Using halogen, kinetic and thermodynamic products

Bromo ketones are commonly employed and are much easier to dehydro-halogenate than chloro ketones, although the latter are not so susceptible to reduction and rearrangement. lodo ketones are generally less accessible and very prone to reduction. 

Enolization is the rate-determining step in the halogenation of normal ketones. Where alternate directions for enolization exist, the preferred direction (and hence the position of kinetic bromination) depends on the substituents and stereochemistry. Furthermore, the orientation of the bromine introduced depends on stereochemical and stereoelectronic factors. 

The most familiar example of these effects is the enolization of A B cw-and A B fra j-3-ketones. Thus, in the 5a-series enolization towards C-2 is favored, while in 5jS-compounds enolization towards C-4 predominates. This preference is attributed to relief of nonbonding interactions in the enol form. Brominations are generally run in the presence of acid in order to 

The A -6-ketone functionality in polyhydroxy cholestanes is common to all the insect moulting hormones of the ecdysone type. Therefore, there has recently been considerable interest in the introduction of A -double bonds into 6-ketones. The different approaches that follow illustrate the utility of isomerization, rearrangement, and blocking groups. 

In the Syntex route the 5-position was blocked by hydroxyl which was removed reductively at a later stage  

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Using halogen, kinetic and thermodynamic products / 268 The use of enol acetates / 271... 

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