Methylation of bases

Methionine, which is involved in methyl group donation for nucleotide synthesis, methylation of bases in DNA (Chapter 20) and conversion of choline to ethanolamine for membrane synthesis (Chapter 11). 

In both eukaryotic and prokaryotic cells, large precursors to rRNA are transcribed and processed to produce the mature rRNAs. Processing involves methylation of bases and/or ribose and endolytic cleavage to cut out unwanted sequences. The eukaryotic products are the 18, 5.8, and 28 S rRNAs. In prokaryotes, the final products are the 16 S rRNA, a spacer region that includes one or two tRNAs, the 23 S rRNA, and the 5 S rRNA and, in some instances, one or two additional tRNAs. 

Eukaryotes Regulation of gene expression at the level ofDNA. In eukaryotes, activation of a gene requires changes in the state of chromatin (chromatin remodeling) that are facilitated by acetylation of histones and methylation of bases. These changes in DNA determine which genes are available for transcription. 

Methionine is an essential amino acid with a unique role in the initiation of protein synthesis, hi addition, by conversion to 5 -adenosyhnethionine, it serves as the major methyl group donor involved in the formation of creatinine and choline, in the methylation of bases in RNA, and as the source of the aminopropyl group in the formation of polyamines. Finally, in relationship to classical homocystinuria, it is converted by way of homocysteine and cystathionine in a series of reactions termed as the transsulfuration pathway (Fig. 20.3). 

In the methylation of DNA, specific methylases catalyse transfer of methyl groups from SAM to the 6-amino groups of adenine residues and C5 of cytosine. A specific pattern of methylation serves to protect DNA from the cell s own Restriction endonucleases (see) these enzymes destroy the DNA of invading viruses. The DNA of viruses that are able to replicate within a particular host are protected from the endonucleases by methylation of bases at the endonuclease-sensitive sites. 

Another circumstance which could change the most commonly observed characteristics of the two-stage process of substitution has already been mentioned it is that in which the step in which the proton is lost is retarded because of a low concentration of base. Such an effect has not been observed in aromatic nitration ( 6.2.2), but it is interesting to note that it occurs in A -nitration. The A -nitration of A -methyl-2,4,6-trinitroaniline does not show a deuterium isotope effect in dilute sulphuric acid but does so in more concentrated solutions (> 60 % sulphuric acid kjj/kjj = 4 8). ... 

The nucleophilic reacbvity of the C-5 oxygen is well documented however, no quantitative data are available. A-2-Thiazoline-5-ones (212) react at oxygen with acetyl chloride or acetic anhydride (447. 452). benzoyl chloride (447). methyl or phenyl isocyanate (467). carbamoyl chloride (453, 467). or phosphorus derivatives (468, 428) in the presence of bases to give 213, 214, 215. or 216 (Scheme 109). Strong bases such as... 

TABLE 1-60. METALATION AND METHYLATION OF 171b WITH BASES OF INCREASING STERIC BULK (444)... 

A primary isotope effect /ch/ d of 6.4 (extrapolated for 35 C) is observed for the metalation and the methylation of 171b when the C-5 position is deuterated. This value is in excellent agreement with the primary isotope effect of 6.6 reported for the metalation of thiophene (392) and it confirms that the rate-determining step is the abstraction by the base of the acidic proton. 

In Table III-33 results for the methylation of thiazoles in acetic acid are given (lead tetraacetate is used as radical source), but in this case some discrepancies appear, the acidic medium being too weak, and the heterocyclic base not fully protonated. Thiazole has also been methylated by the DMSO-H2O2 method (201), and the results are in agreement with those described previously. 

Methyl ketones are cleaved on re action with excess halogen in the presence of base The products are a trihalomethane (haloform) and a carboxylate salt... 

Heteroatom functionalized terpene resins are also utilized in hot melt adhesive and ink appHcations. Diels-Alder reaction of terpenic dienes or trienes with acrylates, methacrylates, or other a, P-unsaturated esters of polyhydric alcohols has been shown to yield resins with superior pressure sensitive adhesive properties relative to petroleum and unmodified polyterpene resins (107). Limonene—phenol resins, produced by the BF etherate-catalyzed condensation of 1.4—2.0 moles of limonene with 1.0 mole of phenol have been shown to impart improved tack, elongation, and tensile strength to ethylene—vinyl acetate and ethylene—methyl acrylate-based hot melt adhesive systems (108). Terpene polyol ethers have been shown to be particularly effective tackifiers in pressure sensitive adhesive appHcations (109). 

Ciesol and xylenol can be prepared by the methylation of phenol with methanol over both acid and base catalysts. It is postulated that phenol methylation on acid catalysts proceeds through the initial formation of anisole (methoxybenzene [100-66-3]) followed by intramolecular rearrangement of... 

An asymmetric synthesis of estrone begins with an asymmetric Michael addition of lithium enolate (178) to the scalemic sulfoxide (179). Direct treatment of the cmde Michael adduct with y /i7-chloroperbenzoic acid to oxidize the sulfoxide to a sulfone, followed by reductive removal of the bromine affords (180, X = a and PH R = H) in over 90% yield. Similarly to the conversion of (175) to (176), base-catalyzed epimerization of (180) produces an 85% isolated yield of (181, X = /5H R = H). C8 and C14 of (181) have the same relative and absolute stereochemistry as that of the naturally occurring steroids. Methylation of (181) provides (182). A (CH2)2CuLi-induced reductive cleavage of sulfone (182) followed by stereoselective alkylation of the resultant enolate with an allyl bromide yields (183). Ozonolysis of (183) produces (184) (wherein the aldehydric oxygen is by isopropyUdene) in 68% yield. Compound (184) is the optically active form of Ziegler s intermediate (176), and is converted to (+)-estrone in 6.3% overall yield and >95% enantiomeric excess (200). 

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 pyrimidin-2(or 4)-amine on a ring-nitrogen gives an imine, e.g. (8), of quite high basic strength (pjSTa 10.7) because its cation, e.g. (13 R = Me), has typical and effective resonance stabilization indeed, methylation of pyrimidine-2,4-diamine gives a still stronger base (pjSTa> 13) due to an even more resonance-stabilized cation (14). 

The 4-thioxoazetidin-2-one (281) (c/. Section 5.09.3.3.5) is reported to undergo exclusive 5-alkyIation on treatment with methyl iodide in the presence of base to yield the 2-methylthio-l-azetin-4-one (282) (80TL4247). 

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