Rearrangements methyl group

BUTENE. As shown in Figure 38, a group attached to C-1 can migrate from position 1 to 3 (1,3 shift) to produce an isomer. If it is a methyl group, we recover a 1-butene. If it is a hydrogen atom, 2-butene is obtained. A third possible product is the cyclopropane derivative. The photochemical rearrangement of 1-butene was studied extensively both experimentally [88]... 

Thiazole-N-oxides are prepared by the action at low temperature (-10°C) of hydrogen peroxide in acetic acid (474). 4-MethyIthiazole and 2,4-dimethylthiazole afforded the corresponding N-oxides with yields of 27 and 58%, respectively (Scheme 88). Thiazole-N-oxides without a methyl group in the 2-position are so unstable that they have a tendency to form 2-hydroxythiazoles and are decomposed by oxidation, whereas a 2-methyl group would prevent such rearrangement (474). 

Sesquiterpenes are formed by the head-to-tad arrangement of three isoprene units (15 carbon atoms) there are, however, many exceptions to the rule. Because of the complexity and diversity of the substances produced in nature, it is not surprising that there are many examples of skeletal rearrangements, migrations of methyl groups, and even loss of carbon atoms to produce norsesquiterpenoids. 

When alkoxypyridazine 1-oxides are heated alone or in the presence of p-toluenesulfonic acid the methyl group migrates from the methoxy group to the A-oxide group. In this manner, 4-methoxypyridazine 1-oxide rearranges to l-methoxypyridazin-4(l//)-one, 5-methoxypyridazine 1-oxide to 2-methylpyridazin-5(2//)-one 1-oxide and substituted 3,6-dimethoxypyridazine 1-oxides to l,3-dimethoxypyridazin-6(l//)-ones. 

The chemical behaviour of the mesoionic pyrazole (459) has been studied by Boyd et al (74JCS(P1)1028). Protonation and alkylation take place on the exocyclic nitrogen atom and a thermal rearrangement of a methyl group is observed when (459) is boiled in benzonitrile for several hours giving (460). 

The 1-azirines obtained from the vapor phase pyrolysis of 4,5-disubstituted 1-phthalimido-1,2,3-triazoles (157) have been found to undergo further thermal reactions (71CC1S18). Those azirines which contain a methyl group in the 2-position of the ring are cleaved to nitriles and phthalimidocarbenes, whereas those azirines which possess a phenyl substituent in the 2-position rearrange to indoles. 

All rearrangements of 20-oximino steroids have thus far given only the product arising from migration of the large steroid residue rather than the smaller C-21 methyl group. This is a consequence of the fact that only the oxime with the hydroxyl furthest removed, and therefore trans, to the bulk of the steroid, is formed. 

The thermal lead tetraaeetate method has no practical value for the substitution of the angular methyl groups from 11/5-hydroxy steroids. The major product is either the 11-ketone" or the lla,l-ether formed by rearrangement if ketone formation is suppressed. " ... 

FIGURE 18.29 Vitamin B19 functions as a coenzyme in intramolecular rearrangements, reduction of ribonucleotides, and methyl group transfers. 

The pyrimidine compounds are known to undergo a rearrangement of the 0-alkyl derivatives to the iV-alkyl ones. The methoxy derivatives of 1,3,5-triazine display a similar behavior. On applying methyl iodide to 2,4-dimethoxy-l,3,5-triazine one of the methyl groups is shifted giving rise to l-methyl-4-methoxy-derivative (22). This compound was also obtained by methylation of 4-methoxy-2-oxo-1,2-dihydro-1,3,5-triazine (18) with diazomethane. At higher temperature (100°C) in presence of methyl iodide a shift of both methyl groups takes place and methiodide is formed simultaneously (23). Similarly,... 

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