Methyl groups derivatives

From the radioactivity of the polymer and of the original methyl chloride it was found that if there was not more than one 14C atom per polymer molecule, 0.27 of the polymer molecules contained a methyl group derived from the solvent. The authors concluded that the formation of this fraction of molecules had been started by initiation according to equation 10, and that the remainder had been started by transfer with monomer. It has been admitted by the authors (private communication, and in press) that this conclusion is not warranted on this evidence, since alkyl halides such as methyl chloride, are known to act as transfer agents by a reaction which can be represented by Equation 7. At best, the evidence shows that methyl chloride was involved in starting - by initiation and/or transfer - about a quarter of the polymer molecules. The results of further studies with 14CH3C1 and CH336C1 are in process of publication [12]. 

The methyl group derived from acetaldehyde in 10 occupies an equatorial position. Steric arguments can be used to explain this result. supported by the fact Lhat in a similar model system with acetone no spiroaeetali/ation occurs.1 ... 

A combination of propionate and acetate units is used to produce the 14-membered macrocyclic ring of oleandomycin (Figure 3.62) from Strep-tomyces antibioticus, but otherwise many of the structural features and the stereochemistry of oleandomycin resemble those of erythromycin A. One acetate provides the starter unit, whilst seven propionates, via methylmalonyl-CoA, supply the extension units (Figure 3.62). One methyl group derived... 

C-2 of the substituted pyridyl ring in (40) derives from lysine, and C-3 and C-4 from C-2 and C-l of acetate, respectively, but the origin of the remainder is obscure (the O-methyl group derives from methionine). Sources likely to... 

CAB 3c derived from N-tosyl (a5 / )-/3-methyltryptophan catalyzes the Diels-Alder reaction of 2-bromoacrolein and furan with 96 4 enantioselectivity, enabling efficient synthesis of numerous chiral 7-oxabicyclo[2.2.1]heptene derivatives (Eq. 17) [18]. Interestingly, the analog 3b of catalyst 3c which lacks the y8-methyl group (derived from M-tosyl-(5)-tryptophan) is not as effective at catalyzing the formation of Diels-Alder product, and the reaction rate and yield are considerably lower. 

Figure 25.13. Thymidylale Synthesis. Thymidylate synthase catalyzes the addition of a methyl group (derived from TV 5 N 10-methylenetertahydrofolate to dUMP to form TMP. The addition of a thiolate from the enzyme activates dUMP. Opening the five-membered ring of the THF derivative prepares the methylene group for nucleophilic attack by the activated dUMP. The reaction is completed by the transfer of a hydride ion to form dihydrofolate.

Monensin A 41 is derived from five acetate, seven propionate and one butyrate units, with an extra 0-methyl group derived from L-methionine [76, 80]. The NAC thioester of a postulated triketide chain elongation intermediate has been incorporated intact into monensin A by Patzelt and Robinson. This experiment required the extremely careful use of several p-oxidation inhibitors and other additives to minimise precursor degradation [77]. 

The quinonoid form of the pteridine might also participate in the flavin requiring methylation of ribothymidine by 5,10-methylene-H4-folate in the tRNA of Streptococcus faecalis [66]. In this case the methyl group derives its third hydrogen from solvent and not the C-6 position of the folate, so that the reduced flavin may trap the quinonoid cofactor of the ion pair before internal hydrogen transfer from C-6. 

The final component of the clavine intermediates is the N(6)-linked methyl group, derived from the iS -methyl group of S -adenosyfinethionine (AdoMet) (77). 

The remethylation cycle allows the conversion of homocysteine back to methionine by two pathways. The first and major pathway is catalyzed by the enzyme, methionine synthase, and links the folate cycle with homocysteine metabolism. Methionine synthase requires the cofactor, meth-ylcobalamin. The second pathway utilizes the enzyme, betaine-homocysteine methyltransfer-ase [8]. This pathway remethylates homocysteine using a methyl group derived from betaine, formed via oxidation of choline, and is presumably responsible for up to 50 % of homocysteine remethylation [10]. Both methionine and homocysteine play important roles in protein synthesis, folding, and function. 

Alkylation of the side chain may occur at C-24 with successive methyl groups derived from 5-adenosyl methionine. The first addition is catalyzed by a sterol-5-adenosylmethio-nine methyl transferase (Goad, 1991b Goodwin, 1985). The side chain of -sitosterol (4) may be derived as in Fig. 23.12. 

In the course of isotopic labeling studies aimed at examining the origin of the isoprene units in the paraherquamide structure, Williams et al. discovered an unexpected stereochemical distribution of the geminal methyl groups derived from DMAPP [42]. These workers carried out feeding experiments in Penicillium... 

The NMR spectrum of the salt obtained from [6-16] upon reaction with silver perchlorate shows only one type of methyl group. This is not noted in the spectrum of the analogous salt protonated with HBF4. Since inversion does not occur, the NMR spectrum reveals two types of methyl groups derived from this syn-anti derivative, equation (6-29). 

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