Enol ethers to protect carbonyl groups

Isobutyl carbamates, to protect amines, 347 Isobutyl carbonates to protect alcohols, 107 Isobutyl enol ethers, to protect carbonyl groups, 220... 

Selective reduction of a benzene ring in the presence of another reducible group is possible if the other group is first protected in some way. Ketones, for example, may be converted to acetals or enol ethers to protect them from reduction. Conversely, reduction of benzene rings takes place only slowly in the absence of a proton donor, and selective reduction of an a,(3-unsaturated carbonyl system can be effected. 

The carbonyl group forms a number of other very stable derivatives. They are less used as protective groups because of the greater difficulty involved in their removal. Such derivatives include cyanohydrins, hydrazones, imines, oximes, and semicarbazones. Enol ethers are used to protect one carbonyl group in a 1,2- or 1,3-dicarbonyl compound. 

A carbonyl group cannot be protected as its ethylene ketal during the Birch reduction of an aromatic phenolic ether if one desires to regenerate the ketone and to retain the 1,4-dihydroaromatic system, since an enol ether is hydrolyzed by acid more rapidly than is an ethylene ketal. 1,4-Dihydro-estrone 3-methyl ether is usually prepared by the Birch reduction of estradiol 3-methyl ether followed by Oppenauer oxidation to reform the C-17 carbonyl function. However, the C-17 carbonyl group may be protected as its diethyl ketal and, following a Birch reduction of the A-ring, this ketal function may be hydrolyzed in preference to the 3-enol ether, provided carefully controlled conditions are employed. Conditions for such a selective hydrolysis are illustrated in Procedure 4. 

Silyl enol ethers are an elegant means to protect the reactive and hence labile enolate moiety [15]. At the time of reaction, the enolate group is generated as an intermediate and reacts with the carbonyl-carrying compound. 

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