19/3-Methyl group removal

Kazansky et al. reported (over platinum on carbon under identical conditions) about 5% C5 cyclic yield from -alkanes 38), 12% from 3-ethylpentane (79), and 35% from 2,2,4-trimethylpentane 38a, 80). The Cj cyclization of 2,5-dimethylhexane did not take place, presumably because the end methyl group removes carbon atom No. 2 from the surface 81) (Scheme VII). The... 

In the chapter on benzene and in Figure 2—7, you saw that toluene disproportionation yielded both benzene and mixed xylenes. When the catalyst-prompted methyl group removes itself from the toluene, it usually attaches itself to another toluene molecule in a way that it forms xylene. That s transalkylation. The freed methyl group might attach itself momentarily to another free benzene molecule, or it might attach itself to the methyl group of another toluene, forming ethylbenzene. However, the creation of benzene and xylenes predominates, and the combined yields of the two are 92-97%. 

Cyclopropylmescaline (CPM) is a molecule that is very simply mescaline itself, with a methyl group removed from an oxygen atom and a cyclopropyl methyl group put on instead. Might CPM be not only inactive, but actually block the action of mescaline Interesting concept. But it turned out to be entirely wrong. 

This compound was synthesized from intermediates of known structure and hence Bertho s structure for conessine is revised to XXI and conkur-chine would be XXI with the methyl groups removed from the nitrogen and another double bond between the pyrrolidine nitrogen and an adjacent carbon. 

Deuteration of the methyl group removes this splitting. °) Labelled derivative. ... 

The anionic methyl group removes a proton from the alcohol carbon, forming the C=0 7T-bond. Dimethylsulfide departs as a leaving group, resulting in the oxidized product. 

Lithium n-propylmercaptide in hexamethylphosphoramide provides a mild system for the rapid dealkylation of quaternary ammonium salts, with high propensity for methyl group removaL. Substituted amidines have been prepared under very mild conditions from nitriles, halides, and amines with the help of a Lewis acid such as ferric chloride . a,a-Dichloro-/ -ketoesters undergo a rapid fragmentation with nucleophiles, even with relatively weak ones such as diethylamine, and can thus serve as acylating agents... 

Brown and his coworkers had rejected steric hindrance to hydration as a main cause of these ortho effects, because they considered that a more gradual increase in the anomalies should be found . Wepster points out that substitution of a hydrogen atom by a methyl group removes at least one water molecule from the solvation shell, so that for instance in the ion of 2-f-butylaniline the solvation energy will be decreased considerably as compared with the 2-isopropylammonium... 

N-Demethylation. Li- -propylmercaptide in hexamethylphosphoramide provides a mild, rapid, and convenient system for dealkylation of quaternary ammonium salts in excellent yield with high propensity for methyl group removal. - E A soln. of phenyldimethylethylammonium iodide in hexamethylphosphoramide treated at 0-5° with LiH followed by n-propylmercaptan, and stirred 0.5 hr. at 0° under N2 phenylmethylethylamine. Y 97% 94% after 5 min. at 85°. F. e. s. R. O. Hutchins and F. J. Dux, J. Org. Chem. 38, 1961 (1973). 

Six protective groups for alcohols, which may be removed successively and selectively, have been listed by E.J. Corey (1972B). A hypothetical hexahydroxy compound with hydroxy groups 1 to 6 protected as (1) acetate, (2) 2,2,2-trichloroethyl carbonate, (3) benzyl ether, (4) dimethyl-t-butylsilyl ether, (5) 2-tetrahydropyranyl ether, and (6) methyl ether may be unmasked in that order by the reagents (1) KjCO, or NH, in CHjOH, (2) Zn in CHjOH or AcOH, (3) over Pd, (4) F", (5) wet acetic acid, and (6) BBrj. The groups may also be exposed to the same reagents in the order A 5, 2, 1, 3, 6. The (4-methoxyphenyl)methyl group (=MPM = p-methoxybenzyl, PMB) can be oxidized to a benzaldehyde derivative and thereby be removed at room temperature under neutral conditions (Y- Oikawa, 1982 R. Johansson, 1984 T. Fukuyama, 1985). 

The least sterically hindered p hydrogen is removed by the base m Hofmann elim matron reactions Methyl groups are deprotonated m preference to methylene groups and methylene groups are deprotonated m preference to methmes The regioselectivity of Hofmann elimination is opposite to that predicted by the Zaitsev rule (Section 5 10) Elimination reactions of alkyltrimethylammonmm hydroxides are said to obey the Hofmann rule, they yield the less substituted alkene... 

Vindoline [2182-14-1] (45), a monomeric Vinca alkaloid intermediate important in the synthesis of antineoplastic alkaloids, is selectively converted in good yield to 0-desmethylvindoline [68687-22-9] (46) by cultures of Sepedonium chrysospermum (17,25), whereas Streptomyces albogriseolus removes only the A[-methyl group to give (47) (91) (see Chemotherapeutics, anticancer). 

Interest in the synthesis of 19-norsteroids as orally active progestins prompted efforts to remove the C19 angular methyl substituent of readily available steroid precursors. Industrial applications include the direct conversion of androsta-l,4-diene-3,17-dione [897-06-3] (92) to estrone [53-16-7] (26) by thermolysis in mineral oil at about 500°C (136), and reductive elimination of the angular methyl group of the 17-ketal of the dione [2398-63-2] (93) with lithium biphenyl radical anion to form the 17-ketal of estrone [900-83-4] (94) (137). 

In the alcohol oxidations, the sulfonium intermediate (2, nucleophile = R2C(OH)) loses a proton and dimethyl sulfide to give the carbonyl compound (42). The most common mechanism for the decomposition of (2) is attack by a mild base to remove a proton from one of the methyl groups. Subsequent cycHc coUapse leads to the carbonyl compound and dimethyl sulfide (eq. 9) ... 

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