4- butyrate esters protect alcohols


Substituent effects come into play at several points in the ester hydrolysis mechanism. In the base-catalyzed reaction, electron-withdrawing substituents in either the acyl or alkoxy group facilitate hydrolysis. Because the tetrahedral intermediate formed in the ratedetermining step is negatively charged, this intermediate, and the corresponding transition state, is stabilized by electron withdrawal. If the carbonyl group is conjugated with an electron-releasing group, reactivity is decreased by ground-state stabilization. The partitioning of the tetrahedral intermediate between reversion to starting material by loss of hydroxide ion and formation of product by expulsion of the alkoxide is strongly affected by substituents in the alkoxy group. Electron-withdrawing groups in the alkoxy group shift the partitioning in favor of loss of the alkoxide and favor hydrolysis. For this reason, exchange of carbonyl oxygen with solvent does not occur in basic hydrolyses when the alkoxy group is a good leaving group. This has been demonstrated, for example, for esters of phenols. Because phenols are stronger acids than alcohols, their conjugate bases are better leaving groups than alkoxide ions. Aryl esters are hydrolyzed faster than alkyl esters and without observable exchange of carbonyl oxygen with solvent  [c.476]

The chemical properties of PVAc are those of an aUphatic ester. Thus, acidic or basic hydrolysis produces poly(vinyl alcohol) and acetic acid or the acetate of the basic cation. Industrially, poly(vinyl alcohol) is produced by a base-catalyzed ester interchange with methanol, where methyl acetate forms iu addition to the polymeric product. The chemical properties of PVAc can be modified by copolymerization. When a comonomer having a carboxyflc acid group or a sulfuric acid group is used, the copolymer becomes soluble iu dilute aqueous alkafl or ammonia. These copolymers also adhere better to metals than homopolymers or neutral copolymers because of the iateraction between the acid groups and the metal surface. Copolymerization with monomers such as butyl acrylate can improve flexibiUty and provide specific adhesion to surfaces. Monomers such as -methylol acrylamide copolymerized with vinyl acetate provide sites for cross-linking either duting polymerization reaction or iu use. Other comonomers such as vinyl Versatate (vinyl neodecanoate) reduce the potential for hydrolytic breakdown of the polymer chain by shielding the VAc group from attack.  [c.463]

In general the Stork reaction gives moderate yields with simple alkyl halides better yields of alkylated product are obtained with more electrophilic reactants such like allylic, benzylic or propargylic halides or an a-halo ether, a-halo ester or a-halo ketone. An example is the reaction of 1-pyrrolidino-l-cyclohexene 6 with allyl bromide, followed by aqueous acidic workup, to yield 2-allylcyclohexanone  [c.268]


Protective groups in organic synthesis (1991) -- [ c.113 ]