The Preparation and Elimination of -Substituted Ketones

The general application of J elimination reactions will be considered later (section VII), however, the elimination from -substituted ketones proceeds particularly readily and is treated separately in this section. 

An illustration of the difference in reactivity of a-and / -halides is provided by the ready elimination of 1,4a-dibromo-5a-cholestan-3-one to 4a-bromo-5a-cholest-l-en-3-one in collidine at room temperature. Calcium carbonate in refluxing DMA is necessary to complete the dehydrobromination to the l,4-dien-3-one.  

A second (y) substituent is frequently present because of the methods of formation commonly employed. 

Eliminations of this type occur very easily under both acidic and basic conditions because of the activation possible through the enol or enolate anion. 

The dehydration of -hydroxy ketones is a closely related reaction. In the case of 5,6-disubstituted 3-ketones, the 6-substituent usually remains in the less stable configuration. With acid catalyzed elimination, prolonged treatment or high concentration may cause epimerization  

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