Methylation with diazomethane, mechanism

This mechanism does not require a decision as to the question of whether the association of imidazole occurs through hydrogen bonding or by ionization. - However, if the methylation with diazomethane is considered together with methylations with dimethyl sulfate, dimethyl sulfate and alkali, and methyl iodide and the silver derivative of the imidazole, then such a comparison is best done using the hydrogen-bonded association model. 

Racemic colchiceine was obtained by Corrodi and Hardegger by a base-catalyzed equilibration of the Schiff base obtained by reacting deacetyl-colchiceine with benzaldehyde (34). Aldimine-ketimine isomerization was found to be the mechanism by which the racemization had occurred (35). The optical resolution of deacetylcolchiceine was accomplished with cam-phorsulfonic acids, affording, after O-methylation with diazomethane, separation of the enolate isomers and after N-acetylation, unnatural (+)-and natural (-)- colchicine (Fig. 1). Racemization of colchicine in refluxing acetic anhydride followed by mild hydrolysis of the intermediate triacetate represents a much improved method of preparing ( )-colchicine (36). The Blade-Font procedure was later extended to the preparation of ( )-3-demethylcolchicine and other racemic analogs (5). 

The chemistry of a fourth coenzyme was at least partially elucidated in the period under discussion. F. Lynen and coworkers treated P-methylcrotonyl coenzyme A (CoA) carboxylase with bicarbonate labelled with 14C, and discovered that one atom of radiocarbon was incorporated per molecule of enzyme. They postulated that an intermediate was formed between the enzyme and C02, in which the biotin of the enzyme had become car-boxylated. The carboxylated enzyme could transfer its radiolabelled carbon dioxide to methylcrotonyl CoA more interestingly, they found that the enzyme-COz compound would also transfer radiolabelled carbon dioxide to free biotin. The resulting compound, carboxybiotin [4], was quite unstable, but could be stabilized by treatment with diazomethane to yield the methyl ester of N-carboxymethylbiotin (7) (Lynen et al., 1959). The identification of this radiolabelled compound demonstrated that the unstable material is N-carboxybiotin itself, which readily decarboxylates esterification prevents this reaction, and allows the isolation and identification of the product. Lynen et al. then postulated that the structure of the enzyme-C02 compound was essentially the same as that of the product they had isolated from the reaction with free biotin, but where the carbon dioxide was inserted into the bound biotin of the enzyme (Lynen et al., 1961). Although these discoveries still leave significant questions to be answered as to the detailed mechanism of the carboxylation reactions in which biotin participates as coenzyme, they provide a start toward elucidating the way in which the coenzyme functions. 

Simply heating this white powder with a ketone leads to efficient carboxylation and the unstable keto-acid may be trapped with diazomethane to form the stable methyl ester. The mechanism is presumably very like that drawn above for the transfer of CO2 from carboxybiotin to acetyl CoA, Reactions like this prove nothing about the biochemical reaction but they at least show us that such reactions are possible and help us to have confidence that we are right about what Nature is doing. 

This acid is a mixture of cis- and trans-isomers. By esterifying the acid to the methyl ester with diazomethane a two compound mixture is formed, whereas ester formation under acidic conditions (MeOH/HCl) gives a single compound. This single compound 115 is thought to be the trans-isomer, because isomerization of the mixture of esters obtained by reaction with diazomethane to a single compound is apparently possible by an esterification relactonization mechanism by treating the mixture with acid in methanol. These results are consistent with the formulation of the ester 115 as the trans-isomer. 

The orientation of methylation using diazomethane is not easy to explain since the mechanism is in doubt. One interpretation of the preferential formation of 5-substituted 1-methylimidazoles (with a highly electron-dense potential 5-substituent) is via the initial formation of an ion pair [Im MeN2 ] in which the cationic portion is situated near to the nitrogen adjacent to the substituent. This may be true, but the results obtained with unsymmetrical pyrazoles (79AJC2203) cannot be explained entirely in this way. 

Recently, treatment of 2-methylthiopurine with diazomethane has been shown to give 9-methyl-2-methylthiopurine as the main product accompanied by the 7-methyl derivative, and also by small but significant amounts of the 7,8- and 8,9-dimethyl-2-methylthiopurines (142) and (143), respectively. Experiments showed that the 8-methyl group was introduced prior to N-methylation (79AJC2771). With diazomethane, however, it is again not possible to eliminate a free radical mechanism. 

Reaction of 12 with diazomethane gives an 0-methyl ether (mp 144-145°) and acetylation of 12 in acetic anhydride-pyridine solution for a short time (5 min) gives the 0-acetyl derivative 13 (mp 174-175°). Prolonged reaction of 12 with acetic anhydride-pyridine at room temperature, however, gives the iV-acetyl compound 14 (4). The formation of 14 can be explained by further acetylation of 13 to give the 7-acetoxy compound followed by elimination of acetic acid to give an unsaturated ketone and finally cleavage of the C-9-N bond by the mechanism indicated in 15. 

The conversion from salicylic acid to methyl 2-methoxybenzoate could best be carried by treating salicylic acid with a quantitative amount of diazomethane (CH2N2) This versatile reagent reacts with both carboxylic acids and phenols to give methyl esters and methyl ethers respectively. The mechanism of these conversions is based upon the acidity of carboxylic acids and phenols. (Alcohols normally do not react with diazomethane.)... 

Interactive mechanism for methylation of carboxylic acid with diazomethane... 

Reaction with Diazomethane (Section 17.7B) Diazomethane is used to form methyl esters from carboxylic acids. The mechanism involves protonation of the diazomethane carbon atom by the carboxylic acid to make a methyldiazonium cation, followed by attack of the resulting carboxylate on the methyldiazonium cation to give the methyl ester and Nj. 

Now that we have examined objectively three mechanisms that agree with the experimental results, we should examine the real problem in biotin chemistry. Indeed, in recent years a controversy has arisen regarding the exact site on biotin to which the carboxyl group is attached. During isolation and characterization by Lynen of the relatively unstable free carboxybiotin, particularly at acidic pH, the product was converted to the more stable dimethyl ester with diazomethane (343, 344). This derivative was subsequently identified as r-N-methoxycarbonyl-( + )-biotin methyl ester. The same product was also obtained by enzymatic degradation of enzyme-bound biotin. Figure 7.16 shows some of these transformations. 

A study of the regioselectivity of the 1,3-dipolar cycloaddition of aliphatic nitrile oxides with cinnamic acid esters has been published. AMI MO studies on the gas-phase 1,3-dipolar cycloaddition of 1,2,4-triazepine and formonitrile oxide show that the mechanism leading to the most stable adduct is concerted. An ab initio study of the regiochemistry of 1,3-dipolar cycloadditions of diazomethane and formonitrile oxide with ethene, propene, and methyl vinyl ether has been presented. The 1,3-dipolar cycloaddition of mesitonitrile oxide with 4,7-phenanthroline yields both mono-and bis-adducts. Alkynyl(phenyl)iodonium triflates undergo 2 - - 3-cycloaddition with ethyl diazoacetate, Ai-f-butyl-a-phenyl nitrone and f-butyl nitrile oxide to produce substituted pyrroles, dihydroisoxazoles, and isoxazoles respectively." 2/3-Vinyl-franwoctahydro-l,3-benzoxazine (43) undergoes 1,3-dipolar cycloaddition with nitrile oxides with high diastereoselectivity (90% de) (Scheme IS)." " ... 

Carboxylic acids can be converted to esters with diazo compounds in a reaction essentially the same as lO-II. In contrast to alcohols, carboxylic acids undergo the reaction quite well at room temperature, since the reactivity of the reagent increases with acidity. The reaction is used where high yields are important or where the acid is sensitive to higher temperatures. Because of availability diazomethane (CH2N2) " is commonly used to prepare methyl esters, and diazo ketones are common. The mechanism is as shown in 10-11. 

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