Acid-catalysis cleavage forming

The term acid catalysis is often taken to mean proton catalysis ( specific acid catalysis ) in contrast to general acid catalysis. In this sense, acid-catalyzed hydrolysis begins with protonation of the carbonyl O-atom, which renders the carbonyl C-atom more susceptible to nucleophilic attack. The reaction continues as depicted in Fig. 7. l.a (Pathway a) with hydration of the car-bonium ion to form a tetrahedral intermediate. This is followed by acyl cleavage (heterolytic cleavage of the acyl-0 bond). Pathway b presents an mechanism that can be observed in the presence of concentrated inorganic acids, but which appears irrelevant to hydrolysis under physiological conditions. The same is true for another mechanism of alkyl cleavage not shown in Fig. 7.Fa. All mechanisms of proton-catalyzed ester hydrolysis are reversible. 

In the initial stages of this analytical method the proanthocyanidins are cleaved by acid catalysis into their constitutive subunits. Proanthocyanidin extension subunits, upon cleavage, form unstable electrophilic intermediates. Phloroglucinol (1,3,5-trihydroxy-benzene) is added to the reaction mixture as a nucleophile, where it combines with extension subunit intermediates to form analyzable adducts. 

The difference between the hydrolysis of an orthoester and the hydrolysis of an acetal is that profanation and C-O bond cleavage occur together in the former. In the hydrolysis of an acetal, the protonated form of the acetal is produced as an intermediate in the reaction. While we will not go into details here, the hydrolysis of the orthoester constitutes a case of what is known as general acid catalysis, while that of an acetal is specific acid catalysis. 

If so, one may expect products to result from chemical bond formation between the cation-radical-anion-radical pair, which are both paramagnetic and of opposite charge. In the latter route, there is a precedent for the formation of dioxetane intermediates of stable olefin cation radicals [51], as in the characterization by Nelsen and coworkers of a dioxetane cation radical from adamantylidene cation radical [52]. If a dioxetane is formed, either in neutral form or as a cation radical, the Ti02 surface can function in an additional role, that is, as a Lewis acid catalyst, to induce decomposition of the dioxetane. Since no chemiluminescence could be observed in these reactions, apparently Lewis acid catalysis provides a nonradiative route for cleavage of this high-energy intermediate. That Ti02 can indeed function in this way can be demonstrated by independent synthesis of the dioxetane derived from 1,1-diphenylethylene, which does indeed decompose to benzophenone when it is stirred in the dark on titanium dioxide. 

Many flavor compounds contain double bonds or aldehyde groups, which are susceptible to oxidation, cleavage, polymerization, or interaction among components (Sinki et al., 1997). Alcohols can be oxidized to the corresponding aldehyde and then acid. Alcohol and acid can react to form ester. Ester can be hydrolyzed to alcohol and acid at neutral or alkaline pH. Aldehyde and alcohol can be dehydrated by catalysis to form hemiacetal, and the reverse reaction can occur in acidic conditions or in water. 

Since macroline (215) does not give ketone (222) under the above conditions, macralstonine (220) must break down by acid catalysis via two distinct processes from one are formed alstophylline (221) and macroline (215), from the other alstophylline, ketone (222), and formaldehyde (Scheme 15). The two modes of cleavage can be understood in terms of structure (220) as shown in Scheme 16. 

Besides specific proton catalysis, general acid catalysis by oximes may contribute to the advanced degradation of concentrated pyridinium aldoxime solutions. In fact, addition of 2-PAM to HI 6 accelerated not only the cleavage of the aminal-acetal bridge of the latter (Eyer et al, 1988), but also isonicotinamide deamination (Korte and Shih, 1993). Due to this molecular canibalism , concentrated solutions of HI 6 and HLd 7 seem to be condemned to increased degradation and cannot be stockpiled in dissolved form. 

There have been no examples of reactions proceeding via general acid catalysis alone by cyclodextrin. In the hydrolysis of trifluoroacetanilide, however, general acid catalysis enhances the cleavage of the tetrahedral intermediate (5) formed by nucleophilic attack by a secondary alkoxide ion. General acid catalysis serves to convert the leaving group from an extremely unstable anion of aniline to a stable neutral aniline molecule (Scheme 2) [14]. 

The mechanism of hydrolysis of [99] must involve attack of the ureido oxygen in-line with the exocyclic leaving group to form the initial TBP intermediate (Scheme 41, shown for the monomethyl ester of [99]). Intramolecular proton transfer (or acid catalysis in the case of the diester) and loss of aleohol leads to cyclic 0-phosphourea, which is rapidly hydrolysed, leading to ring cleavage. The overall stereochemistry of this double-displacement mechanism is retention of configuration at phosphorus. 

Another route from five-membered 0-heterocycles to oxepines uses 2,3-dihydrofurans as starting materials and involves their [2 -I- 2] cycloaddition reaction with ethyne or ethylene compounds, followed by cleavage of bicyclic compounds formed by thermolysis or Lewis acid catalysis <83CB1691, 87TL1501,92JOC5102). These transformations are presented in Scheme 29 by starting from dimethyl acetylenedicarboxylate and 2,3-dihydrofuran or its 5-substituted derivatives <87TL1501>. 

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