Methylation Dimethyl sulfate

Alkylation of compound 2 has been confirmed at position 1 as predicted for methylation (dimethyl sulfate) and ethylation (triethyloxonium fluoroborate) (93T4307), and in the simple case of compound 193 (94AJC1009), and in the compound 194 (82MI1). The ylide 195 (84JCS(P1)1891) is substituted at position 3. 

The reagent is prepared from (R,R)-( + )-diethyl tartrate by reaction with di-methylamine followed by methylation (dimethyl sulfate and 2,2-dimethoxypro-pane). 

Dihydroxycoumarin (55) underwent selective methylation (dimethyl sulfate) to the 7-methoxy derivative (56), which upon benzylation and oxidation (selenium dioxide), afforded the 4-formyl coumarin (74). Conversion to the acetal (75) occured upon treatment with triethyl orthoformate, and subsequent catalytic hydrogenation served the dual purpose of removal of the benzyl group and reduction of the coumarin double bond, to give (76). Hydride reduction of the derived acetate (77), followed by acidic workup, gave directly the furobenzofiiran (75) [presumably through the hydroxy aldehyde (75)]. Comparison of the spectra of racemic (75) with those of the naturally derived material showed the compounds to be identical. 

Brucine is oxidized by 5 V nitric acid (or chromic acid (159)) at 0-5° to a rose-colored solution from which a red bruciquinone (bis-desmethylbrucine, (C21H20O4N2), XCIII) can be isolated as its perchlorate. It is a typical quinone because it is reduced by sulfurous acid to a hydroquinone, bis-apomethylbrucine (diacetate formation brucine yields two moles of methyl chloride when heated with hydrochloric acid but bis-apomethylbrucine could not be isolated from the reaction mixture (191) ). No other alterations occur in the brucine molecule during these transformations since methylation (dimethyl sulfate) converts bis-apomethylbrucine to the quaternary salt of brucine. The hydroquinone has been oxidized, with... 

The synthesis of Gates and Tschudi (Scheme 13.44) began with 2,6-dihydroxy-naphthalene, which was converted to its monobenzoate and nitrosated. Reduction of the nitroso (H2, Pd/C) yielded the corresponding a-aminophenol, the oxidation of which, with iron(III) chloride, produced a quinone, and reduction of the quinone with sodium hydrosulfite followed by methylation (dimethyl sulfate [(CH3)2S04]) yielded a dimethoxybenzoate. Removal of the benzoate protecting group and repetition of the entire sequence outlined above produced a dimethoxyquinone. 

The most stable protected alcohol derivatives are the methyl ethers. These are often employed in carbohydrate chemistry and can be made with dimethyl sulfate in the presence of aqueous sodium or barium hydroxides in DMF or DMSO. Simple ethers may be cleaved by treatment with BCI3 or BBr, but generally methyl ethers are too stable to be used for routine protection of alcohols. They are more useful as volatile derivatives in gas-chromatographic and mass-spectrometric analyses. So the most labile (trimethylsilyl ether) and the most stable (methyl ether) alcohol derivatives are useful in analysis, but in synthesis they can be used only in exceptional cases. In synthesis, easily accessible intermediates of medium stability are most helpful. 

J. Rebek, Jr., (1987) first developed a new synthesis of Kemp s acid and then extensively explored its application in model studies. The synthesis involves the straightforward hydrogenation (A. Steitz, 1968), esterification and methylation of inexpensive 1,3,5-benzenetricar-boxylic acid (trimesic acid 30/100 g). The methylation of the trimethyl ester with dimethyl sulfate, mediated by lithium diisopropylamide (V. J. Shiner, 1981), produced mainly the desired aff-cis-1,3,5-trimethyl isomer, which was saponified to give Kemp s acid. 

Alkylation of bis(4-methyl-2-thiazolyl)urea (257) with dimethyl sulfate gives product 258 dimethylated on the ring nitrogens (Scheme 154) (488). Alkylation of l-alkyl-3-(2-thiazolyl)urea from its derived anion formed by NaH gives 259 (Scheme 155). 

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). 

QuaterniZation. Quaternary ammonium compounds are formed by alkylation of alkyl, alkyl dimethyl, dialkyl, and dialkylmethyl fatty amines with methyl chloride, dimethyl sulfate, or benzyl chloride (1,3,7,12,29). 

V,/V-dimethy1amino)pheno1 (177). In addition, 3-aminophenol may be methylated with dimethyl sulfate under neutral conditions, or its hydrochloride salt heated with methanol at 170°C under pressure for 8 h to give the desired product (178). The compound is used primarily as an intermediate in the production of basic (Red 3 and Red 11) and mordant (Red 77) dyes. 

Fats, Oils, or Fatty Acids. The primary products produced direcdy from fats, oils, or fatty acids without a nitrile iatermediate are the quatemized amidoamines, imidazolines, and ethoxylated derivatives (Fig. 3). Reaction of fatty acids or tallow with various polyamines produces the iatermediate dialkylarnidoarnine. By controlling reaction conditions, dehydration can be continued until the imidazoline is produced. Quaternaries are produced from both amidoamines and imidazolines by reaction with methyl chloride or dimethyl sulfate. The amidoamines can also react with ethylene oxide (qv) to produce ethoxylated amidoamines which are then quaternized. 

Etherification. The reaction of alkyl haUdes with sugar polyols in the presence of aqueous alkaline reagents generally results in partial etherification. Thus, a tetraaHyl ether is formed on reaction of D-mannitol with aHyl bromide in the presence of 20% sodium hydroxide at 75°C (124). Treatment of this partial ether with metallic sodium to form an alcoholate, followed by reaction with additional aHyl bromide, leads to hexaaHyl D-mannitol (125). Complete methylation of D-mannitol occurs, however, by the action of dimethyl sulfate and sodium hydroxide (126). A mixture of tetra- and pentabutyloxymethyl ethers of D-mannitol results from the action of butyl chloromethyl ether (127). Completely substituted trimethylsilyl derivatives of polyols, distillable in vacuo, are prepared by interaction with trim ethyl chi oro s il an e in the presence of pyridine (128). Hexavinylmannitol is obtained from D-mannitol and acetylene at 25.31 MPa (250 atm) and 160°C (129). 

All lation. In alkylation, the dialkyl sulfates react much faster than do the alkyl haHdes, because the monoalkyl sulfate anion (ROSO ) is more effective as a leaving group than a haHde ion. The high rate is most apparent with small primary alkyl groups, eg, methyl and ethyl. Some leaving groups, such as the fluorinated sulfonate anion, eg, the triflate anion, CF SO, react even faster in ester form (4). Against phenoxide anion, the reaction rate is methyl triflate [333-27-7] dimethyl sulfate methyl toluenesulfonate [23373-38-8] (5). Dialkyl sulfates, as compared to alkyl chlorides, lack chloride ions in their products chloride corrodes and requires the use of a gas instead of a Hquid. The lower sulfates are much less expensive than lower bromides or iodides, and they also alkylate quickly. 

The nitrogen of aHphatic and aromatic amines is alkylated rapidly by alkyl sulfates yielding the usual mixtures. Most tertiary amines and nitrogen heterocycles are converted to quaternary ammonium salts, unless the nitrogen is of very low basicity, eg, ia tn phenylamine. The position of dimethyl sulfate-produced methylation of several heterocycles with more than one heteroatom has been examined (22). Acyl cyanamides can be methylated (23). Metal cyanates are converted to methyl isocyanate or ethyl isocyanate ia high yields by heating the mixtures (24,25). 

Carbon is alkylated ia the form of enolates or as carbanions. The enolates are ambident ia activity and can react at an oxygen or a carbon. For example, refluxing equimolar amounts of dimethyl sulfate and ethyl acetoacetate with potassium carbonate gives a 36% yield of the 0-methylation product, ie, ethyl 3-methoxy-2-butenoate, and 30% of the C-methylation product, ie, ethyl 2-methyl-3-oxobutanoate (26). Generally, only one alkyl group of the sulfate reacts with beta-diketones, beta-ketoesters, or malonates (27). Factors affecting the 0 C alkylation ratio have been extensively studied (28). Reaction ia the presence of soHd Al O results mosdy ia C-alkylation of ethyl acetoacetate (29). 

Sulfates having alkyl groups from methyl to pentyl have been examined. With methyl as an example, the hydrolysis rate of dimethyl sulfate iacreases with the concentration of the sulfate. Typical rates ia neutral water are first order and are 1.66 x lO " at 25°C and 6.14 x lO " at 35°C (46,47). Rates with alkaH or acid depend on conditions (42,48). Rates for the monomethyl sulfate [512-42-5] are much slower, and are nearly second order ia base. Values of the rate constant ia dilute solution are 6.5 X 10 L/(mol-s) at 100°C and 4.64 X 10 L/(mol-s) at 138°C (44). At 138°C, first-order solvolysis is ca 2% of the total. Hydrolysis of the monoester is markedly promoted by increasing acid strength and it is first order. The rate at 80°C is 3.65 x lO " ... 

SuIfona.tlon, Sulfonation is a common reaction with dialkyl sulfates, either by slow decomposition on heating with the release of SO or by attack at the sulfur end of the O—S bond (63). Reaction products are usually the dimethyl ether, methanol, sulfonic acid, and methyl sulfonates, corresponding to both routes. Reactive aromatics are commonly those with higher reactivity to electrophilic substitution at temperatures > 100° C. Tn phenylamine, diphenylmethylamine, anisole, and diphenyl ether exhibit ring sulfonation at 150—160°C, 140°C, 155—160°C, and 180—190°C, respectively, but diphenyl ketone and benzyl methyl ether do not react up to 190°C. Diphenyl amine methylates and then sulfonates. Catalysis of sulfonation of anthraquinone by dimethyl sulfate occurs with thaHium(III) oxide or mercury(II) oxide at 170°C. Alkyl interchange also gives sulfation. 

Miscellaneous. Halogenation of the methyl group of dimethyl sulfate by chlorine and by fluorine has been described (64,65). Reduction of dimethyl sulfate by hydhodic acid occurs in a way that is comparable to that shown by sulfuric acid and results in reduction to sulfur (66). 

The only commercially important dialkyl sulfates are dimethyl sulfate and diethyl sulfate. Estimated worldwide production in 1996 for dimethyl sulfate was 90,000 metric tons per year. Dimethyl sulfate was initially made by vacuum pyrolysis of methyl hydrogen sulfate ... 

Later it was synthesized in a batch process from dimethyl ether and sulfur thoxide (93) and this combination was adapted for continuous operation. Gaseous dimethyl ether was bubbled at 15.4 kg/h into the bottom of a tower 20 cm in diameter and 365 cm high and filled with the reaction product dimethyl sulfate. Liquid sulfur thoxide was introduced at 26.5 kg/h at the top of the tower. The mildly exothermic reaction was controlled at 45—47°C, and the reaction product (96—97 wt % dimethyl sulfate, sulfuhc acid, and methyl hydrogen sulfate) was continuously withdrawn and purified by vacuum distillation over sodium sulfate. The yield was almost quantitative, and the product was a clear, colorless, mobile Hquid. A modified process is deschbed in Reference 94. Properties are Hsted in Table 3. 

An alternative route from l-aminoanthraquinone (17) has been proposed. Methylation is preferably carried out usiag dimethyl sulfate or methyl iodide ia an organic solvent ia the presence of alkah metal hydroxide and a catalytic amount of quaternary ammonium compound (98). 

IV-Methylated pyridazinones can be obtained from 3,6-dialkoxypyridazines by treatment with alkyl halides or dialkyl sulfates. Methyl iodide and dimethyl sulfate are most frequently used. According to the proposed mechanism, an intermediate quaternary pyridazinium salt is formed, followed by elimination of a group R from the alkoxy group. At higher temperature, l,2-dimethylpyridazine-3,6(l//,2//)-dione is formed with dimethyl sulfate. 

When large groups, such as phenyl, bromo, ethoxycarbonyl or nitro are attached at position 3, the principal products are l-alkylcinnolin-4(l/f)-ones. Cyanoethylation and acetylation of cinnolin-4(l/f)-one takes place exclusively at N-1. Phthalazin-l(2/f)-ones give 2-substituted derivatives on alkylation and acylation. Alkylation of 4-hydroxyphthala2in-l(2/f)-one with an equimolar amount of primary halide in the presence of a base leads to 2-alkyl-4-hydroxyphthalazin-l(2/f)-one and further alkylation results in the formation of 4-alkoxy-2-alkylphthalazinone. Methylation of 4-hydroxy-2-methyl-phthalazinone with dimethyl sulfate in aqueous alkali gives a mixture of 4-methoxy-2-methylphthalazin-l(2/f)-one and 2,3-dimethylphthalazine-l,4(2//,3//)-dione, whereas methylation of 4-methoxyphthalazin-l(2/f)-one under similar conditions affords only 4-methoxy-2-methylphthalazinone. 

Alkylation of pyrazinones and quinoxalinones may be carried out under a variety of conditions and it is usually observed that while O-alkylation may occur under conditions of kinetic control, to yield the corresponding alkoxypyrazines or alkoxyquinoxalines, under thermodynamic control the A-alkylated products are formed. Alkylation using trialkyl-oxonium fluoroborate results in exclusive O-alkylation, and silylation under a variety of conditions (75MI21400) yields specifically the O-silylated products. Alkylation with methyl iodide or dimethyl sulfate invariably leads to A-methylation. 

N-Unsubstituted 1,2,3-triazoles are methylated mainly in the 1-position with methyl iodide and silver or thallium salts, but mainly in the 2-position by diazomethane. There is also some steric control. For example, 4-phenyl-l,2,3-triazole with dimethyl sulfate gives the 2-methyl-4-phenyl (38%) and l-methyl-4-phenyl isomers (62%), but none of the more hindered 1-methyl-5-phenyltriazole (74AHC(16)33). JV-Unsubstituted 1,2,4-triazoles are generally alkylated at N-1. 

Dimethyl sulfate in basic medium Methyl iodide in a sealed tube Methyl iodide in aqueous ethanolic potash Dimethyl sulfate in basic medium... 

Treatment of 2-isoxazolines with acid usually leads to ring rupture and formation of chalcone products 62HC(l7)l), although 5-methyl-3-phenyl-2-isoxazoline forms a quaternary salt with dimethyl sulfate in the presence of perchloric acid (Scheme 51) (73BSF1390). 

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