Benzilic acid rearrangements

Normant et al.19), the migration power decreases in the order given underneath Eq. (15), and is reminiscent of that in other base-induced sextet rearrangements (benzilic acid, Favorsky-type). 

Benary Reaction Benkeser Reduction Benzidine Rearrangement Benzil-Benzilic Acid Rearrangement Benzilic Acid Rearrangement Benzoin Condensation Bergius Process Bergman Reaction... 

The mechanism of the formation of the saccharinic acids and their derivatives remains unclear. Nef has suggested that the reaction proceeds through the diketone produced by removal of a molecule of water. This suggestion was combined by Isbell (68) with current concepts of the benzilic acid rearrangement. Benzilic acid rearrangements of the diketones would produce the saccharinic acids, the nature of the acid being determined by the position of dehydration. 

When benzii is heated with potassium hydroxide solution, it undergoes a molecular rearrangement with the formation of the potassium salt of benzilic acid, or diphenyl lycollic acid ... 

This reaction applies to many i,2 diketones, and is termed the Benzilic Acid Rearrangement. It provides a ready method for the preparation of disubstituted a4iydroxy-carboxylic acids. When applied to a cyclic 1,2-diketone, the ring system is necessarily reduced by one carbon atom for example, cyclohexan-i,2 ... 

Benzil (and other a-diketones Ar—CO—CO—Ar) upon refluxing with aqueous-alcoholic potassium hydroxide undergo the beozilic acid rearrangement. Thus benzil is converted into a salt of benzilic acid ... 

Benzilic acid rearrangement Benzoin reaction (condensation) Blanc chloromethylation reaction Bouveault-Blanc reduction Bucherer hydantoin synthesis Bucherer reaction Cannizzaro reaction Claisen aldoi condensation Claisen condensation Claisen-Schmidt reaction. Clemmensen reduction Darzens glycidic ester condensation Diazoamino-aminoazo rearrangement Dieckmann reaction Diels-Alder reaction Doebner reaction Erlenmeyer azlactone synthesis Fischer indole synthesis Fischer-Speior esterification Friedel-Crafts reaction... 

The benzilic acid rearrangement of steroid a-diketones is often the preferred method for ring contraction. Yields are generally excellent, and in several instances the reaction has been shown to be stereospecific. Preparation of the starting a-diketones is discussed below and illustrated with several excellent experimental procedures. 

Hirschmann and co-workers have reported the synthesis of another A-nor analog of cortisone obtained from the benzilic acid rearrangement of a 2,3-diketo-A -steroid. 17a,20 20,21-Bismethylenedioxypregna-L4-diene-... 

In systems which preclude retro-aldol condensations, benzilic acid rearrangement of 11,12-diketones affords normal C-norsteroids in fair yields. For example, 11,12-diketotigogenin (82) is converted to the C-nor-(5oc,9(, 22a)-spirostane (83) in 65 % yield by barium oxide in boiling aqueous methyl-cellosolve. ... 

An interesting one-step combined oxidation and benzilic acid rearrangement has been described by Holden and Kerwin in connection with the synthesis of C-norpregnanes from 3a,20yS-dihydroxy-5j9-pregnan-12-one (partial structure 85). 

Ring contraction by benzilic acid rearrangement of a-diketones... 

Baeyer-Villager oxidation, 10, 151, 433 Bamford-Stevens reaction, 402 Barton reaction, 253 Beckmann rearrangement, 140 Benzilic acid rearrangement, 418, 435 3 -Benzoyloxy-7-methylenecholest-5-ene, 60 Benzylmagnesium chloride, 64 3-Benzyloxycholesta-3,5-diene, 342... 

Diketones 1 can be converted into the salt of an a-hydroxy carboxylic acid upon treatment with alkali hydroxide after acidic workup the free a-hydroxy carboxylic acid 2 is obtained. A well-known example is the rearrangement of benzil (R, R = phenyl) into benzilic acid (2-hydroxy-2,2-diphenyl acetic acid). The substituents should not bear hydrogens a to the carbonyl group, in order to avoid competitive reactions, e.g. the aldol reaction. 

Finally a proton transfer leads to formation of carboxylate anion 3. Of particular interest is the benzilic acid rearrangement of cyclic diketones such as 4, since it... 

A variant is represented by the benzilic ester rearrangement, where an alkoxide is used as nucleophile. The alkoxide should not be sensitive towards oxidation. The reaction product is the corresponding benzilic acid ester 5 ... 

Substrates can be 1,2-diketones with aryl groups as well as some aliphatic substituents, cyclic and heterocyclic diketones. However the benzilic acid rearrangement is of limited preparative importance. 

Mechanistically closely related is the benzilic acid rearrangement, where an alkyl or aryl group migrates instead of the hydrogen. 

The following reaction, called the benzilic acid rearrangement, tajkes place by typical carbonyl-group reactions. Propose a mechanism fPh = phenyl). 

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