Rearrangements protic acid catalyzed

Let us turn to experiments that probe in more detail the mechanism of this tetrahydrofuran synthesis. Three mechanistic possibilities are suggested in Figure 7 and these parallel mechanisms we recently considered for the formation of 3-acylpyrrolidines from protic acid-catalyzed rearrangement of 5-vinyloxazolidines (Figure 2, X = NR). [8] The first two possibilities seem to us most likely and one approach to probing these is to examine the stereochemical outcome of the... 

The Curtius rearrangement can be catalyzed by Lewis acids or protic acids, but good yields are often obtained also without a catalyst. From reaction in an inert solvent (e.g. benzene, chloroform) in the absence of water, the isocyanate can be isolated, while in aqueous solution the amine is formed. Highly reactive acyl azides may suffer loss of nitrogen and rearrange already during preparation in aqueous solution. The isocyanate then cannot be isolated because it immediately reacts with water to yield the corresponding amine. 

Aza-Cope rearrangement.7 This reaction proceeds readily when catalyzed by Pd(0) and a strong protic acid, particularly trifluoroacetic acid or methanesul-fonic acid. This isomerization is involved in a direct synthesis of S,e-unsaturated imines from allylamines with carbonyl compounds (equation I). The unsaturated... 

Nafion-H is a convenient acid catalyst for pinacolone rearrangements. Hydration of acetylenes can be conducted with Nafion-H impregnated with mercury(II) ions. Diels-Alder catalyst. This protic resin catalyzes Diels-Alder reactions, but longer reaction times are needed than in reactions catalyzed by Lewis acids. The reactions are generally conducted in refluxing benzene or chloroform. In the case of dienes that polymerize readily, the reaction is conducted at room temperature for 1-2 days.°... 

The presentation of the rearranging intermediates as mesoionic ground-state species (102 and 106) has gained wide acceptance as a tool for interpretation and prediction of structural changes in this field. The intervention of such intermediates is suggested by the facile incorporation of nucleophilic solvent molecules in the course of the photochemical transformation of dienones, and by a number of acid-catalyzed non-photolytic reactions which either parallel the phototsomerizations or correlate photoproducts from reactions in protic and aprotic media. 

In protic solvents, the allylic carbonium ion formed by acid-catalyzed alkyl carbon-oxygen bond fission can recombine either with the carboxylic acid molecule or with a solvent molecule. The electrostatic attraction between the carbonium and carboxylate ions, which is a major factor in isomerization of allylic esters by ion-pair internal return during solvolysis, is absent in the acid-catalyzed reaction. The more numerous, usually more nucleophilic, solvent molecules in the solvation shell of the carbonium ion should compete effectively with the departed carboxylic acid molecule and solvolysis rather than isomerization should be the predominant reaction. For example, in the presence of 0.05 M perchloric acid, solvolyses of cis- and //- //7.s-5-methyl-2-cyclohexenyl p-nitrobenzoates are not only very much faster than in the absence of the acid, but polarimetric and titrimetric rates of solvolysis of optically-active esters were identical within experimental error. For these esters, the acid-catalyzed solvolysis was not accompanied by a detectable amount of isomerization. Braude reported, on the basis of indirect evidence, that isomerization accompanies acid-catalyzed hydrolysis of a-ethynyl-y-methylallyl acetate in aqueous dioxane. It was shown that, under some experimental conditions, the spectrophotometrically determined rate of appearance of the rearranged 1 -yne-3-ene chromophore exceeds the titrimetrically determined rate of hydrolysis,... 

It has been suggested that both types of compounds, 101 and 102, are originating from the ANRORC reactions through intermediacy of the common open-chain intermediate 103. In protic solvents the acid-catalyzed cyclization appears to occur, thus leading to the Dimroth rearrangement product 101 (Scheme 64, Path A). In aprotic DMF the alkylamino group of 103 is active enough for the... 

Bronsted Acids. Sulfuric acid (H2SO4) is an inexpensive, easy to handle protic acid used widely as catalyst in hydrolysis, hydration and dehydration, elimination, substitution, and rearrangements. It also catalyzes aromatic electrophilic substitutions mostly Friedel-Crafts acylations and alkylations (22). A very important application of sulfuric acid is its use in commercial isoalkane-alkene alkylation technologies. These commercial processes are still based on the use of sulfuric acid (and hydrogen fluoride) catalysts (23). 

Bicyclic ethers can also be prepared from the direct acid-catalyzed reaction of 1,2-cycloalkandiols with aldehydes or ketones. [13] Both protic acids and Lewis acids can be employed. The stereoselectivity of these rearrangements is also high and the bicyclic products obtained have the same stereochemical features (a cis ring fusion with the C-2 substituent trans to the ring fusion) as the products formed in the related trans-Aio series (compare Figures 13 and 14). 

An acid (protic or Lewis) catalyzed conversion of an oxime to a substituted carboxylic amide. See Darling, C.M. and Chen, C.P., Rearrangement ofV-benzyl-2-cyano-(hydroxyimino)acetamide,... 

Before the introduction of metal-ammonia solutions for the reduction of a,p-unsaturated carbonyl compounds,sodium, sodium amalgam, or zinc in protic media were most commonly employed for this purpose. Some early examples of their use include the conversion of carvone to dihydrocarvone with zinc in acid or alkaline medium, and of cholest-4-en-3-one to cholestanone with sodium in alcohol. These earlier methods are complicated by a variety of side reactions, such as over-reduction, dimerization, skeletal rearrangements, acid- or base-catalyzed isomerizations and aldol condensations, most of which can be significantly minimized by metal-ammonia reduction. 

According to this mechanism, it is anticipated that methoxy and methyl substituents must exert a similar influence on the course of the photochemical rearrangement in protic solvents. Indeed, the 4-methoxydienone 38 yielded exclusively the 5/7-fused compound 39 on irradiation in acetic acid, while under similar conditions, the 2-methoxydienone 40 gave the spiro enone 41 in high yield (Scheme 7). The use of aqueous acetic acid in these cases led to a decrease in the amounts of rearranged products due to a Hght-catalyzed hydrolysis of the enol ether function. 

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