Acetals, acid catalyzed hydrolysis

Many carbamates have been used as protective groups. They are arranged in this chapter in order of increasing complexity of stmcture. The most useful compounds do not necessarily have the simplest stmctures, but are /-butyl (BOC), readily cleaved by acidic hydrolysis benzyl (Cbz or Z), cleaved by catalytic hy-drogenolysis 2,4-dichlorobenzyl, stable to the acid-catalyzed hydrolysis of benzyl and /-butyl carbamates 2-(biphenylyl)isopropyl, cleaved more easily than /-butyl carbamate by dilute acetic acid 9-fluorenylmethyl, cleaved by /3-elimination with base isonicotinyl, cleaved by reduction with zinc in acetic acid 1-adamantyl, readily cleaved by trifluoroacetic acid and ally], readily cleaved by Pd-catalyzed isomerisation. 

The rate constant for the acid-catalyzed hydrolysis of ethyl acetate is... 

The carbonyl group can be deprotected by acid-catalyzed hydrolysis by the general mechanism for acetal hydrolysis (see Part A, Section 7.1). A number of Lewis acids have also been used to remove acetal protective groups. Hydrolysis is promoted by LiBF4 in acetonitrile.249 Bismuth triflate promotes hydrolysis of dimethoxy, diethoxy, and dioxolane acetals.250 The dimethyl and diethyl acetals are cleaved by 0.1-1.0 mol % of catalyst in aqueous THF at room temperature, whereas dioxolanes require reflux. Bismuth nitrate also catalyzes acetal hydrolysis.251... 

Acid-catalyzed hydrolysis of the latter followed by Jones oxidation furnished racemic peshawarine (43) (Scheme 12). Simanek et al. (59) transformed the same amino acetal 47, obtained in optically active form from the Emde degradation of rhoeadine methiodide (48), to ( )-43, also by hydrolysis and oxidation. However, the optical activity was lost during hydrolysis (Scheme 12). 

Acetals Removing the acetal protecting group is easily achieved by acid-catalyzed hydrolysis, although catalytic hydrogenolysis is better for acid-sensitive compounds. The oxygen connected to the... 

The concept of zeolite action was tested in a particular reaction where the enzyme is exposed from the beginning to an acidic environment the esterification of geraniol with acetic acid catalyzed by Candida antarctica lipase B immobilized on zeolite NaA [219]. Lipases have been used for the hydrolysis of triglycerides and due to their ambivalent hydrophobic/hydrophilic properties they are effective biocatalysts for the hydrolysis of hydrophobic substrates [220]. When water-soluble lipases are used in organic media they have to be immobilized on solid supports in order to exhibit significant catalytic activity. 

The mechanism of the acid-catalyzed hydrolysis of cellulose is based on that normally expected for an acetal (see Scheme 11). This involves formation of a conjugate acid by protonation of either of the acetal oxygen atoms at C-1, and formation of a carbonium ion, followed by stabilization of the product by heterolysis of a participating water molecule. The car-... 

The glassware employed in the distillation should be washed first with ammonium hydroxide and then water and finally dried in an oven before use to avoid the possibility of acid-catalyzed hydrolysis or rearrangement of the enol acetate during the distillation. 

Hydrolysis of acetals yields aldehydes, which are intermediates in the biochemical /3-oxidation of hydrocarbon chains. Acid catalyzed hydrolysis of unsubstituted acetals is generally facile and occurs at a reasonable rate at pH 4-5 at room temperature. Electron-withdrawing substituents, such as hydroxyl, ether oxygen, and halogens, reduce the hydrolysis rate, however [50]. Anionic acetal surfactants are more labile than cationic [40], a fact that can be ascribed to the locally high oxonium ion activity around such micelles. The same effect can also be seen for surfactants forming vesicular aggregates. 

The most convenient route to the 3-hydroxyquinolizinium ion (Scheme 94) involves the quaternization of a 2-picolinaldehyde acetal with bromoacetone followed by acid-catalyzed hydrolysis and intramolecular condensation of the salt (170) (62AG(E)593). Good yields have been reported (65JOC526) for both steps when R—R = CH2CH2. 

The introduction of CHj requires that the terminal alkyne C first become a carbanion and then be methylated. Such a carbanion, acting like the R group of RMgX, would react with the C==0 group of another molecule before it could be methylated. To prevent this, C==0 is protected by acetal formation before the carbanion is formed. The acetal is stable under the basic conditions of the methylation reactions. The aldehyde is later unmasked by acid-catalyzed hydrolysis. 

Cumulenes such as butatrienes and hexapentaenes can undergo cycloaddition at several possible double-bond sites. The electrophilic l,l-diphenyl-4,4-bis(trifluoromethy )butatriene (34), however, reacts with ketene acetals and geminal enediamines at the central double bond exclusively.25 In the case of the ketene acetal cycloadduct 35 (R1 = H R2 = R3 = OMe). acid-catalyzed hydrolysis gives the cyclobutanone. 

Indanone was first prepared by distillation of the calcium salt of o-phenylenediacetic acid2 3 and, more recently, by the action of acetic anhydride on its potassium salt.4 It has been obtained by the dilute sulfuric acid-catalyzed hydrolysis and decarboxylation of 2-iminoindan-l-carboxylate6 and ethyl 2-indanone-l-carboxylate.6 2-Indanone is commonly obtained by acid-catalyzed dehydration of an indene glycol,7 8 as illustrated in this preparation. Indene glycol has been obtained from indene via the bromohydrin.9-12 The most recent preparation of 2-indanont is by Curtius degradation of 2-indenecarboxylic acid.13... 

Mixed acid-base ion-exchange resins.1 Rexyn ion-exchange resins (Fisher Scientific Co.) consist of sulfonic acid beads and quaternary ammonium hydroxide beads. Stowcll and Hauck have used such a resin to convert the keto acetal 1 in one operation to 2-methyl-2-cyclopenlenone-l (2). Two steps are involved acid-catalyzed hydrolysis of the acetal group and hasc-cataly/cd aldol condensation. The first step is reversible, but the second is irreversible. 

In (65) only electrophilic catalysis of the scission of the S-S bond is involved, a situation also encountered in the acid-catalyzed hydrolysis in weekly nucleophilic media (acetic acid-1% water) of certain aryl sulfinyl sulfones substituted with electron-donating substituents (66)164, viz. 

EFFECT OF SOLVENT ON THE ACID-CATALYZED HYDROLYSIS OF /-BUTYL ACETATE ... 

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