Unsaturated carboxylic acids, Favorskii rearrangement

Another useful route to cyciopentanes is the ring contraction of 2-bromo-cydohexanones by a Favorskii rearrangement to give csrdopcntanecarboxylic acids. If a 0 dibromoketones are used, ring opening of the intermediate cydopropanone leads selectively to, y-unsaturated carboxylic acids (S.A, Achmad, 1963, 1965 J. Wolinsky, 1965). 

The Favorskii rearrangement also occurs with di-, tri- and tetrahalo ketones and gives rise to (halogenated) unsaturated carboxylic acid derivatives. 

White and coworkers used a Favorskii rearranjgement in their synthesis of the natural product ( )-byssochlamic acid 17. 4-Ethylcyclohexanone 18 synthesized by Jones oxidation of the corresponding alcohol, was carboxylated to 19 and then dibrominated to 20. The Favorskii rearrangement was carried out with NaOMe to form the unsaturated diester... 

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