Phenacyl esters Phenols

The protection of carboxylic acids as their phenacyl esters has been described regeneration is effected with great facility at room temperature by zinc in glacial or dilute aqueous acetic acid. Of notable applicability is the formation of y-hydroxyesters from y-lactones, an otherwise indirect transformation an example is the obtention of the ester (13) from the sodium salt of santonin (12). This protection can be extended to phenols. 

C) Phenacyl and p-Bromophenacyl esters. Ammonium salts in aqueous-ethanolic solution do not however usually condense satisfactorily with phenacyl and />-bromophenacyl bromide. The aqueous solution of the ammonium salt should therefore be boiled with a slight excess of sodium hydroxide to remove ammonia, and the solution then cooled, treated with hydrochloric acid until just alkaline to phenol-phthalein, and then evaporated to dryness. The sodium salt is then treated as described (p. 349) to give the ester. Filter the ester, and wash with water to remove senium halide before recrystallisation. 

Phenacyl ethers, to protect phenols, 153 Phenacylsulfonamides, to prote ct amines, 385 Phenols, protection of, 143-170 as esters, 162-170 as ethers, 145-162 Reactivity Chart 4, 425-428 Phenothiazinyl-(10)-carbonyl derivatives, to protect amines, 346... 

Acetals of a-bromo aldehydes are obtainable from <%,/ -dibromoalkyl acetates by an analogous procedure in which these esters are prepared from enol acetates and bromine and are then converted into acetals by means of alcohols.939 A simple synthesis of <%-keto aldehydes is to treat a-brominated phenacyl phenyl ethers with alkoxides or phenoxides.940 This method is particularly useful for the preparation of aryl acetals because it avoids the side reactions that can complicate reactions between phenols and aldehydes in an acidic medium. 

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