Named rearrangements Favorskii

Translation of these results into compound I leads to structure X. Unraveling of the strained zwitterion XI derived from this would yield keto aldehyde XII, a structure that plays a central role in the various possible reaction mechanisms that branch off from the starting material I. Furthermore, under photo-lytic conditions, some alkenes react with carbonyl compounds to form four-membered cyclic ethers, namely, oxetanes, by way of a [2-1-2] cycloaddition reaction known as the Patemo-Buchi process. Such a reaction would be all that is necessary to convert XII into the bicyclic cyclopropanone XIII required for the Favorskii-type rearrangement (see Scheme 42.3). Splitting by methanol attack would directly yield compound II. 

Several structural requirements appear to be necessary in order to be able to observe the semibenzilic mechanism. Thus the acidity of the a hydrogen plays an important role. Indeed, it should be weakly acid in order to favor nucleophilic attack at the carbonyl carbon. Of course, this kind of mechanism will be facilitated by the absence of an a hydrogen the rearrangement is then known under the name of quasi-Favorskii. Since both substrates just discussed react via the semibenzilic mechanism, we shall discuss them simultaneously. 

Favorskii rearrangement is the base-catalyzed transformation of a-haloketones into the corresponding carboxylic acid or carboxylic acid derivative. Alexey Favorskii described the rearrangement that has his name more than a century ago. Since then, this reaction has been used in a variety of different manners, particularly in the synthesis of highly branched systems. Herein, examples regarding the ring contraction will be discussed. 

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