-free three-component condensation of β,δ'-triketones, H₂O₂, and an NH-group source such as aqueous ammonia or ammonium salts. This procedure is scalable and can produce gram quantities of tricyclic heterocycles, containing peroxide, nitrogen, and oxygen cycles in one molecule. Amazingly, such complex tricyclic molecules are selectively formed despite the multitude of alternative reaction routes, via equilibration of peroxide, hemiaminal, monoperoxyacetal, and peroxyhemiaminal functionalities! The reaction is initiated by the "stereoelectronic frustration" of H₂O₂ and combines elements of thermodynamic and kinetic control with a variety of mono-, bi-, and tricyclic structures evolving under the conditions of thermodynamic control until they reach a kinetic wall created by the inverse α-effect, that is, the stereoelectronic penalty for the formation of peroxycarbenium ions and related transition states. Under these conditions, the reaction stops before reaching the most thermodynamically stable products at a stage where three different heterocycles are assembled and fused at the acyclic precursor frame.

Marriage of Peroxides and Nitrogen Heterocycles: Selective Three-Component Assembly, Peroxide-Preserving Rearrangement, and Stereoelectronic Source of Unusual Stability of Bridged Azaozonides

Stable bridged azaozonides can be selectively assembled via a catalyst-free three-component condensation of 1,5-diketones, hydrogen peroxide, and an NH-group source such as aqueous ammonia or ammonium salts. This procedure is scalable and can produce gram quantities of bicyclic stereochemically rich heterocycles. The new azaozonides are thermally stable and can be stored at room temperature for several months without decomposition and for at least 1 year at -10 °C. The chemical stability of azaozonides was explored for their subsequent selective transformations including the first example of an aminoperoxide rearrangement that preserves the peroxide group. The amino group in aminoperoxides has remarkably low nucleophilicity and does not participate in the usual amine alkylation and acylation reactions. These observations and the 15 pK_a units decrease in basicity in comparison with a typical dialkyl amine are attributed to the strong hyperconjugative n_N→σ*_C-O interaction with the two antiperiplanar C-O bonds. Due to the weakness of the complementary n_O→σ*_C-N donation from the peroxide oxygens (a consequence of "inverse α-effect"), this interaction depletes electron density from the NH moiety, protects it from oxidation, and makes it similar in properties to an amide.

Exploration of artemisinin derivatives and synthetic peroxides in antimalarial drug discovery research

Malaria is a life-threatening infectious disease caused by protozoal parasites belonging to the genus Plasmodium. It caused an estimated 405,000 deaths and 228 million malaria cases globally in 2018 as per the World Malaria Report released by World Health Organization (WHO) in 2019. Artemisinin (ART), a "Nobel medicine" and its derivatives have proven potential application in antimalarial drug discovery programs. In this review, antimalarial activity of the most active artemisinin derivatives modified at C-10/C-11/C-16/C-6 positions and synthetic peroxides (endoperoxides, 1,2,4-trioxolanes, 1,2,4-trioxanes, and 1,2,4,5-tetraoxanes) are systematically summarized. The developmental trend of ART derivatives, and cyclic peroxides along with their antimalarial activity and how the activity is affected by structural variations on different sites of the compounds are discussed. This compilation would be very useful towards scaffold hopping aimed at avoiding the unnecessary complexity in cyclic peroxides, and ultimately act as a handy resource for the development of potential chemotherapeutics against Plasmodium species.

Artemisinin-derived antimalarial endoperoxides from bench-side to bed-side: Chronological advancements and future challenges

According to WHO World Malaria Report (2018), nearly 219 million new cases of malaria occurred and a total no. of 435 000 people died in 2017 due to this infectious disease. This is due to the rapid spread of parasite-resistant strains. Artemisinin (ART), a sesquiterpene lactone endoperoxide isolated from traditional Chinese herb Artemisia annua, has been recognized as a novel class of antimalarial drugs. The 2015 "Nobel Prize in Physiology or Medicine" was given to Prof Dr Tu Youyou for the discovery of ART. Hence, ART is termed as "Nobel medicine." The present review article accommodates insights from the chronological advancements and direct statistics witnessed during the past 48 years (1971-2019) in the medicinal chemistry of ART-derived antimalarial endoperoxides, and their clinical utility in malaria chemotherapy and drug discovery.

Current scenario of artemisinin and its analogues for antimalarial activity

Human malaria, one of the most striking, reemerging infectious diseases, is caused by several types of Plasmodium parasites. Whilst advances have been made in lowering the numbers of cases and deaths, it is clear that a strategy based solely on disease control year on year, without reducing transmission and ultimately eradicating the parasite, is unsustainable. Natural products have served as a template for the design and development of antimalarial drugs currently in the clinic or in the development phase. Artemisinin combine potent, rapid antimalarial activity with a wide therapeutic index and an absence of clinically important resistance. The alkylating ability of artemisinin and its semi-synthetic analogues toward heme related to their antimalarial efficacy are underlined. Although impressive results have already been achieved in malaria research, more systematization and concentration of efforts are required if real breakthroughs are to be made. This review will concisely cover the clinical, preclinical antimalarial and current updates in artemisinin based antimalarial drugs. Diverse classes of semi-synthetic analogs of artemisinin reported in the last decade have also been extensively studied. The experience gained in this respect is discussed.

11-Azaartemisinin cocrystals with preserved lactam : acid heterosynthons

The R22(8) lactam : acid hetero-synthon, found in several new 1 : 1 and 2 : 1 cocrystals between the anti-malarial 11-azaartemisinin and organic acids.

Enantioselective Rh-Catalyzed Carboacylation of C═N Bonds via C-C Activation of Benzocyclobutenones

Herein we describe the first enantioselective Rh-catalyzed carboacylation of oximes (imines) via C-C activation. In this transformation, the benzocyclobutenone C1-C2 bond is selectively activated by a low valent rhodium catalyst and subsequently the resulting two Rh-C bonds add across a C═N bond, which provides a unique approach to access chiral lactams. A range of polycyclic nitrogen-containing scaffolds were obtained in good yields with excellent enantioselectivity. Further derivatization of the lactam products led to a rapid entry to various novel fused heterocycles.

Nucleophile-catalyzed additions to activated triple bonds. Protection of lactams, imides, and nucleosides with MocVinyl and related groups

Additions of lactams, imides, (S)-4-benzyl-1,3-oxazolidin-2-one, 2-pyridone, pyrimidine-2,4-diones (AZT derivatives), or inosines to the electron-deficient triple bonds of methyl propynoate, tert-butyl propynoate, 3-butyn-2-one, N-propynoylmorpholine, or N-methoxy-N-methylpropynamide in the presence of many potential catalysts were examined. DABCO and, second, DMAP appeared to be the best (highest reaction rates and E/Z ratios), while RuCl3, RuClCp*(PPh3)2, AuCl, AuCl(PPh3), CuI, and Cu2(OTf)2 were incapable of catalyzing such additions. The groups incorporated (for example, the 2-(methoxycarbonyl)ethenyl group that we name MocVinyl) serve as protecting groups for the above-mentioned heterocyclic CONH or CONHCO moieties. Deprotections were accomplished via exchange with good nucleophiles: the 1-dodecanethiolate anion turned out to be the most general and efficient reagent, but in some particular cases other nucleophiles also worked (e.g., MocVinyl-inosines can be cleaved with succinimide anion). Some structural and mechanistic details have been accounted for with the help of DFT and MP2 calculations.

Antimalarial peroxides

The problem of endemic malaria continues unabated globally. Malaria affects 40 % of the global population, causing an estimated annual mortality of 1.5-2.7 million people. The World Health Organization (WHO) estimates that 90 % of these deaths occur in sub-Saharan Africa among infants under the age of five. While a vaccine against malaria continues to be elusive, chemotherapy remains the most viable alternative towards treatment of the disease. During last years, the situation has become urgent in many ways, but mainly because of the development of chloroquine-resistant (CQR) strains of Plasmodium falciparum (Pf). The discovery that artemisinin (ART, 1), an active principle of Artemisia annua L., expresses a significant antimalarial activity, especially against CQR strains, opened new approaches for combating malaria. Since the early 1980s, hundreds of semi-synthetic and synthetic peroxides have been developed and tested for their antimalarial activity, the results of which were extensively reviewed. In addition, in therapeutic practice, there is no reported case of drug resistance to these antimalarial peroxides. This review summarizes recent achievements in the area of peroxide drug development for malaria chemotherapy.

Preparation ofN-Sulfonyl- andN-Carbonyl-11-Azaartemisinins with Greatly Enhanced Thermal Stabilities: in vitro Antimalarial Activities

As the clinically used artemisinins do not withstand the thermal stress testing required to evaluate shelf life for storage in tropical countries where malaria is prevalent, there is a need to develop thermally more robust artemisinin derivatives. Herein we describe the attachment of electron-withdrawing arene- and alkanesulfonyl and -carbonyl groups to the nitrogen atom of the readily accessible Ziffer 11-azaartemisinin to provide the corresponding N-sulfonyl- and -carbonylazaartemisinins. Two acylurea analogues were also prepared by treatment of the 11-azaartemisinin with arylisocyanates. Several of the N-sulfonylazaartemisinins have melting points above 200 degrees C and possess substantially greater thermal stabilities than the artemisinins in current clinical use, with the antimalarial activities of several of the arylsulfonyl derivatives being similar to that of artesunate against the drug-sensitive 3D7 clone of the NF54 isolate and the multidrug-resistant K1 strain of P. falciparum. The compounds possess relatively low cytotoxicities. The carbonyl derivatives are less crystalline than the N-sulfonyl derivatives, but are generally more active as antimalarials. The N-nitroarylcarbonyl and arylurea derivatives possess sub-ng ml(-1) activities. Although several of the azaartemisinins possess log P values below 3.5, the compounds have poor aqueous solubility (<1 mg L(-1) at pH 7). The greatly enhanced thermal stability of our artemisinins suggests that strategic incorporation of electron-withdrawing polar groups into both new artemisinin derivatives and totally synthetic trioxanes or trioxolanes may assist in the generation of practical new antimalarial drugs which will be stable to storage conditions in the field, while retaining favorable physicochemical properties.

QSAR for anti-malarial activity of 2-aziridinyl and 2,3-bis(aziridinyl)-1,4-naphthoquinonyl sulfonate and acylate derivatives

Quantitative structure-activity antimalarial relationships have been studied for 63 analogues of 2-aziridinyl and 2,3-bis(aziridinyl)-1,4-naphthoquinonyl sulfonate and acylate derivatives by means of multiple linear regression (MLR) and artificial neural networks (ANN). The antimalarial activity [-log(IC50x10(6))] of the compounds studied were well correlated with descriptors encoding the chemical structure. Using the pertinent descriptors revealed by a stepwise procedure in the multiple linear regression technique, a correlation coefficient of 0.9394 (s=0.2121) for the training set was obtained for the ANN model in a [3-5-1] configuration. The results show that the antimalarial activity of 2-aziridinyl and 2,3-bis(aziridinyl)-1,4-naphthoquinonyl sulfonate and acylate derivatives is strongly dependent on hydrophobic character, hydrogen-bond acceptors and also steric factors of the substituents.

On the Susceptibility of Organic Peroxy Bonds to Hydride Reduction

Reduction of organic molecules that contain a peroxy bond is broadly considered as a "risky" and uncertain operation when cleavage of the peroxy linkage is not desired. For this reason, such reduction steps are normally avoided at the planning stage of the synthesis when possible. As a natural consequence, the information in the literature about the susceptibility of organic peroxy bonds to reducing species is scant. In this work the tolerance of organic peroxy bonds to some common hydride reductants was examined systematically for the first time. Using reduction of ester group to alcohol as a probe, LiAlH(4), LiAlH(O(t)()Bu)(3), LiBHEt(3), and LiBH(4) were found to be significantly better than other reductants examined when taking into consideration both the completeness of the reduction of ester groups and the peroxy bond survival rate. LiBH(4) appeared to be the most suitable reductant for the reduction under discussion, not only because of the high reduction yields/excellent compatibility with peroxy bonds, but also because of the advantages in practical aspects. The results disclosed herein may (hopefully) provide a handy reference for dealing with reduction of other peroxy bond-containing molecules in the future.

Development of artemisinin and its structurally simplified trioxane derivatives as antimalarial drugs

Artemisinin and simplified trioxane analogs constitute a promising class of antimalarial chemotherapeutic agents. Their development since the early 1970s into clinical trials and clinical use has drawn much attention from medical scientists worldwide although the crude extract containing artemisinin has been used in China for treatment of fever for many centuries. Many research groups have independently and collaboratively conducted various studies on the artemisinin system both in search for the new compounds more antimalarially active than the parent artemisinin and in an attempt to understand its molecular mechanism(s) of action. Ongoing studies have provided a better understanding of the putative intermediates essential for the antimalarial activity and have led to designer trioxanes whose chemical structures have been simplified and modified to increase efficacy while lowering toxicity. Other desirable features beneficial to clinical uses such as bioavailability, drug stability and water solubility have been considered, and portions of the trioxane skeleton have been added or modified to accommodate these parameters accordingly.

Antimalarial and antiproliferative evaluation of bis-steroidal tetraoxanes

Several cis and trans bis-steroidal 1,2,4,5-tetraoxanes possessing amide terminus were synthesised and evaluated as antimalarials and antiproliferatives. The compounds exhibited submicromolar antimalarial activity against Plasmodium falciparum D6 and W2 strains. The existence of HN-C(O) moiety was found necessary for pronounced antimalarial and antiproliferative activity. In antiproliferative screen, the trans tetraoxane 6 was found to exhibit a pronounced cytotoxicity on 14 cancer cell lines. In addition, tetraoxanes 11 and 12 exhibited significant cytotoxic activity too; microscopic examination of treated HeLa cells showed morphological appearance reminiscent for apoptosis (condensed and/or fragmented nuclei).

Antimalarial, antimycobacterial and antiproliferative activity of phenyl substituted mixed tetraoxanes

Mixed tetraoxanes of the 4"-phenyl-substituted cyclohexyl-spirotetraoxacyclohexyl- spirocholate series have been prepared and evaluated as possible antimalarials, antiproliferatives and antimycobacterials. The activity of the (4"R or S)-phenyl series against P. falciparum D6 and W2 strains was found to be at the level of artemisinin with two compounds, the acid 4 and the amide 6, exhibiting encouraging anti-TB activity as well. Very promising in vitro results of the said tetraoxanes were obtained against solid tumours and, in some instances, the activity against a selected number of cell lines was higher than that of the antitumor drug Paclitaxel.

Antimalarial chemotherapeutic peroxides: artemisinin, yingzhaosu A and related compounds

Mechanism-based rational design and gram-scale chemical synthesis have produced some new trioxane and endoperoxide antimalarial drug candidates that are efficacious and safe. This review summarises recent achievements in this area of peroxide drug development for malaria chemotherapy.

Antimalarial peroxides: The first intramolecular 1,2,4,5-tetraoxane

An intramolecular steroidal 1,2,4,5-tetraoxane has been synthesised in six steps starting from methyl3-oxo-7?,12?-diacetoxy-5?-cholan-24-oate. The synthesised 1,2,4,5-tetraoxane has moderate in vitro antimalarial activity against P. falciparum strains (IC50 (D6) = 0.35 ?g/mL; IC50 (W2) = 0.29 ?g/mL).

Artemisinin and derivatives: the future for malaria treatment?

The isolation in 1972 of artemisinin by Chinese scientists, and their development of all the derivatives now used in the treatment of malaria today, were of outstanding importance. The results which have accumulated both from the Chinese work and from that subsequently conducted on a worldwide basis provide for a relatively comprehensive understanding of the chemistry, pharmacological profiles, toxicology, metabolism, and effects on the malaria parasite. The optimal regimens for use in the field are also apparent, particularly in combinations with longer half-life quinoline antimalarials. Thus the future use of the artemisinin class of drug appears assured. However, the mechanism of action needs to be clarified. More importantly from a clinical viewpoint, problems inherent in the current derivatives must be addressed, particularly that of neurotoxicity, if new artemisinin derivatives are to be introduced in a normal drug regulatory environment. The application of established principles of modern drug design should indeed allow for the first truly rationally designed, in so far as the target is still unknown, derivatives to come to hand.

Molecular modeling and 3D-QSAR studies in 2-aziridinyl-and 2,3-bis(aziridinyl)-1,4-naphthoquinonyl sulfonate and acylate derivatives as potential antimalarial agents

Malaria is still continuing to be one of the most dreadful diseases of the tropical countries particularly due to the development of resistance to the existing antimalarials. From observed, antimalarial activity of 2-aziridinyl- and 2,3-bis(aziridinyl)-1,4-naphthoquinonyl sulfonate and acylate derivatives acting through redox cycling mechanism, molecular modeling and three dimensional-quantitative structure activity relationship (3D-QSAR) studies have been carried out on a set of 63 compounds to identify important pharmacophors. Among several 3D-QSAR models generated, three models with correlation coefficient r > 0.82, match > 0.60 and chance = 0.00 have shown two common biophoric sites: one being the oxygen atom at position 1 of the naphthoquinone ring in terms of pi-population, charge and electron donating ability while the second being the center of the phenyl ring in terms of its 6pi-electrons. In addition to these sites, the models also share two common secondary sites: one positively contributing H-acceptor site while the second site contributing negatively in terms of steric refractivity. All these models showed good agreement between the experimental, calculated and predicted antimalarial activities.