Intramolecular reactions benzilic acid rearrangement

We have already discussed a large group of reactions in which carbanions add to the C=0 group (cf. pp. 221-234), including examples of intramolecular carbanion addition, e.g. an aldol reaction (p. 226), Dieckmann reaction (p. 230), and the benzilic acid rearrangement (p. 232), and also to the C=C—C=O system, the Michael reaction... 

Fragmentation and recombination reactions predominate in nearneutral, alkaline solutions. Fragments from the cleavage of the carbohydrate, and from its dehydration products, undergo further dehydration, condensation, and intermolecular, Cannizarro-type reactions. The benzilic acid rearrangement, an intramolecular Cannizarro reaction, seems to be inoperative. 

That intermediate is a three-membered cyclic ketone, a cyclopropanone the alkoxide acts not as a nucleophile (its role in the benzilic acid rearrangement) but as a base, enolizing the ketone. The eno-late can alkylate itself intramolecularly in a reaction that looks bizarre but that many chemists think is not unreasonable. The product is the same cyclopropanone in each case. 

The prototype reaction is the conversion of glyoxal into glycolic acid (equation 2), and here the benzilic acid rearrangement mechanism coincides with that for an intramolecular Cannizzaro reaction. The reaction is observed with other purely aliphatic a-diketones such as f-butyl 2,3-dioxobutyrate and cyclohexane-1,2-dione (equations 3 and 4), but the scope is limited in the aliphatic series by competing (c.g. aldol) reactions. Suitably constructed heterocyclic systems also rearrange, and the conversion of alloxan (3) into alloxanic acid (4) was among the first of the benzilic acid rearrangements to be discovered (equation 5). ... 

In addition to these reactions in which the carbanions are supplied from carbonyl compounds, we will discuss in this chapter Grignard reactions, the benzilic acid rearrangement, the benzoin condensation, and the Kolbe synthesis of hydroxy aromatic acids. These reactions illustrate the addition of other kinds of carbanions to carbonyl groups. The benzilic acid rearrangement is an example of the intramolecular addition of a group with its pair of electrons to a carbonyl carbon atom. 

The Benzilic Acid Rearrangement When benzil is treated with sodium hydroxide, benzilic acid is fornied and the reaction appears to proceed by the intramolecular addition of a group with an electron pair to the carbonyl carbon atom. A mechanism which seems quite plausible for the reaction is the following.50 It is identical mechanistically with the glyoxal-glyoolic acid rearrangement (p. 107). 

The released end-group forms an a-dicarbonyl structure which rearranges by an intramolecular, Cannizzaro type of reaction to yield saccharinates. Intramolecular reactions of the Cannizzaro type can occur not only with dialdehydes but also with a-ketoaldehydes and a-diketones ( benzilic acid rearrangement ) they can be exemplified by the base-catalyzed rearrangement of phenylglyoxal (VIII) [or of (2,4,6-trimethylphenyl)-glyoxal ] to the salt of mandelic acid (IX) (or of 2,4,6-trimethylmandelic acid). 

Liebig observed the first intramolecular rearrangement in 1838 when he found that benzil in basic solution forms a new compound.136 In 1870 Jena correctly established the product of the reaction as benzilic acid, but proposed an incorrect structure for the starting material to avoid postulating a skeletal rearrangement.137 In 1928 Ingold proposed the mechanism shown in Equation 6.50, which today is solidly supported by experimental evidence.138... 

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