Enol ethers electrophilic halogenation

Reactions of enol ethers with halogen and sulfur electrophiles... 

Enol ethers are readily attacked in buffered medium by electrophilic reagents such as halogens, A -haloamides, perchloryl fluoride and organic peracids to give a-substituted ketones. Similarly, electrophilic attack on... 

Asymmetric halogenation of chiral acetals has been realized by C. Giordano (refs. 2-7). Using alkyl esters of optically active tartaric acids as chiral auxiliaries, a high diastereoselectivity is obtained even at room temperature. The results are best explained by a fast electrophilic addition of bromine on the electron rich enol ether, originating from an acid-catalyzed equilibrium with the chiral acetal. If (2R, 3R)-tartaric acid is involved, a S-configuration prevails at the new stereogenic center. Finally, cautious hydrolysis provides a set of 2-bromo alkyl aryl ketones, which can be obtained in enantiomerically pure form after crystallization (Fig. 2) ... 

Reaction with Electrophiles. The electrophilic substitution of a-silyl ketones gives the same products as those derived from the corresponding silyl enol ether. Thus reaction with bromine or thionyl chloride gives the a-bromo (or chloro) ketone, with the halogen replacing the trimethylsilyl group (eq... 

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