Proposals for New Antimalaria Drugs
Malaria is the most devastating disease afflicting humans , having been so since the dawn of history and continuing today . Statistics on malaria's toll vary from source to source , but in any case are shocking : over half a billion people are infected with the malaria parasites at any one time , and more than a million per year die from its effects . Children are particularly vulnerable : on average 3000 children per day (mostly in Africa) one every thirty seconds die from malaria . The deadliest pandemic was undoubtedly the Black Death (bubonic plague) , which in the 1340s erased a third of Earth's human population . Over the centuries malaria has killed many more people , tens of billions , in an endless plague .
Malaria is caused by an infection of the body by Plasmodia protozoa , a type of single-cell animal microorganism . There are many types of Plasmodia ; at least ten varieties affect humans , causing the disease symptoms collectively referred to as "malaria" . The four principal strains infecting humans are falciparum , vivax , ovale , and malariae . Falciparum causes the most fatalities , its infections leading to the often-fatal cerebral malaria . Although Plasmodia are very simple organisms , they have a complex life cycle with many different forms . The reader is referred to the excellent Wikipedia review articles for a discussion of the biology of the Plasmodia parasites and of the epidemiology of malaria . The "Malaria Site" website also has much information on the subject . For a photograph of a Plasmodium falciparum , see this web page . For a diagram of a Plasmodium merozoite (the form that attacks the red blood cells) , see this web page .
There are four important reasons why malaria is such a severe health problem . First , Plasmodia are extraordinarily adept at evading the antibodies of the human immune system . You would think that as fairly large antigens they would be quickly detected , enveloped , and eliminated by the body's cellular defence mechanisms , but unfortunately this isn't the case . A certain amount of immunity to malaria can , and does occur in populations living in malarial regions of the world , but such immunity usually takes years and even generations to acquire in the survivors .
Second , Plasmodia have shown a remarkable skill in developing resistance to several of the more important antimalaria drugs such as quinine and chloroquine . In the last couple of decades there has been a worrisome resurgence of malaria in Africa and Asia caused by newly resistant strains of the parasite . This reversal of fortune in the war against malaria has stimulated a revival of research into new drugs to treat it .
Third , Plasmodia are transmitted to humans by the bite of infected female Anopheles mosquitos , their primary host and carrier ("vector") , a sort of insect Typhoid Mary . It's essentially impossible to eradicate these mosquitos , and it might even be unwise to do so as they are an integral part of the natural ecology . The best we can do with respect to the mosquitos is to suppress them locally with insecticides , drain their watery breeding grounds , keep them at bay with chemical repellents , and ensure personal protection at night with mosquito netting .
Fourth , malaria has often been referred to as a "disease of poverty" , afflicting as it does the poor and powerless people of the tropical regions . Malaria also causes poverty in those areas , by debilitating their labour forces and retarding their economies . Modern (20th Century) drug research and development has been carried out in the First World , addressing First World health problems , which mostly didn't include malaria , a disease of the Third World . There was little economic incentive , therefore , to spend millions of dollars on drug discovery for a disease with little or no impact on Western society .
That attitude has been changing with "globalization" , the increasing human interaction and intermingling brought about by advances in communications technology and travel . Climate change also threatens , among many other negative impacts , to widen the range of the Anopheles mosquito and spread malaria into southern regions of the First World . The affluent western nations can no longer afford to be complacent about the threat of malaria . It is in their increasing self-interest , not only for humane reasons , to accelerate research into new , more effective antimalaria drugs .
The naturally-occurring alkaloid quinine was for about three centuries the only effective antimalaria drug known to Europeans . It was used by South American indigenous peoples as an extract from the bark of the cinchona tree , and was brought to Europe in 1632 . In the 1920s and 1930s several synthetic drugs based on the the quinine structure were developed . When the quinine supply from Southeast Asia was cut off during World War II , frantic efforts were made to discover new , more effective antimalaria drugs , the most prominent of which were quinacrine (1942) and chloroquine (1945) . Mefloquine (1977) , a third noteworthy quinine analogue , is also highly effective against Plasmodia , but unfortunately it can produce distressing side effects with prolonged usage . Quinine , chloroquine , and mefloquine are all modifications of the heterocyclic quinoline molecule [quinacrine (atabrine , mepacrine) is an acridine derivative ; it was widely used during World War II , but apparently is no longer prescribed for the treatment of malaria] :
There is some debate as to how the quinoline drugs affect Plasmodia . Earlier theories proposed that they bond to the parasite's DNA , inhibiting its replication . Other references suggested that they interfere with Plasmodia's processes for disposing of the heme waste product , or prevent it from attacking hemoglobin . In any case , the three quinoline drugs shown above are the most commonly used medications for the treatment of malaria .
Antimalaria chemotherapy research has advanced steadily since World War II , and we now have an impressive array of proven drugs of various chemical classes to combat Plasmodia . The reader is referred to the excellent reviews cited in the References section below for more detailed discussions of the various antimalaria drugs and antiprotozoal drugs now in use or in current development .
Artemisinin and related peroxide-based antimalaria drugs
Unknown to western society , the Chinese had been using an herbal extract , qinghaosu (from the Artemisia annua plant) , to successfuly treat malaria for over 2000 years (Butler,Wu) . In the 1960s there was a dramatic resurgence of malaria in Southeast Asia , including southern China , caused by a growing resistance to chloroquine by the deadly Plasmodia falciparum . Chinese scientists began an intense research program in 1967 to isolate the active component of qinghaosu , which they succeeded in doing . It proved to be a sesquiterpene , a natural product composed of fifteen carbon atoms , based on three isoprene molecules usually joined head-to-tail , with additional oxygen atom functionality . The really striking feature of qinghaosu was the peroxide bridge spanning one of the molecule's rings , which was shown to be its "active ingredient", the key part of the drug that made it so effective against Plasmodia .
The pure active compound isolated from qinghaosu in 1972 was officially called artemisinin , and it was soon being studied by several research groups . Artemisinin derivatives were prepared , and several were found to have even greater activity against Plasmodia than the parent compound :
Artemisinin and its derivatives are now widely used in the treatment of malaria . They are usually mixed in a cocktail with another non-peroxide antimalarial drug ("artemisinin-based combination therapy" , ACT) , since the artemisinin compounds , while fast-acting and highly effective against Plasmodia , have rather short half-lives in the human body (they are rapidly metabolized and excreted) , while complete elimination of the parasites usually requires several days to a week . The non-peroxide drugs have longer half-lives , and so are often used together with the artemisinins .
A significant economic disadvantage of the artemisinin family of drugs is that they are natural products , derived from a plant (Artemisia annua) , and their supply is controlled by its limited cultivation and extraction in Southeast Asia . The restricted supply of the artemisinins has been an incentive for medicinal chemists to design and prepare purely synthetic peroxide-based antimalarial drugs in their laboratories , hoping that their compounds , if effective , can be economically produced on a large scale in chemical factories . As Rosenthal has pointed out in his excellent overview of antimalaria drugs ,
The critical consideration in antimalarial drug development is economic . Financial constraints are relevant in two key regards . First, to be widely useful , antimalarial drugs must be very inexpensive so that they are routinely available to populations in need in developing countries . Indeed , even a price of $1 per treatment is probably unacceptable in many regions , considering severe poverty in most of the malarious world and familiarity with available drugs , especially chloroquine , that are very inexpensive (less than $0.10 per treatment) , albeit increasingly ineffective . Second , since malaria markets are primarily in poor countries , marketing opportunities have generally been considered to be limited , and so investment in antimalarial drug discovery and development has been small . Thus , drug discovery directed against malaria is particularly reliant upon shortcuts that may obviate excess cost (p. 3735) .
After invading a human , Plasmodia parasites [merozoites] attack the host's red blood cells [erythrocytes] , puncturing them and ingesting their hemoglobin molecules . Hemoglobin consists of four associated coiled protein strands (globin) , to each of which is bonded a heme [iron(II) porphyrin] molecule . Plasmodia's enzymes break down the globin proteins into their component amino acids , which they use in the growth of new parasites . The heme molecules are indigestible , so Plasmodia crystallizes them into a solid waste substance , hemozoin , that it expels from its interior . One theory proposes that the peroxide drugs , by bonding to the heme , inhibit the formation of hemozoin . The accumulating waste heme then poisons the parasite .
Artemisinin and other peroxide-based antimalaria drugs appear to bond to the iron atom in heme by one of the peroxide oxygens :
R1OOR2 + Fe2+heme ---------------> R1 O + R2OFe3+heme
The highly energetic free radicals so produced then react with Plasmodia's cellular components . It is currently thought that the radicals bond to and deactivate the parasite's SERCA enzyme (sarco-endoplasmic reticulum calcium-dependent ATPase) , which functions as a sort of "calcium ion pump" in the organism (Ridley) .
An important point about the artemisinins and related peroxide-based antimalaria drugs is that Plasmodia are apparently unable to build up any significant resistance to them :
A key advantage of these endoperoxide-containing antimalarial agents , which have been used for nearly two decades , is the absence of any resistance to them (Robert et al. , p.1179) .
Researchers seemed to have stumbled onto Plasmodia's Achilles heel ! Whatever the mechanism of the peroxide reaction inside the organism may be , whether by deactivating SERCA , heme , or any other cellular process , the parasite seems to be helpless against it , in contrast to the classical quinoline drugs , to which they can , and do , build up resistance . Plasmodia fundamentally consume human hemoglobin to obtain their necessary amino acids . The waste by-product of hemoglobin consumption is heme , so there will always be a source of iron(II) inside the parasite . This iron(II) can be used either as a reagent or catalyst with appropriately-designed drug molecules to generate highly reactive intermediates , such as free radicals , that will attack and destroy or disable Plasmodia's "internal organs" (organelles) or metabolic processes (such as its enzymes) . As this web page unfolds I'll propose a wide variety of new peroxide-based drugs and novel reactive systems to deliver "poison packages" selectively targeting the iron(II) inside the parasites , from red blood cells they have invaded and ruptured .
The question arises : won't the peroxide , which generally is toxic to humans , damage the body's cells as it circulates in the blood , gradually infiltrating the Plasmodia ? The answer is yes , but humans are vastly larger than Plasmodia , and can more readily tolerate and detoxify peroxide , which would be present in the blood in (usually) nanomolar concentrations . Humans at least those with reasonably mature and healthy immune systems have elaborate cellular repair mechanisms which the extremely simple microorganisms like Plasmodia and other protozoa , bacteria , viruses , etc. lack . Over the course of evolutionary time , humans have developed several defense mechanisms for cellular protection and repair against oxidative damage by various agents such as oxygen free radicals and peroxides (E. Niki) . Thus , the artemisinins and the purely synthetic peroxide drugs seem to be well tolerated by humans with no appreciable toxic side-effects .
An Antimalaria Drug Model
Development of new peroxide-based antimalaria drugs seems to have proceeded in the last twenty years or so in three stages : first , modification of the artemisinin molecule , of which several examples are sketched above . More examples are provided by Lin and co-workers :
The numerical data refer to IC50 values , " 50% inhibitory concentration" of the drug studied . The fluorophenyl derivative is quite potent .
The second stage is the preparation of new compounds whose endoperoxide ring system looks somewhat like that of artemisinin . Posner's research group provides several examples :
Finally , in the third stage , the peroxide drug candidates are purely synthetic , with no resemblance to artemisinin at all (except for their peroxide bridge) . Vennerstrom(1) and co-workers have been pioneers in the development of radically new peroxide-based antimalaria drug compounds . They studied the effect , for example , of the steric hindrance of alkane groups surrounding the peroxide bridge , successfully incorporating the unusual adamantane group in the highly active compound OZ277 :
It's interesting to compare the structure of OZ277 and related ozonides with that of a 1,2,4,5-tetraoxane (bis-peroxide) synthesized by O'Neill's research group :
Although adamantyl is considered to be a valuable hydrophobic group in antimalaria drug design , we see that it's not really critical in achieving a high effectiveness in the synthesized compounds . Rather , an important component in antimalaria drugs is an extended "tail" structure , comprised partly of lipophilic (hydrocarbon) and hydrophilic (oxygen and nitrogen atoms and groups , such as carbonyl and amide) . If the reader scrolls up the page and examines the highly potent compounds prepared by Posner and Lin's groups , and the artemisinin derivatives , an emerging pattern will be observed . The biologically active part of the molecule , the peroxide group , is surrounded by a hydrophobic group or groups . Attached to it is a moderately long extension of substiuents , partly hydrophilic and partly lipophilic . The researcher can alter the efficacy of the candidate compound by making subtle adjustments to the sterically-hindering hydrophobic groups around the peroxide oxygens , and to the "tail" part of the molecule .
The structure of typical highly active antimalarial drugs , that of a hydrophobic "head" containing a chemically reactive group , and a partially hydrophilic tail , is reminiscent of the general structure of surfactants (I have discussed surfactants in another web page) . However , the surfactant structure is inverse to that of antimalaria drugs : the former always have a hydrophilic head [anionic , cationic , amphoteric , or nonionic] and a long lipophilic tail . The surfactant concept of the hydrophilic/lipophilic balance (HLB) of a molecule may be useful in the design of new drug candidates , too .
Miltefosine is an effective antiprotozoal drug , currently used in the treatment of leishmaniasis (although apparently not for malaria) . It actually has the structure of an amphoteric surfactant , with a long lipophilic tail and a hydrophilic betaine head composed of phosphocholine :
Certain cationic surfactants have been found to be very potent antimalarial compounds . Vial and co-workers synthesized a variety of compounds with long (C12 and longer) "bodies" , and having a cationic "head" at each end of the molecule :
While remarkably effective in vitro , these cationic compounds were less effective in vivo , being poorly absorbed through the intestinal wall of the test animals . Vial's group did some brilliant biochemistry , designing a nonionic "prodrug" , TE3 (readily absorbed) which is transformed in mammalian bodies by thioesterase enzymes into the bis-quat T3 . This latter thiazolium compound proved to be as effective as the more conventional bis-quats studied previously .
Vial's team developed a simple model of the quat receptors in Plasmodia's enzymes , perhaps located in the apicoplast organelle , in which lipophilic molecules are processed . These particular enzymes are involved in the synthesis of phosphatidylcholine , a fatty quaternary compound used in constructing the parasite's outer membrane . Vial's cationic surfactants interfere with the the phosphatidylcholine synthesis , thus terminating development of the organism . Such a model , however , is unlikely to apply to the peroxide-based antimalarial compounds we are studying in this essay , whose activity is based primarily in Plasmodia's food vacuole , where the hemoglobin is digested .
Vial's systematic methodology in developing his model of the bis-quat receptor is reminiscent of the research carried out by other French chemists , Tinte and Nofre , who studied the human sweetness receptor . By a progressive series of design , synthesis , and testing they were able to refine their three-dimensional model of the sweetness receptor to an extraordinary extent , identifying eight distinct molecular interaction sites on it , having varying degrees of polarity , lipophilicity , and hydrophilicity . They simultaneously prepared molecules of increasing sweetness , culminating in a glycine derivative they called "sucrononic acid" :
They determined that it was about 200,000 times sweeter than sucrose . To put this into perspective : a two kilogram bag of sucrononic acid (the size of a typical bag of sugar in the supermarket) could replace 400 tonnes of sugar , an entire warehouse full ! I think this sweetness research is worth mentioning in connection with the development of new antimalaria drugs , because it's an excellent illustration of the scientific method (deduction/induction) : proposing an explanatory model of a natural phenomenon , then refining it and redefining it , if necessary in a series of experiments which progress slowly but surely toward confirming , and perfecting , the proposed model . Along the way the original model may even have to be discarded in favour of a new one .
Apart from Vial's bis-quat model of the cationic inhibitors of phosphatidylcholine synthesis , no model seems to have been developed yet of an idealized antimalarial drug applicable to the hemoglobin aspect of Plasmodia's "modus operandi" inside the human body . Granted , several avenues of attack on Plasmodia have been identified , but hemoglobin destruction by the parasite is not only its fundamental activity and the root cause of malaria , but it also provides an especially promising entry for new , reactive drugs . The very waste Plasmodia creates as it consumes hemoglobin can be used to activate , or catalyze , the drug molecules sent to destroy the parasite . Even better , it has no cellular mechanisms to resist these drugs , or to repair the damage they do inside the organism .
Returning to the idealized structure of an antimalarial drug , the candidate molecule will carry a chemical species that can react with heme's Fe(II) ; this is generally a peroxide or a related group . The reactive center is surrounded by sterically-repulsive hydrophobic alkyl/aryl groups , to help protect it as the compound disperses throughout the body . Finally , it seems that some sort of "tail" should be attached to the reactive "head" of the drug molecule , enhancing its activity in the cellular environment , undoubtedly by assisting in its solution and dispersal in blood . Conventional surfactants form "molecular clusters" (micelles) when dissolved in a liquid phase . However , I'm not sure if antimalaria drugs with long tails actually would form micelles when dissolved in human blood and tissue . I'm guessing that optimum molecular tails , based on the results obtained for the remarkably active antimalaria drugs developed so far (several of which are sketched above) , should have a roughly equal HLB . The picture of the proposed drug model that emerges might look something like this :
Although some writers have referred to the peroxide-based antimalaria drugs in military terms , such as a "missile with a toxic payload", I like to think of the idealized model above as a sort of "molecular tadpole", swimming in human blood , which will enter Plasmodia and selectively attack and destroy its basic life processes .
In the following narrative I'll discuss a series of new compounds with various types of "poison packages" intended for interaction with the iron(II) component of heme . The chemically reactive poison packages might also disable other of Plasmodia's cellular components , such as its sensitive enzymes . As a standard tail for all the compounds I will be using the carbitol molecule . I use the word "carbitol" as shorthand for the more formal name , diethylene glycol monoethyl ether ; carbitol is actually a trademarked name ™ ® of the Union Carbide Corporation now part of Dow Chemical Corporation the chemical company which manufactures the industrial solvent in the United States . There are a family of carbitols , including the monomethyl and monobutyl members , but I've arbitrarily selected the monoethyl ether to serve as a tail in the proposed new compounds . I estimate it has a roughly equal hydrophilic and lipophilic balance , has approximately the right chain length , and is obtained from an inexpensive industrial chemical . Ethyl carbitol has a relatively low mammalian toxicity of LD50 = 8.69 g/kg (Merck Index , 14th ed. , 2006 , p. 291) , so its milligram-level residues in the body probably wouldn't be harmful to humans .
In organic chemistry the peroxides and related compounds have traditionally been shunned by most chemists as dangerously unstable materials , prone to explode with tremendous force in an unpredictable manner . Artemisinin and its relatives , with their peroxide group , undoubtedly raised a few eyebrows when their structures were first revealed . I was intrigued when I first read about Vennerstrom's OZ277 , as its pharmacologically active part (pharmacophore) is the ozonide group (sketch above) . The low molecular weight ozonides are notorious for their highly explosive nature . However , the larger , heavier organic ozonides are stabler , probably because the non-oxygen atoms in the structure act as "shock absorbers" , dispersing thermal and mechanical energy imparted to the molecule in an orderly manner . This is probably true for most of the organic peroxides .
I was also surprised to discover the extensive chemical literature on organic peroxides . A remarkable amount of research has been carried out over the last century or so on these forbidding and unforgiving compounds . There is a large , bewildering array of types of organic peroxides ; for example , a Kirk-Othmer Encyclopedia of Chemical Technology review describes fifteen different varities of alpha-oxy peroxides alone ! In this same article (p. 83) it is stated ,
There are more than 65 commercially available organic peroxides in over 100 formulations .......
The peroxide structures below are commonly found in present-day research into new antimalaria drugs :
The "arteflene" mentioned in the sketch is a potent antimalaria drug :
(I understand that arteflene , despite its effectiveness against Plasmodia , has been discontinued by its manufacturer , the Hoffmann-LaRoche Corporation) .
Artemisinin (sketch above) is an a-peroxyketal ; however , there is no particular reason to restrict the design and synthesis of new antimalaria drug candidates to that class , or to any of the others now being investigated , such as the simple cyclic peroxides (1,2-dioxanes) , 1,2,4-trioxanes , 1,2,4,5-tetraoxanes , or ozonides (1,2,4-trioxolanes) .
Vennerstrom(2) has commented that hydrogen peroxide and t-butyl hydroperoxide both have a certain degree of antimalarial activity ; the latter has a measured IC50 = 203-240 µM in vitro against falciparum strains (as can be seen in the sketches of various antimalarial drugs above , the "best" candidates will typically have IC50 values in the nanomolar range) . Perhaps if the hydroperoxide group , which should be as reactive as a peroxide , was used as the "poison package" in the model structure sketched above , it would be a more effective antiplasmodial phamacophore than in hydrogen peroxide and t-butyl hydroperoxide . Here are three suggestions for possible hydroperoxide drug candidates :
and , using the starting material methyl salicylate (naturally-occurring in the flavouring ingredient , oil of wintergreen) :
This would be an appropriate point to comment briefly on the safety aspect of working with hydrogen peroxide , organic hydroperoxides , and peroxides . Throughout this report I indicate the use of hydrogen peroxide of around 30% strength or so . I have used 30% H2O2 in my own research (with an acid catalyst) , and experienced no problems with it . In their "mini-review" of hydrogen peroxide , Fieser and Fieser advise ,
Experiments with sizable amounts of even 30% reagent [hydrogen peroxide] should be carried out behind an explosion-proof safety screen (Reagents for Organic Synthesis , vol. 1 , 1967 , p. 457) .
Sharpless , while reviewing many reactions of t-butyl hydroperoxde , provides useful advice in the safe handling of this versatile organic reagent that would be quite relevant to the preparation of the hydroperoxide drug candidates .
Hiatt specifically cautions ,
......... addition of an alcohol to preacidified concentrated hydrogen peroxide is usually safe ; adding concentrated acid to alcohol / concentrated hydrogen peroxide almost always results in an explosion , particularly if the mixture is partially inhomogeneous and the alcohol is a solid . Safety shields should always be employed in such reactions , and the scale of the reaction should be minimal (p. 29 ; his emphasized words in red) .
As mentioned above , the larger , heavier molecules incorporating OO functionalities are less sensitive to mechanical shock and thermal decomposition than are the smaller , lighter molecules . Nevertheless , researchers working with all such compounds should observe basic safety measures : use a safety screen , wear safety glasses and goggles (and heavy gloves if feasible) , with initial preparations on a semi-micro scale (no more than a gram of product anticipated) , until the stability properties of the material have been determined and its safe handling assured .
Hiatt has pointed out ,Generally speaking , acidified concentrated hydrogen peroxide yields hydroperoxides only from those carbinols that bear three alkyl substituents or one aryl and at least one other alkyl or aryl substituent.......a useful exception is a variation on the Mannich reaction discovered by Rieche and co-workers (p. 33) .
Under acidic conditions the substrate alcohol or alkene is converted into the corresponding carbocation ; and it appears that only the tertiary or aryl secondary carbocations are stable enough to react with the nucleophilic hydrogen peroxide . I chose the carbitol esters as intermediates , since the reaction of a carboxylic ester with two equivalents of a Grignard reagent generally results in the production of a tertiary alcohol . That two identical alkyl or aryl groups are introduced at the hydroperoxide group is also helpful , as no asymmetric carbon atom will be produced there . This is desirable , if possible , since optically active enantiomers often have a highly selective action , as drugs , in biological systems . Frequently one enantiomer is the "bioactive" one , and the other is inactive , and sometimes even harmful , as was the case with the disastrous thalidomide . Achiral syntheses produce 50 : 50 mixtures of both enantiomers , resulting in a theoretical maximum 50% yield of active product . This may or may not be a problem with the highly reactive molecules we are considering here .
In my review of peroxide chemistry I came across a reference to dioxazolidines , which are similar to the ozonides , but substituting an NH for the ether oxygen atom . One such compound , 1,1'-peroxydicyclohexylamine , is a key intermediate in the production of nylon-6 polymer :
Peroxydicyclohexylamine is an a-aminoperoxide , a relative of the a-aminoethers , an obscure , little-studied class of compounds . a-Aminoethers , first prepared around 1921 , are formed by the methylolation of alcohols and secondary amines . Methylolation is a very general type of chemical reaction in which two substrates , AH and BH , combine with the reactive electrophile formaldehyde , such that AH and BH are linked together by a methylene bridge and a water molecule is expelled :
AH + CH2=O + HB ----------------> ACH2B + H2O
The Mannich reaction (1912) is an especially valuable type of methylolation in which carbon-carbon bonds are formed ; for example ,
CH3COCH2H + HCHO + HN(Et)2 ----------------> CH3COCH2CH2N(Et)2 + H2O
a-Aminoethers (and sulfides) are similarly prepared :
ROH + HCHO + HN(Et)2 -----------------> ROCH2N(Et)2 + H2O
Primary or secondary amides can be used in place of the amines . If the AH component is omitted , N-methylolamides are produced in good yields . They can be used to alkylate a wide variety of substrates , a reaction called amidoalkylation .
In the late 1950s German chemists found that hydrogen peroxide and alkyl hydroperoxides (usually t-butyl hydroperoxide) could participate in methylolations as the AH component , with BH amines and amides :
The peroxides , both in the reactions sketched above and in their other reactions described in the chemical literature , are seen to be extraordinarily reactive nucleophiles , and they participate in methylolations with great ease , even requiring some cooling of their reaction mixtures . The common oxygen anions such as hydroxide and acetate are mediocre or weak nucleophiles . The peroxides have a much higher nucleophilicity , probably comparable to that of cyanide and iodide anions . This can be attributed to the alpha effect , in which the lone pair (or pairs) of electrons on the neighbouring (alpha) atom to the nucleophilic atom enhances the nucleophilicity of the latter's electron pair . For example , either of the NH2 groups on the hydrazine molecule , H2NNH2 , would be more nucleophilic than the ammonia molecule , NH3 , which lacks a neighbouring atom with a lone pair of electrons . Similarly , the oxygens in t-butyl hydroperoxide should be more nucleophilic than an alcohol's oxygen atom such as in t-butanol .
A second point of interest raised by one of the reactions sketched above is the strongly acidic reaction medium (2N sulfuric acid) in the last methylolation , that of urea and t-butyl hydroperoxide . a-Aminoethers are very sensistive to dilute acid :
They are very rapidly hydrolyzed in dilute aqueous acid at even room temperature ...... (Stewart and Bradley , p. 4172) .
On the other hand , the a-amidoethers , or at least -peroxides , seem to be quite resistant to hydrolysis even in relatively strong aqueous acid . This is more than just of academic interest . The antimalaria drug candidates must be reasonably stable to both acid and base-catalyzed hydrolysis , since such processes will tend to degrade them as they circulate in human blood and cellular fluids . Vennerstrom(2) prepared a series of a-aminoperoxides by the same methylolation technique used by the earlier German chemists . He found them to be about ten times as effective as t-butyl hydroperoxide against two Plasmodia strains :
However , the lowest IC50 value found for this series of compounds was 2.7 µM , making them far less effective than the artemisinin drugs , whose IC50 values are typically in the nanomolar range . Vennerstrom speculated that the a-aminoperoxides might have been degraded in the acidic environment of Plasmodia's food vacuole :
It is conceivable , however , that amine peroxides 4-8 may be hydrolyzed preferentially at pH 5.2 of the parasite digestive vacuole vs pH 7.4 of the extracellular fluid (p. 66) .
He found that all of his a-aminoperoxide drug candidates were indeed readily hydrolyzed under mild conditions . Since then there apparently hasn't been any further investigation of the potential of the amine peroxides as antimalaria agents . The reported acid stability of the amide peroxides - at least one of them , but this is probably true of the entire class - prompts me to propose the following a-amidoperoxides as candidate compounds for antimalaria evaluation .
First , the ammonia component of the nylon-6 intermediate , 1,1'-peroxydicyclohexylamine , could be replaced by primary amides . Various ketones could be examined in the reaction . Menthone , occurring naturally in oil of peppermint , has a sterically-hindering isopropyl group next to the carbonyl :
The extraordinary nucleophilic strength of the peroxide oxygens permits a reaction even with the usually less-reactive ketones . Since the optically-active menthone with its asymmetric carbons will result in the production of enantiomers , several symmetrical ketones lacking asymmetric centers could also be tried in the synthesis .
The carbitol amide used in the above series of experiments could also be methylolated with a commercially available hydroperoxide such as t-butyl hydroperoxide or cumene hydroperoxide :
CarbitolCH2CONH2 + HCHO + HOO-t-Bu ---------> CarbitolCH2CONHCH2OO-t-Bu + H2O
In the following example a variation of the Mannich reaction is proposed in the synthesis of the first intermediate . In the Mannich reaction a ketone (usually) is methylolated together with a secondary amine , in acidic conditions , to produce a b-aminoketone . In my proposed variation an alcohol - the ethyl carbitol tail - is substituted for the sec-amine . The ketone substrate , acetylacetone , has two very labile methylene hydrogen atoms at C3 which should readily participate in the methylolation :
Acetylacetone is known to react readily with hydrogen peroxide to give a good yield of the 1,2-trioxolane derivative . To avoid the nuisance of isomers , the trioxolane diol could be further condensed with urea in the Rieche reaction , which should produce the symmetrical molecule shown at the left above . This will be the acid-stable a-amidoperoxide . The twin carbitol "tails" are reminiscent of the gemini surfactants , which similarly have twin (although lipophilic) tails . The s-trioxane shown in the sketch is the anhydrous trimer of formaldehyde , which I have found in my own work to be a convenient form of formaldehyde , readily soluble in most organic solvents (although it's somewhat "rubbery" and annoying to scrape out of its storage bottle !) . The polymeric paraformaldehyde powder can also be used in methylolations , but is insoluble and only slowly reacts with , and dissolves in , the reaction mixture .
The compound p-acetamidophenol (yes , the familiar Tylenol ™ ® headache medicine !) might serve as the substrate for an antimalaria drug candidate :
Acetamidophenol , as a laboratory organic chemical , is actually rather inexpensive (according to my Aldrich Catalog Handbook of Fine Chemicals) . Its phenolic hydroxyl group could be alkylated with "carbityl chloride" [1-chloro-3,6-dioxo-octane] , possibly utilizing a phase transfer catalysis procedure , which limits the exposure of base-sensitive parts of the substrate to the alkaline water phase (this was similarly recommended for the alkylation of methyl salicylate , as sketched above for the wintergreen hydroperoxide) . I wonder if the 4-alkylacetamidophenol , with its long carbitol tail , would retain its effectiveness as an antipyretic and analgesic (?) . Condensation of the 4-carbitylacetamidophenol with formaldehyde and t-butyl hydroperoxide in the Rieche reaction would complete the synthesis of the drug candidate compound .
Ridley observed ,
....... the parasite [Plasmodia] requires enormous amounts of glucose uptake from the [human] host to support its growth ....... (p. 690) ,
which suggests another peroxide delivery system worth investigation : glucose ! To the best of my knowledge no peroxide derivatives of glucose have been reported in the literature . On the contrary , all attempts to react glucose (and other simple sugars) with peroxides have resulted in the complete degradation of the sugars into small molecules . Since the one primary and five secondary carbon atoms in the glucose molecule apparently can't support a stable carbocation , we can see from our experience above with the hydroperoxides why a peroxide or hydroperoxide compound with glucose , and indeed any of the sugars , can't be formed . However , a poison package can be attached to the sugar molecule , as an aglycon , in a glycoside derivative . An example is provided below :
The French chemist Glacet and co-workers prepared a-aminoethers and a-amidoethers in a simple procedure and in good yields by reacting various amines and amides with 2-dihydrofuran and 2-dihydropyran , and with their corresponding hemiacetals (eg. 2-hydroxypyran) . It should be possible to similarly condense a primary amide such as acetamide or benzamide with D-glucose , using an acid catalyst , at the anomeric C1 position of the sugar . The hemiacetal hydroxyl group can be displaced in an acid environment by various nucleophiles , for example alcohols , to form glycosides . The a-amido group (the aglycon) would assume the more stable equatorial conformation at C1 , to form the "beta-glycoside" . In the second step , the peroxide poison package is introduced in a Rieche methylolation with formaldehyde and t-butyl hydroperoxide . In a variation , the carbityl amide used in the peppermint peroxide synthesis sketched above could be substituted for the acetamide or benzamide , so the resulting product would have a long carbitol tail . With such drug candidates we could go fishing for Plasmodia , with glucose as the bait and the attached peroxide as the hook !
This web page continues in Part 2 with a discussion of N-O compounds that might have antimalaria activity .
antimalaria drugs : J. Wiesner , R. Ortmann , H. Jomaa , and M. Schlitzer ,"New Antimalarial Drugs", Angew. Chem. Internat. Ed. Engl. 42 (43) , pp. 5274-5293 (2003) ; A. Robert , F. Benoit-Vical , O. Dechy-Cabaret , and B. Meunier , "From Classical Antimalarial Drugs to New Compounds Based on the Mechanism of Action of Artemisinin", Pure Appl. Chem. 73 (7) , pp. 1173-1188 (2001) [available as a free download (PDF, 392 KB) from : http://www.iupac.org/publications/pac/2001/pdf/7307x1173.pdf ] ; L.W. Kitchen , D.W. Vaughn , and D.R. Skillman , "Role of US Military Research Programs in the Development of US Food and Drug Administration-Approved Antimalarial Drugs", Clinical Infect. Diseases 43 (1) , pp. 67-71 (2006) [available as a free download (PDF, 82 KB) from : http://www.journals.uchicago.edu/doi/pdf/10.1086/504873 ] .
antiprotozoal drugs : M. Khaw and C.B. Panosian , "Human Antiprotozoal Therapy : Past , Present , and Future", Clinical Microbio. Rev. 8 (3) , pp. 427-439 (1995) [available as a free download (PDF, 262 KB) from : http://cmr.asm.org/cgi/reprint/8/3/427.pdf ] .
Butler,Wu : A.R. Butler and Y.-L. Wu ,"Artemisinin (Qinghaosu) : A New Type of Antimalarial Drug", Chem. Soc. Rev. 21 (2) , pp. 85-90 (1992) ; D.L. Klayman , "Qinghaosu (Artemisinin) : An Antimalarial Drug from China", Science 228 (4703) , pp. 1049-1055 (1985) .
used together : P.L. Olliaro and W.R.J. Taylor , "Antimalarial Compounds : From Bench to Bedside", J. Experimental Biol. 206 , pp. 3753-3759 (2003) [available as a free download (PDF, 49 KB) , from : http://jeb.biologists.org/cgi/reprint/206/21/3753 ] .
Rosenthal : P.J. Rosenthal , "Antimalarial Drug Discovery : Old and New Approaches", J. Experimental Biol. 206 , pp. 3735-3744 (2003) [available as a free download (PDF, 55 KB) , from : http://jeb.biologists.org/cgi/reprint/206/21/3735 ] .
Ridley : R.G. Ridley , "To Kill a Parasite", Nature 424 (6951) , pp. 887-889 (2003) [available as a free download (PDF, 97 KB) , from http://www.nature.com/nature/journal/v424/n6951/pdf/424887a.pdf ] ; U. Eckstein-Ludwig et al. , "Artemisinins Target the SERCA of Plasmodia Falciparum", ibid. , pp. 957-961 [available as a free download (PDF, 365 KB) , from http://www.nature.com/nature/journal/v424/n6951/pdf/nature01813.pdf ] .
Robert et al. : see above in antimalaria drugs .
E. Niki : E. Niki , "Formations and Reactions of Peroxides in Biological Systems", Ch. 17 , pp. 917-936 in The Chemistry of Hydroxyl , Ether , and Peroxide Groups , S. Patai (ed.) , John Wiley , Chichester (UK) , 1993 .
Vennerstrom(1) : J.L. Vennerstrom et al. , "Identification of an Antimalarial Synthetic Trioxolane Drug Development Candidate", Nature 430 (7002) , pp. 900-904 (2004) [available as a free download (PDF, 365 KB) , from http://www.nature.com/nature/journal/v430/n7002/pdf/nature02779.pdf ] . Discussion of OZ277 : P.M. O'Neill , "A Worthy Adversary for Malaria", ibid. , pp. 838-839 [available as a free download (PDF, 146 KB) , from http://www.nature.com/nature/journal/v430/n7002/pdf/430838a.pdf ] .
1,2,4,5-tetraoxane : R. Amewu et al. , "Design and Synthesis of Orally Active Dispiro 1,2,4,5-Tetraoxanes ; Synthetic Antimalarials with Superior Activity to Artemisinin", Org. Biomol. Chem. 4 (24) , pp. 4431-4436 (2006) ; J.L. Vennerstrom et al. , "Synthesis and Antimalarial Activity of Sixteen Dispiro-1,2,4,5-Tetraoxanes : Alkyl-Substituted 7,8,15,16-Tetraoxadispiro[184.108.40.206]hexadecanes", J. Med. Chem. 43 (14) , pp. 2753-2758 (2000) .
Vial : X.-J. Salom-Roig , A. Hamzé , M. Calas , and H.J. Vial , "Dual Molecules as New Antimalarials", Combinatorial Chem. & High Throughput Screen. 2005 (8) , pp. 49-62 [available as a free download (PDF , 129 KB) , from http://www.bentham.org/cchts/samples/cchts8-1/0007A.pdf ] ; M. Calas et al. , "Antimalarial Activity of Molecules Interfering with Plasmodia Falciparum Phospholipid Metabolism . Structure-Activity Relationship Analysis", J. Med. Chem. 40 (22) , pp. 3557-3566 (1997) ; M. Calas et al. , "Antimalarial Activity of Molecules Interfering with Plasmodia Falciparum Phospholipid Metabolism . Comparison Between Mono- and Bisquaternary Ammonium Salts", J. Med. Chem. 43 (3) , pp. 505-516 (2000) .
simple model : see the reference immediately above , Fig. 4 , p. 511 . Also in Vial's "Dual Molecules" paper , Fig. 2 , p. 54 , and Fig. 13 , p. 60 .
three-dimensional model : A. van der Heijden , "Historical Overview on Structure-Activity Relationships Among Sweeteners", Pure & Appl. Chem. 69 (4) , pp. 667-674 (1997) ; Fig. 3 , p. 672 [available as a free download (PDF, 605 KB) from : http://www.iupac.org/publications/pac/1997/pdf/6904x0667.pdf ] .
several avenues : R.G. Ridley , "Medical Need , Scientific Opportunity , and the Drive for Antimalarial Drugs", Nature 415 (6872) , pp. 686-693 (2002) [Box 2 , "Sites of Drug Action and New Drug Targets", p. 689] [available as a free download (PDF, 390 KB) from : http://www.nature.com/nature/journal/v415/n6872/pdf/415686a.pdf ] ; G. Padmanaban , V.A. Nagaraj , and P.N. Rangarajan , "Drugs and Drug Targets Against Malaria", Current Science , 92 (11) , pp. 1545- 1555 (2007) . [available as a free download (PDF, 218 KB) from : http://www.ias.ac.in/currsci/jun102007/1545.pdf ] . See also the review by Rosenthal , above .
extensive chemical literature : D. Swern (ed.) , Organic Peroxides , vol. 1 (1970) , vol. 2 (1971) , and vol. 3 (1972) , Wiley-Interscience , New York ; E.G.E. Hawkins , Organic Peroxides , Their Formation and Reactions , D. Van Nostrand , Princeton (NJ) , 1961 ; A.G. Davies , Organic Peroxides , Butterworths , London (UK) , 1961 ; S. Patai (ed.) , The Chemistry of Peroxides , John Wiley , Chichester (UK) , 1983 ; ibid. , see above for E. Niki .
alpha-oxy peroxides : C.S. Sheppard and O.L. Mageli , "Peroxides and Peroxy Compounds , Organic", vol. 17 , pp. 27-90 , Kirk-Othmer Encyclopedia of Chemical Technology , third ed. , M. Grayson and D. Eckroth (eds.) , John Wiley , New York (1982) . Table 2 , pp. 30-31 , a list of many different organic peroxides ; Table 16 , p. 82 , a list of commercial organic peroxides .
Vennerstrom(2) : J. L. Vennerstrom , "Amine Peroxides as Potential Antimalarials", J. Med. Chem. 32 (1) , pp. 64-67 (1989) .
Sharpless : K.B. Sharpless and T.R. Verhoeven , "Metal-Catalyzed , Highly Selective Oxygenations of Olefins and Acetylenes with tert-Butyl Hydroperoxide . Practical Considerations and Mechanisms", Aldrichimica Acta 12 (4) , pp. 63-74 (1979) [available as a free download (PDF, 3942 KB the entire issue 4 has to be downloaded if you want the article) from : http://www.sigmaaldrich.com/aldrich/acta/al_acta_12_04.pdf ] .
Hiatt : R. Hiatt , "Hydroperoxides", Ch. 1 , pp. 1-151 in Organic Peroxides , vol. 2 , D. Swern (ed.) , Wiley-Interscience , New York , 1971.
a-aminoethers : C.M. McLeod and G.M. Robinson , Pseudo-bases . III . Dialkylaminomethyl Alkyl Ethers and Sulfides, J. Chem. Soc. 119 , pp. 1470-1476 (1921) ; T.D. Stewart and W.E. Bradley , "The Mechanism of Hydrolysis of Dialkylaminoethyl Ethers", J. Amer. Chem. Soc. 54 (11) , pp. 4172-4183 (1932) .
formaldehyde : J.F. Walker , Formaldehyde , second ed. , Reinhold , New York , 1953 . See also Walt Volland's web page , "Formaldehyde" (Molecule of the Month website) , at : http://www.800mainstreet.com/formaldehyde/form-0.html , and the Wikipedia web page .
Mannich : F.F. Blicke , "The Mannich Reaction", Organic Reactions , Vol. 1 , Ch. 10 , pp. 303-341 , R. Adams (ed.) , John Wiley , New York , 1942 . Three practical examples of the Mannich reaction are cited in b-aminoketone , below .
amidoalkylation : H.E. Zaugg and W.B. Martin , "a-Amidoalkylations at Carbon", Organic Reactions , Vol. 14 , Ch. 2 , pp. 52-269 , A.C. Cope (ed.) , John Wiley , New York , 1965 .
nucleophilicity : M.B. Smith and J. March , March's Advanced Organic Chemistry , Reactions , Mechanisms , and Structure , sixth ed. , John Wiley , Hoboken (NJ) , 2007 ; Table 10.8 , "Nucleophilicities of Some Common Reagents", p. 494 .
alpha effect : Smith and March (above) , p. 495 .
Stewart and Bradley : see above for a-aminoethers .
b-aminoketone : A.L. Wilds , R.M. Nowak , and K.E. McCaleb , "1-Diethylamino-3-Butanone", Org. Synth. Coll. Vol. 4 , pp. 281-282 (1963) [available as a free download (PDF, 121 KB) from : http://www.orgsyn.org/orgsyn/pdfs/CV4P0281.pdf ] ; C.E. Maxwell , "b-Dimethylaminopropiophenone Hydrochloride", Org. Synth. Coll. Vol. 3 , pp. 305-306 (1955) [available as a free download (PDF, 142 KB) from : http://www.orgsyn.org/orgsyn/pdfs/CV3P0305.pdf ] ; E.L. Eliel and M.T. Fisk , "5-Methylfurfuryldimethylamine", Org. Synth. Coll. Vol. 4 , pp. 626-627 (1963) [available as a free download (PDF, 112 KB) from : http://www.orgsyn.org/orgsyn/pdfs/CV4P0626.pdf ] .
gemini surfactants : M.J. Rosen ,"Geminis : A New Generation of Surfactants", Chemtech 23 (3) , pp. 30-33 (March , 1993) .
phase transfer catalysis : E.V. Dehmlow , "Phase Transfer Catalysis", Chemtech , pp. 210-218 (April 1975) ; this is essentially the same article as : idem. , "Phase-Transfer Catalyzed Two-Phase Reactions in Preparative Organic Chemistry", Angew. Chem. Internat. Ed. Engl. 13 (3) , pp. 170-179 (1974) ; R.A. Jones , "Applications of Phase-Transfer Catalysis in Organic Synthesis", Aldrichimica Acta 9 (3) , pp. 35-45 (1976) [available as a free download (PDF document , 3317 KB) at http://www.sigmaaldrich.com/aldrich/acta/al_acta_09_03.pdf ] ; K. Sjöberg , "PTC in Practice", Aldrichimica Acta 13 (3) , pp. 55-58 (1980) [available as a free download (PDF document , 2990 KB) at http://www.sigmaaldrich.com/aldrich/acta/al_acta_13_03.pdf ] ; M. Makosza and A. Jonczyk , "Phase-Transfer Alkylation of Nitriles : 2-Phenylbutyronitrile", Org. Synth. Coll. Vol. 6 , p. 897-900 (1988) [available as a free download (PDF document , 183 KB) at http://www.orgsyn.org/orgsyn/pdfs/CV6P0897.pdf ] .
all attempts : J.H. Payne and L. Foster , "The Action of Hydrogen Peroxide on Carbohydrates", J. Amer. Chem. Soc. 67 (10) , pp. 1654-1656 (1945) . Sugars degrade stepwise , producing HCHO which is then oxidized by the peroxide to formate and - surprisingly - hydrogen , or water . There are earlier references in Chemical Abstracts to studies of the reactions of sugars with hydrogen peroxide ; for example , C.W. Schonebaum , Chem. Abs. 17 , p. 1786 (1923) . However , no mention of the formation of a discrete , well-characterized peroxide compound with a sugar was made in any of these articles .
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