Agents, alkylating

Alkylating agents are chemically reactive eompounds that combine most readily with nucleophilic centres a fully saturated carbon atom of the alkylating group becoming attached to the nueleophile. 

These are further sub-divided into four eategories, namely  

After the discovery of the antileukemic activity of mustard gas (Cl CH2 CH2)2S Bis- 3-chloroenthyl sulphide (Mustard Gas) in human being, its clinical application for the treatment of neoplasms could not be persued further due to its high toxicity, low solubility in water, oily nature and blister-producing properties. 

Nitrogen mustards were selected for the clinical application for the treatment of neoplasms because they presented fewer problems in handling, besides their respective hydrochlorides and other salts are generally stable solids having low vapour pressure and high solubility in water. 

A few important nitrogen mustards used as antineoplastic agents are discussed below, for instance Mechlorethamine hydrochloride, Mephalan, Cyclophosphamide and Chlorambucil. 

All alkylating agents are toxic to mammals methyl iodide and diazomethane provide examples. However, there are substances with two alkylating groups which are selectively toxic to mammalian tumours. In fact, these nitrogen-mustards as they came to be called, were the first drugs of all anti-cancer drugs. How they were discovered, under wartime conditions about 1942, is told in Section 6.3.6. 

In the S type of reaction characteristic of the nitrogen-mustards, the cation of the drug, e.g. (13.43) which is formed relatively slowly, attacks the anion of the biopolymer rapidly, and the product is neutral alternatively this cation 

A clinically valuable alkylating agent of quite another kind is busulphan 13.46) (1,4-dimethanesulfonyloxobutane), discovered by G. Timmis. It is highly effective in, and almost specific for, chronic myelocytic leukaemia, and has few side effects (Haddow and Timmis, 1953). Its mode of action is apparently the alkylation of, and eventual stripping of sulfur from, cysteine residues of proteins and peptides (Roberts and Warwick, 1961). Busulphan acts, without prior ionization, by an S 2 reaction, i.e. one whose rate depends on the concentrations of both the alkylating agent and the nucleophilic site on the biopolymer. 

Cyclophosphamide can be given orally and produces good clinical results in several malignant conditions including Hodgkin s disease, lymphosarcoma, Burkitt s lymphoma, acute lymphoblastic leukaemia of childhood, and after 

Mitomycin C 4.26) is a violet-coloured antibiotic which mammalian metabolism converts to a bifunctional alkylating agent that irreversibly unites two strands of DNA (Iyer and Szybalski, 1964). It has found limited application in cancer of the stomach and bowel but has a high toxicity for the patient. 

All alkylating agents (e.g. methyl sulphate, diazomethane) are toxic to mammals, but several examples with two alkylating groups show a selective toxicity towards mammalian tumours. The chemical nature of alkylating 

The Sn2 mechanism is a second-order nucleophilic substitution whose rate depends on the concentrations of both the alkylating agent and the nucleophilic site. Busulphan 12.31), which is one of the few agents reacting by this mechanism, achieves this result by a propinquity effect without prior ionization, thus  

Chlormethinum (4.19) is not in circulation long enough to be fully effective. The Japanese drug Nitromin (12.35) was introduced to overcome this disadvantage the slow reduction of the AT-oxide ftmction gradually liberates the true drug (Druckrey, 1955). It is as effective orally as parente-rally. Its action is mild and it is in regular clinical use for the more easily controlled conditions. 

Nitromin is an example of chemical change as a step in the activation of a drug which, imtil activation occurs, exists in a stable and beneficial depot (a principle discussed in Section 3.5). Cyclophosphamide (3.28) ( Endoxan , Cytoxan ) is another pro-drug of this kind. This substance cannot form an aziridiniiun cation, like (/2. j2), until the phosphonolactone ring is opened, as happens in the liver. It can be given orally, and produces very good 

Dimethyl sulfate and diethyl sulfate are commonly used as methylating and ethylating agents, respectively. Again, the possibility of sulfate salts giving rise to these compounds in the presence of methanol or ethanol must be considered. 

Ring opening of an epoxide results in a reactive ion that can alkylate DNA. Quinolines can be metabolised to form an epoxide on the nitrogen ring, although substituents in the 2, 3 and 4 positions reduce the likelihood of this. 

The strained ring in aziridines makes them reactive, with ring opening under attack from any nucleophile to create a new nucleophUe-carbon bond. Aziridine rings can be formed from intramolecular substitution of a halogen atom by the nitrogen in nitrogen mustards. 

Usually, three groups of chemosterilants are distinguished alkylating agents, antimetabolites and miscellaneous compounds. 

13) (Ross, 1962 Rotz and Grundmann, 1957 LaBrecque, 1961). They are prepared by the general method of synthesis of phosphoroamidates and phosphorothio-amidates. 

Oral LDjo values for metepa, TEPA and apholate for rats are 136, 37 and 98 mg/kg, respectively (Gaines and Kimbrough, 1964). 

Characteristic examples of derivatives in which the nitrogen atom of the azi-ridine ring is linked to a carbonyl group are N,N -hcxamethylene-bis(l-aziridinecarboxamide) (14), N,N -p-phenylene-bis(l-aziridinecarboxamide) (ENT-50848, 15) and N,N -l,5-naphthylene-bis(2-methyl-l-aziridinecarboxamide) (ENT-50664, 16) (Ham, 1964 Crystal, 1963 1967 Hendry e/a/., 1951 Eye, 1967 Eye and LaBrecque, 1967). 

Aziridine derivatives where the aziridine groups are linked to nitrogen-containing heterocyclic carrier groups are 2,6-bis(l-aziridinyl)pyrazine (ENT-50457, 17), 4,8-bis(l-aziridinyl)pyrimidino-[5,4d]-pyrimidine (ENT-50792,18) and 2,4,6-tris(l-aziridinyl)-j-triazine (triethylene-melamine, tretamine, 19). 

Alkylation is defined as the replacement of hydrogen on an atom by an alkyl group. The alkylation of nucleic acidi, or proteins involves a substitution reaction in which a nucleophilic atom (nu) of the biopolymer di.splaces a leaving group from the alkylating agent. 

In contrast, sulfur mustard forms the less stable episulfon-ium ion more slowly than this ion reacts with biologlci nucleophiles. Thus, the neighboring-group reaction is rak limiting, and the kinetics are first order.  

Aryl-subslituled nitrogen mustards such as chlorambucil are relatively stable to aziridinium ion fonnation because (he aromatic ring decreases the nucleophilicity of the nitni-gen atom. These mustards react according to first-order ki-netics. The stability of chlorambucil allows it to be taken orally, whereas mechlorethamine is given by intravenous administration of freshly prepared solutions. The Kquiic-ment for fre.shly prepared solutions is bused on (he gnidml decomposition of the aziridinium ion by interaction willi water. 

A somewhat different type of alkylating agent is the N-alkyl- -nitrosourea. Compounds of this cla.ss are unstable in aqueous solution under physiological conditions. They produce carbonium ions (also called carhen mm ions) that can alkylate and isocyanates that can carbamoylate. For example. melhylnitrosourea decompo.scs initially to form iso-cyanic acid and mcthyldiazohydroxide. The latter species decomposes further to methyidiazonium ion and finally to methyl carbonium ion. the ultimate alkylating species.  

Some clinically important alkylating agents arc not active until they have been transformed by metabolic processes. The leading example of this group is cyclopho.sphamide. 

In most cases, olefins are used to only a relatively limited extent for alkylations other than the carbon-carbon type, althou they are employed to make other alkylating agents such as ethyl chloride and isopropyl hydrogen sulfate. 

Alcohols. Methanol and ethanol have long been important alkylating agents, especially for nitrogen bonding. In practically every case, a catalyst is necessary to cause the alkylation to proceed smoothlyvand in many instances, this is a mineral acid. Alcohols are used in the manufacture of ethers, such as ordinary ethyl ether, isopropyl ether, Carbitol, Cellosolve, and naphthyl methyl ether. It should be noted that, although naphthols react with alcohol in the presence of a mineral acid, the aryl alkyl ethers cannot be formed by this reaction in the case of phenols. 

Dimethylaniline is made from aniline and methanol in the presence of a small amount of sulfuric add, whereas diethylaniline is prepared from aniline and ethyl alcohol using hydrochloric add. The ethylation does not proceed so completely as the methylation. 

The alcohols are employed to effect the replacement of aromatically bound halogen atoms by heating in the presence of an alkali. In this way, p-nitrophenetole and o-nitroanisole are made by treating p-nitro-chlorobenzene with ethanol and o-nitrochlorobenzene with methanok  

The lower alcohols have also been employed extensively for the catalytic vapor-phase synthesis of alkylamines and for the alkylation of phenols. 

HPir GH is a 48-year-old woman who presemswidi stage Hi epithelial enocardhoma of the owme She 

Clinical uses Cancer of the testes, Mechlorethamine lymphoma Carmustine and lomustine Busulfan leukemia 

MOA Form intrastrand and interstrand cross-links in DNA which interferes with DNA replication. Usually considered cell cycle  

Pharmacokinetics Only available as IV Mechlorethamine IV only Carmustine renally cleared. Busulfan may give IV or PO  

Adverse reactions Cisplatin severe nausea/ Mechlorethamine vesicant. Myelosuppression Myelosuppression, NA/ 

Alternatively, the drug could link two guanine groups on the same chain such that the drug is attached like a limpet to the side of the DNA helix. Such an attachment would mask that portion of DNA and block access to the necessary enzymes required for DNA function. 

Miscoding due to alkylated guanine units is also possible. The guanine base usually exists as the keto tautomer and base pairs with cytosine. Once alkylated however, guanine prefers the enol tautomer and is more likely to base pair with thymine. Such miscoding ultimately leads to an alteration in the amino acid sequence of proteins and enzymes which in turn leads to disruption of protein structure and function. 

Since alkylating agents are very reactive, they will react with any good nucleophile and so they are not very selective in their action. They will alkylate proteins and other macromolecules as well as DNA. Nevertheless, alkylating drugs have been useful in the treatment of cancer. Tumour cells often divide more rapidly than normal cells and so disruption of DNA function will affect these cells more drastically than normal cells. 

The nitrogen mustard compound mechlorethamine (Fig 6.10) was the first alkylating agent to be used (1942). The nitrogen atom is able to displace a chloride ion intra-molecularly to form the highly electrophilic aziridine ion. Alkylation of DNA can then take place. Since the process can be repeated, cross-linking between chains will occur. 

Cisplatin (Fig. 6.13) is a very useful antitumour agent for the treatment of testicular and ovarian tumours. Its discovery was fortuitous in the extreme, arising from research carried out to investigate the effects of an electric current on bacterial growth. During these experiments, it was discovered that bacterial cell division was inhibited. Further research led to the discovery that an electrolysis product from the platinum electrodes was responsible for the inhibition and the agent was eventually identified as ds-diamino platinum dichloride, known as cisplatin. 

1-methylguanine, and 3-methylthymine. This mechanism is unprecedented because it involves an iron-oxo intermediate to oxidize the methyl substituents that leads to the regeneration of the normal nucleobases [67, 68], 

This then leads on further replication to the replacement of the original GC pair by an AT palr-a base substitution mutation. Whether such mutations need to occur in particular regions of the DNA is not yet clear. 

The hypothesis of the 0-6 guanine Involvement does provide an extra bonus In that a mechanism of action can be postulated, and tested, which allows an explanation of the selective destruction of cancer cells. If the cancer cell becomes so because of its inability to repair the 0-6 guanine lesion caused by a carcinogen then it may also be unable to repair the cisplatin induced damage. But the normal cells have Intact repair mechanisms and can repair the damage prior to DNA replication and 

ACS Symposium Series American Chemical Society Washington, DC, 1980. 

The discovery of a new class of anticancer drugs based on metal complexes affords us a new opportunity to reexamine the problems of cancer chemotherapy. It is obvious that interactions of metals with DM is largely an undeveloped field of study, but is too important to remain so. 

---

CHaSO OfCHjUOSOjCHa. A cytotoxic alkylating agent used clinically to treat leukaemia. 

FS(0)20CH3. Colourless liquid, b.p. 94°C. Functions as a powerful methylating agent, even for amides and nitriles which are not attacked by conventional alkylating agents like dialkyl sulphates. 

ON A alkylating agents, intercalating agents, wrong substrates (Trofan horses, e.g, S-fluorouracil)... 

The above is a general procedure for preparing trialkyl orthophosphates. Similar yields are obtained for trimethyl phosphate, b.p. 62°/5 mm. triethyl phosphate, b.p. 75-5°/5 mm. tri-n-propyl phosphate, b.p. 107-5°/5 mm. tri-Mo-propyl phosphate, b.p. 83-5°/5 mm. tri-wo-butyl phosphate, b.p. 117°/5-5 mm. and tri- -amyl phosphate, b.p. 167-5°/5 mm. The alkyl phosphates are excellent alkylating agents for primary aromatic amines (see Section IV,41) they can also be ua for alkylating phenols (compare Sections IV,104-105). Trimethyl phosphate also finds application as a methylating agent for aliphatie alcohols (compare Section 111,58). 

Other catalysts which may be used in the Friedel - Crafts alkylation reaction include ferric chloride, antimony pentachloride, zirconium tetrachloride, boron trifluoride, zinc chloride and hydrogen fluoride but these are generally not so effective in academic laboratories. The alkylating agents include alkyl halides, alcohols and olefines. 

The formation of isopropylbenzene when n-propyl chloride is employed as the alkylating agent is readily accounted for by the isomerisation of the alkyl carbonium (or alkylium) ion ... 

Esters of /i-toluenesulphonic acid, which are of great value as alkylating agents, may be prepared by interaction of p-toluenesulphonyl chloride and the alcdiol in the presence of sodium hydroxide solution or of pyridine, for example ... 

Studying alkylations, we developed a series of selective ionic alkylating agents. Although Meerwein s trialkyloxonium and dialkoxycar-benium salts are widely used as transfer alkylating agents, they lack selectivity and generally are incapable of C-alkylation. 

In eontrast, dialkylhalonium salts sueh as dimethylbromonium and dimethyliodonium fluoroantimonate, whieh we prepared from excess alkyl halides with antimony pentafluoride or fluoroantimonie acid and isolated as stable salts (the less-stable chloronium salts were obtained only in solution), are very effective alkylating agents for heteroatom eompounds (Nu = R2O, R2S, R3N, R3P, ete.) and for C-alkylation (arenes, alkenes). 

Alkylation with other alkylating agents such as ethyl iodide (43. 180, 181j, chloracetic acid and its esters (182). and dialkylaminoalkylhalides (40.43) occurs also on the ring nitrogen. 

With the more acidic 2-acetamido-4-R-thiazoles. using the weaker base NaOH as condensation agent, a mixture of ring (45) and exocyclic N-alkylation (46) may be observed (Scheme 33) (121). Reaction of 2-acetamido-4-methylthiazole in alcoholic sodium ethoxide solution with a variety of alkylating agents has been reported (40-44). 

The reactivity of sulfathiazoles has been reviewed (65). Methylation in alkaline solution with dimethyl sulfate gives only the ring methylated derivative (85). Mixtures of products are obtained with diazomethane as alkylating agent (see p. 37). Other alkyl halides in aqueous alkali lead also to ring-alkylated products (85. 251, 650. 669-671). 

The most useful synthetic method involves the reaction of A-4-thiazoline-2-thione with the appropriate alkylating agent (see Section I.l.C). An example is given Scheme 54. 

Quaternarj salts are obtained by alkylation of selenazole bases, the heterocyclic nitrogen atom playing the role of nucleophile with regard to the electrophilic carbon of the alkylating, agent. 

Next an alkyl halide (the alkylating agent) is added to the solution of sodium acetylide Acetylide ion acts as a nucleophile displacing halide from carbon and forming a new carbon-carbon bond Substitution occurs by an 8 2 mechanism... 

Alkyl halides by themselves are insufficiently electrophilic to react with benzene Aluminum chloride serves as a Lewis acid catalyst to enhance the electrophihcity of the alkylating agent With tertiary and secondary alkyl halides the addition of aluminum chlonde leads to the formation of carbocations which then attack the aromatic ring... 

We see that a secondary alkyl halide is needed as the alkylating agent The anion of diethyl malonate is a weaker base than ethoxide ion and reacts with secondary alkyl halides by substitution rather than elimination Thus the synthesis of 3 methylpentanoic acid begins with the alkylation of the anion of diethyl mal onate by 2 bromobutane... 

The use of epoxides as alkylating agents for diethyl malonate provides a useful route to y lactones Wnte equations illustrating such a sequence for styrene oxide as the starting epoxide Is the lactone formed by this reaction 3 phenylbutanohde or is it 4 phenylbutanohde ... 

Even the tertiary amine competes with ammonia for the alkylating agent The product is a quaternary ammonium salt... 

The alkyl halide must be one that reacts readily by an 8 2 mechanism Thus methyl and primary alkyl halides are the most effective alkylating agents Elimination competes with substitution when secondary alkyl halides are used and is the only reac tion observed with tertiary alkyl halides... 

Geranyl pyrophosphate is an allylic pyrophosphate and like dimethylallyl pyrophosphate can act as an alkylating agent toward a molecule of isopentenyl pyrophosphate A 15 carbon carbocation is formed which on deprotonation gives/ar nesyl pyrophosphate... 

The hydroxyl groups can be alkylated with the usual alkylating agents. To obtain aryl ethers a reverse treatment is used, such as treatment of butynediol toluenesulfonate or dibromobutyne with a phenol (44). Alkylene oxides give ether alcohols (46). 

Mannich polyacrylamides can react with alkylating agents such as methyl chloride [74-87-3], CH Cl, methyl bromide [74-83-9], CH Br, and dimethyl... 

Tertiary, benzyl, and aHyhc nitro compounds can also be used as Friedel-Crafts alkylating agents eg, reaction of (CH2)3CN02 (2-nitro-2-methyl propane [594-70-7]) with anisole in the presence of SnCl gives 4-/-butylanisole [5396-38-3] (7). SoHd acids, such as perfluorodecanesulfonic acid [335-77-3], and perfluorooctanesulfonic acid [1763-23-1] have been used as catalysts for regio-selective alkylations (8). 

However, strong protic acid catalysts are needed when 7T- or CJ-donor alkylating agents are used to produce carbocationic or highly polarized donor-acceptor-complexes as the reactive alkylating iatermediates ... 

Haloall lation. Haloalkyl groups can be introduced directiy by processes similar to Friedel-Crafts alkylation into aromatic and, to some extent, ahphatic compounds. Because halo alkylations involve bi- or polyfunctional alkylating agents, they must be performed under conditions that promote the initial halo alkylation but not, to any substantial degree, subsequent further alkylations with the initially formed haloalkylated products. 

The most frequentiy used halo alkylating agents are aldehydes and hydrogen haUdes, haloalkyl ethers, haloalkyl sulfides, acetals and hydrogen haUdes, di- and polyhaloalkanes, haloalkenes, haloalcohols, haloalkyl sulfates, haloalkyl -tosylates, and miscellaneous further haloalkyl esters. Haloalkylations include halomethylation, haloethylation, and miscellaneous higher haloalkylations. Under specific conditions, bis- and polyhaloalkylation can also be achieved. 

Selective monohalo alkylations were achieved when chlorobromoalkanes such as l-chloro-S-bromo-S-methjIbutane were used as halo alkylating agents (47). The bromine is replaced preferentially ... 

---