Methyl group reactions

A similar intramolecular oxidation, but for the methyl groups C-18 and C-19 was introduced by D.H.R. Barton (1979). Axial hydroxyl groups are converted to esters of nitrous or hypochlorous acid and irradiated. Oxyl radicals are liberated and selectively attack the neighboring axial methyl groups. Reactions of the methylene radicals formed with nitrosyl or chlorine radicals yield oximes or chlorides. 

Migration of the Methyl Groups. Reactions that involve migration of the methyl groups include isomerization, disproportionation, and... 

Relative rates of reactions of dialkylbenzenes are not easily explained. The presence of p-alkyl-substitution generally causes a large decrease in rate, whereas o-xylene. which should have the same inductive effects as p-xylene, reacts very rapidly. Possibly this rapid reaction of o-xylene is caused by an intramolecular transmetalation between the carbanion adduct with ethylene and the remaining methyl group [Reaction (22)]. 

The pyrrolopyridimidazole (36) from (199 R1 = H R = Me) (Section 3.09.4.3.3) bromi-nates (bromine in chloroform) and acetylates (acetic anhydride) at position 3, but when C-3 is blocked by a methyl group reaction occurs, under the same conditions, at C-8 (77JCS(P1)465). The 5-cyanopyrrolo[2,3-d]pyrimidine (37 R = /3-D-ribofuranosyl) is bromi-nated at C-6 by bromine water <75jcs(Pi)i253). 

UV irradiation of aq solns of TNT has yielded a mixt of products, 15 of which have been identified (Ref 85). The methyl group has undergone reaction in all of these compds (to alcohol, aldehyde, carboxyl, or their derivatives). A nitro group has reacted in some cases this is always ortho and never para to the methyl group. Reaction of TNT which involve the intermediacy of anion (D), such as the reaction with 4-nitroso-... 

The allylic radical formed upon H-abstraction from the methyl group [reaction (60)] has neither reducing nor oxidizing properties, and its yield may be deducted from the difference of the sum of reducing and oxidizing radicals with respect to the total OH yield. A series of such experiments has been carried out (Fujita and Steenken 1981 Al-Sheikhly and von Sonntag 1983 Hazra and Steen-ken 1983), and their results are compiled in Table 10.7. 

An interesting additional aspect offers the reaction of CH2OH with l,3Me2Thy (Schuchmann et al. 1986). Here, not only is an addition to the C(6) position [reaction (197)] observed, but as much as 25% of CH2OH abstract an H atom from the methyl group [reaction (198) see also Leonov et al. 1973 Livneh et al. 1982]. 

A full account of the scope and limitations of the amination chemistry of these ligands has recently appeared [158]. With ligand 15, a number of aminations were conducted at room temperature. In the absence of an ortho substituent, couplings of primary amines with unactivated aryl chlorides at room temperature required 5 mol % catalyst. However, a variety of secondary amines, both cyclic and acyclic, reacted with activated or deactivated aryl chlorides at room temperature. Thirteen examples were demonstrated. The scope of this process was broader, however, when reactions were conducted at 80 or 110 °C. Under these conditions, unactivated aryl chlorides reacted with a variety of amines in high yields. In favorable cases, such as reactions of aryl chlorides bearing one ortho methyl group, reactions of N-methyl... 

Chloroethylation (using acetaldehyde instead of formaldehyde) involves a less reactive electrophile (the positive charge being better delocalized by the methyl group). Reaction of 2-methoxycarbonylthiophene with ZnCl2/HCl and either formaldehyde or acetaldehyde gave the 5-chlo-romethyl and 5-(l-chloroethyl) derivatives in 72 and 32% yields, respectively (62MI2). 

A molecular model shows that approach to the upper face of the double bond is hindered by a methyl group. Reaction with RCO3H occurs at the lower face of the double bond to produce epoxide A. 

For R = hydrogen, methyl or ethyl, the e.s.r. spectrum of the corresponding vinyl radical was observed, showing that no reaction had occurred. This lack of reaction is not surprising as the radicals cannot adopt a suitable configuration for reaction. It also shows that the vinyl radicals are sufficiently isolated from unreacted vinyl halide molecules to prevent the occurrence of intermolecular hydrogen abstraction. However, when R = propyl, a six-membered cyclic transition state can be formed and in fact the spectrum of the vinyl radical is completely replaced by that of the alkyl radical formed by abstraction from the terminal methyl group (reaction 32). 

The oxygen atoms of (27) are, however, quite different. One is next to an axial methyl group and is quite open to attack, the other is hindered by an equatorial methyl group. Reactions of compounds of this series with various aluminum hydrides (HAlBr2, HAlCh and DIBAL-H are best) at low temperatures give good yields of the products expected from the retention mechanism (equation 26 Table 1 l).99,ioo jp. version (equation 27) is obtained with triethylsilane and various Lewis acid partners. 

Methanogens can use only a limited number of Ci compounds and acetate as substrates for growth and methane formation (Reactions 1-11 of Table 1). Very recently pyruvate was shown to be a methanogenic substrate [35]. The substrates can be divided into two groups CH4 is foiTned either by the reduction of CO2 (Reactions 1-4) or by the reduction of a methyl-group (Reactions 5-11) with different electron donors. For a distribution of... 

Those where the substrate and the product have the same electrical state a proton in the substrate being exchanged for a positively charged methyl group (reactions 1-3 and 5). 

Methylation of amines in nucleotides and proteins plays important roles in biological function. Methyl transferases accept a wide range of nucleophiles such as halides, amines, hydroxyls, and enolates [reactions (a) and (b), Scheme 8.6] [42-44], For example, in the biosynthesis of novobiocin, methylation takes place at only one phenolic carbon and not the remaining three hydroxyl groups [45, 46]. On the other hand, methyl transfer to electron-deficient substrates often occurs under radical mechanisms requiring methylcobalamin as the cofactor, as shown in the biosynthesis of fosfomycin, where only one of the two enantiotopic hydrogen was replaced by the methyl group [reaction (c), Scheme 8.6] [47]. 

Reactions 148 and 149 are thought to proceed analogously however, the methide ion must find different reaction pathways to escape as methane from the intermediate complex. It does so by proton abstraction from a methyl group (reaction 151 358->359) or by inducing an SN2 displacement reaction on the methoxyl group (358->330)156b 164b. 

In this reaction the iodine atom in ethyl iodide is replaced by a methyl group. Reactions similar to this are much used in the synthesis of organic compounds. The building up of hydrocarbons in this way was first accomplished by Wurtz, a French chemist, in 1855. The method is usually called the Wurtz synthesis. 

C-, 0-, and A -methylations are encountered frequently in the biosynthesis of secondary metabolites. All appear to involve nucleophilic substitution on the 5-methyl group of 5-adenosyl-L-methionine (7.59) (Scheme 1.12). In biosynthetic feeding experiments it is normal to use [m / /- C]methionine [as (7.55)] as a precursor for methyl groups reaction in vivo with ATP and Mg affords (7.59). Particu-... 

Donors such as pyridine or PPhs catalyzed a rapid isomerization which resulted in nmr equivalence for the methyl groups. Reactions were first... 

Polymerization of 3f4-dibromo-2,5-dimethylphenol (I) provided a route to polymer in which the bromine was only on the aromatic rings and not on the methyl groups (reaction 1). The reaction was found to proceed under conditions. similar to the reaction described by Staffin and Price for the polymerization of 4-bromo-2,6-dimethylphenol, III (reaction 2). Poly(3-bromo-... 

When the 3- and 5-positions in 1,4-dinitropyrazole are blocked by methyl groups, reaction with piperidine or morpholine in simple alcohol solvents ROH... 

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