Elimination methanol

Bischler-Napieralski reaction conditions can be attributed, again, to the destabilizing ability of the trifluoromethyl group to the cationic transition state of the acid catalyzed elimination. Formation of compound 29 was ultimately accomplished by base catalyzed methanol elimination-conditions conditions that are quite unusual for isoquinoline formation. 

Reaction of 2,3-dimethyl- 1-butene with HBr leads to an alky) bromide, CfcH Br. On treatment of this alkyl bromide with KOH in methanol, elimination of HBr to give an alkene occurs and a hydrocarbon that is isomeric with the starting alkene is formed. What is the structure of this hydrocarbon, and how do you think it is formed from the alkyl bromide ... 

Boruah, R. C. Skibo, E. B. Determination of the pKa values for the mitomycin C redox couple by tritration, pH rate profile, and Nemst-Clark fits. Studies of methanol elimination, carbocation formation, and the carbocation/quinone methide equilibrium. J. Org. Chem. 1995, 60, 2232-2243. 

Danishefsky dienes [98] cycloadd to Cjq in refluxing toluene or benzene [5, 38, 99-101]. The diene 103 adds in 60% yield to Cjq to give the desilylated ketone 104 [5,101]. Acid-catalyzed methanol elimination then furnishes the enone 105 in 82% yield (Scheme 4.17). As already described, this enone can be reduced by DIBAL-H to the corresponding alcohol for further functionalization. The same a,(3-un-saturated alcohol can also be obtained in better yield by Diels-Alder reaction of Cg0 with butadiene, followed by oxidation with singlet oxygen to the allylic hydroperoxide and PPhj reduction to the desired alcohol [101]. This sequence yields the allylic alcohol in 53%, starting from Cjq without the need of isolating intermediates. 

In contrast, the regioselectivity of the reaction with acetoacetaldehyde dimethylacetal under the same conditions is poor, resulting in 67% 5-methyl-TP (89JHC1393). In the presence of sodium ethoxide, the reaction leads to 5-methyl-TP exclusively. Here, in the present authors opinion, the regioselection may be controlled by the reaction of the AT anion with the product of base-induced methanol elimination from the keto acetal. 

The quaternary ammonium salt (29) was prepared from 2-methyl-3-carbethoxyfuroic acid. Similarly, the acid-catalyzed methanol eliminations from 2,5-dimethoxyfurans are 1,4 elimination processes.238 242 243 They will be discussed in Section VIII. 

Treatment begins with supportive care, and enteric suction is useful only if recent ingestion or retained gastric methanol is suspected. To prevent formation of toxic metabolites, fomepizole (4-methyl pyrazole or 4-MP) or ethanol to inhibit ADH should be administered. Fomepizole is preferred over ethanol if available [93]. It is well tolerated and may improve initial visual defects [94]. Mild methanol poisoning with levels below 20 mg/ dL can likely be treated with fomepizole only and bicarbonate however a methanol elimination half-life of over 50 hours with fomepizole requires prolonged infusion and monitoring [95]. Folic or folinic acid should be administered to promote conversion of formate to water and carbon dioxide [96]. 

In this reaction the 2-carbon of the allene is protonated by hydrochloric acid, the resulting carbocation 35 reacting with one molecule of water to give the tetrahedral intermediate 36. This forms the a,(3-unsaturated ketone 23 upon tautomerization, methanol elimination and regeneration of the initial proton in quantitative yield. 

Addition to the double bond of enecarbamates is observed during anodic oxidation in methanol in presence of an ammonium halide, leading to the product of halomethox-ylation [Eq. (28)] [155]. The enecarbamates are available through electrochemical a-meth-oxylation of the carbamate followed by methanol elimination (see Sec. VIII.B). 

The methoxylation of A-ethyl carboxamides has been studied especially intensively, as the produced A-(a-methoxyethyl)amides may be transformed to the technically interesting vinyl amides by methanol elimination [209]. Glassy carbon anodes together with graphite or steel cathodes and tetramethylammonium methyl sulfate as conducting salt proved to be the best conditions. Alternatively, the SPE cell using Nafion as polyelectrolyte is practicable [218,219]. 

Other fragments seen in the pyrogram of poly(methyl methacrylate) and given in Table 6.7.20 include smaller fragments than the monomer. They are the result of backbone fragmentation followed by other reactions such as methanol elimination and decarboxylation of the resulting acid. Some examples of small molecules are shown below ... 

A similar removal of benzeneselenenic acid from l,l-dichloro-la-phenylseleninyl-la,9b-di-hydro-l//-cyclopropa[/]phenanthrene (2) at ambient temperature was used to generate 1,1-dichloro-l//-cyclopropa[/]phenanthrene as a short-lived intermediate which was trapped by addition of methanol.Elimination of the elements of HSe Mcj from 3, derived from la-methylselenacyclopropa[/]phenanthrene and la,9b-dihydro-l//-cyclopropa[/]phenanthrene after alkylation, gave a mixture of the alternative cyclopropene, which were both trapped with furan. 

The analysis of NMR spectra of high resolution in solid phase as well as the results of mass-spectrometry investigation of water and methanol desorption from trimethylsilylated aerosil surface with no OH-groups [64,115] testify a nonhydroxylic nature of the sites of adsorption of small water amounts. A correspondence has been fixed of three characteristic regions of the sorbed water and methanol elimination with desorption maxima related to probable destruction of hydrogen-bonded (low-temperature maximum) and coordinate adsorption complexes of various structure (high-temperature maximum) by the help of desorption-field mass-spectrometry [116,117]. 

Methyl 6-ketopentadecanoate (Figure 2.75) also gives a spectrum exhibiting the recurrence of methanol elimination. The molecular ion, an odd-electron ion at miz... 

When 35a (100 °C) and 36a (70 °C) are heated under vacuum, they slowly eliminate water and methanol, respectively, to form 33a (ca. 35% conversion of 35a in 1 h). Thus, the oxidative addition of water or methanol to 33a and 33b to afford 35a, 35b, 36a, and 36b is reversible and methanol elimination proceeds more readily than water elimination. 

The iridium amido complexes (255) without P ligands, prepared from (256) and (257) by methanol elimination in THF, showed excellent enantioselectivities and activities in the asymmetric hydrogenation of simple ketones to alcohols. ... 

Methyl ( )-fran5-4-Hydroxy-2,3-dimethyl-2-cyclohexenecarboxylate heated 14 hrs. with a mixture of N,N-dimethylacetamide dimethyl acetal and its methanol elimination product 1-methoxy-l-dimethylaminoethene at 140° in xylene -> product. Y 76%. Also with the ci5-isomer and f. e. s. A. E. Wick et al., Helv. 47, 2425 (1964). 

The investigation by DFT theory, including solvent effects (molecular solvents and ILs), shows that these cycloadditions proceed by a concerted but asynchronus reaction mechanism. The lowest activation energies for concerted reactions are obtained. However, the stepwise additions have significantly lower activation energy lead to substantially less stable products. Moreover, the primary cycloadducts could never be isolated but were converted into 5-hydroxybenzofurans by subsequent extrusion of nitrous acid, hydrolysis of the silyl enolether, and elimination of methanol. Elimination of nitrous acid is calculated to have lower overall barriers than cycloaddition reactions and is strongly exothermic, thus explaining the preferred reaction pathway. 

Enders et al. isolated the first triazol-5-ylidene 68 by thermal methanol elimination from 5-methoxytriazoline in 1995 (see Scheme l.lOa). 68 was stable up... 

The human toxicology of methanol has been studied [6,71,72]. The skin absorption rate has been reported to be 0.19 m cm2/min [73]. Methanol vapor uptake by the lungs is effective, usually 7080% (74). In the liver, methanol goes through oxidation metabolism catalyzed by alcohol dehydrogenase (an enzyme), producing toxic formaldehyde and formic acid. The accumulation of formic acid leads to acidosis, dama g the nervous system, particularly the optic nerves, and the retina. In the copresence of ethanol, ethanol is selectively metabolized by alcohol dehydrogenase over methanol this delays methanol intoxication and allows detoxication by the natural elimination of methanol via respiration and urination. The methanol elimination half-life is about 23 h [6]. Because of the slow elimination, methanol can be re rded as a cumulative poison [68]. Chronic oq)osure may result in sufficient methanol accumulation in the body, and illness. 

Scheme 3.62) (66, 71), Reduction of the amide followed by protection of the indole nitrogen in the tetracyclic lactam 354 with subsequent hydrolysis of the enol ether function afforded ketone 364. Isomerization to give the corresponding a,)8-unsaturated ketone was followed by treatment with basic hydrogen peroxide to provide epoxy ketone 365. Epoxide opening followed by methanol elimination afforded a-methoxy enone 366 which was depro-tected to yield 367. 

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