Birch reduction aryl ethers

The Birch reduction has been used by several generations of synthetic organic chemists for the conversion of readily available aromatic compounds to alicyclic synthetic intermediates. Birch reductions are carried out with an alkali metal in liquid NH3 solution usually with a co-solvent such as THF and always with an alcohol or related acid to protonate intermediate radical anions or related species. One of the most important applications of the Birch reduction is the conversion of aryl alkyl ethers to l-alkoxycyclohexa-l,4-dienes. These extremely valuable dienol ethers provide cyclohex-3-en-l-ones by mild acid hydrolysis or cyclohex-2-en-l-ones when stronger acids are used (Scheme 1). 

Reduction of aryl ethers is a particularly useful application of the Birch reduction. Thus, methoxybenzenes are reduced to 1,4-dienes, as expected, but one of the double bonds is enol ether and is readily hydrolyzed, followed by rearrangement to give the corresponding a,P-unsaturated-ketone h Li, NH,... 

A very useful method to prepare carbocyclic enol ethers is the Birch reduction of aryl ethers. In the simplest example, metal ammonia reduction of methoxybenzene gives methoxycyclohexadiene (21). The reaction has been applied to numerous aryl ethers of complex structures. 

One of the most important applications of the Birch reduction is to convert aryl alkyl ethers into 1-alkoxycyclohexa-l,4-dienes which are then used as the starting materials in organic synthesis. Compared to the benzene series, the Birch reduction of naphthalenes may afford different products, depending on the position of the alkoxyl group. 

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