Classical Favorskii rearrangement

A version of this type of oxidation, which used a combination of lead tetraacetate and boron trifluoride etherate, allowed the transformation of enamines of cyclic ketones into esters of the corresponding contracted ring, as in the classical Favorskii rearrangement of a-haloketones under basic conditions72 (Scheme 51). 

If there are no enolizable hydrogens present, the classical Favorskii rearrangement is not possible. Instead, a semi-benzylic mechanism can lead to a rearrangement referred to as quasi-Favorskii. 

The Favorskii rearrangement results from the action of base on an a-halo ketone. When applied to cyclic ketones, ring contraction results, as shown in the classic case of cyclohexanone formulated below. 

Partial retrons are more numerous than retrons for the quasi-Favorskii rearrangement. One can focus on the possible carbonyl product and consider all those groups that could be derived from those carbonyl groups. Furthermore, one of the classic partial retrons for a quasi-Favorskii rearrangement is sirrply a tertiary or quaternary carbon embedded in a polycyclic carbon framework. These synthetic strategies will be illustrated with exanples later in this chapter. 

There are three significant variations on the Favorskii rearrangement, the homo-, quasi-, and photo-Favorskii rearrangements. The homo-Favorskii and quasi-Favorskii rearrangements oecur if the precursor does not possess the classic a-hydrogen and a -halide. The photo-Favorskii is a variation involving a light-induced radical mechanism. 

The base-catalysed rearrangements of cz-halo ketones are classical examples of the reactions of ambident enolate anions in solution. The extent of each of the two reactions shown in Equations [11] and [12] is principally a function of the type of solvent used. A protic solvent solvates more strongly at the oxygen centre of the ambident anion and thus reaction proceeds through the carbanion centre to yield the Favorskii species as the major product (Eqn [11]). In marked contrast, the Favorskii rearrangement does not occur in the gas phase. Here,... 

The Favorskii rearrangement of a-haloketones in the presence of base yields carboxylic acids or carboxylate esters. The intermediate in the classic version of the reaction is a cyclopropanone, but an alternative mechanism, related to that of the benzylic acid rearrangement, operates in other cases, with analogous outcome. 

The classic labeling studies of Loftfield 49a> have demonstrated that cyclopropanones are intermediates in the Favorskii reaction 49b) — the base-induced rearrangement of a-haloketones (Scheme 8). The related reaction of a,a -dibromoketones has, in fact, become a convenient preparative route for cyclopropenones, e.g., 48 ->- 49 via 50.50>... 

Classic work by Loftfield on the Favorskii reaction showed that cyclopropanones are intermediates in the base-induced rearrangement of a-haloketones (l heme 4). Isolation of such an intermediate was accomplished in the reaction of the sterically hindered a-bromodineopentyl ketone (9) with potassium p-chlorophenyldimethylcarbinolate. The identity of the product (10), rrans-2,3-di-t-butylcyclopropanone, was established by independent synthesis of 1,3-di-t-butylallene oxide (11) which underwent valence isomerization to (10) ... 

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