Benzilic acid, preparation rearrangement

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 ... 

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. 

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. 

Beckmann rearrangement of oximes, 572 Benzidine rearrangement, 682 Benzilic acid rearrangement, 435 Benzils, see a-Diketones Benzofurans, preparation, by dehydro-cyclization, 842... 

Dienone-phenol rearrangements are mechanistically diverse. They may involve 1,2-shifts of the Wag-ner-Meerwein type, or of the benzil-benzilic acid kind 1,3-shifts by a Claisen-Cope mechanism 1,5-sigmatropic shifts Favorskii-like reactions and other types. They may also be induced photochemically. A number of reviews are available, which discuss mechanistic aspects in detail. In this chapter emphasis is put on preparative aspects of these reactions and the examples are organized on a structural basis, stressing the new bond(s) formed. 

By ESR studies, the benzilic acid rearrangement in solution has been proven to proceed via a radical intermediate [14], For the benzilic acid reanangement in the solid state, a radical intermediate was also detected. For example, a freshly prepared mixture of finely powdered 35e and KOH showed a strong ESR signal (g=2.0049), and this signal declined as the reaction proceeded. 

In the benzil analog (107) a benzilic acid rearrangement leading to the acid (108) can be done and the acid isolated either as a salt or as the methyl ester prepared with diazomethane at a low temperature. The surprise lies in the fact that the acid (108) is unexpectedly unstable in the free state what... 

Disubstituted hydantoins and 2-thiohydantoins, especially their 5,5-diaryl derivatives, have been prepared by means of a condensation that involves a rearrangement similar to that in which benzilic acid is formed from benzil.1,3 Examples include the synthesis of fluorinated hydantoins,35 spirofluorene-,36 and spiroacenaphthylenehydantoins,37 2-thiohydantoins,30 38 39 and 5,5-diphenylhydantoin-2,4,5-13C3.40... 

This reaction has general application for preparing benzilic acids and their esters which have important biological activities. The formed benzilic acid can be used as reducing reagent to transform Qf, -unsaturated ketones into saturated ketones.In addition, the benzilic acid rearrangement on cyclic of-diketones will lead to ring contracted products 2ii, 2kk, 2nn... 

Acetyllactic acid results from an acyloin condensation and loss of carbon dioxide, followed by a very interesting ketol rearrangement, which proceeds in a stereochemically uniform manner. Herein, the hydroxy- and keto-groups are oriented syn-periplanar, so that the methyl group is transferred suprafacially on the (Jle)-side, a mechanism, which is supported by data from NMR spectroscopy on model compounds [260] and by preparative examples. [261, 262] The ketol rearrangement is to some extent related to the benzil-benzilic acid rearrangement. 

In this experiment, benzilic acid will be prepared by causing the rearrangement of the a-diketone benzil. Preparation of benzil is described in Experiment 32B. The rearrangement of benzil proceeds in the following way ... 

Guirado et al reported a one-pot preparative process involving a benzilic acid rearrangement step followed by a spontaneous epoxidation of the intermediates. Treatment of diketone 32 with sodium hydroxide at room temperature gave 2,5,5-trichloro-l,2-epoxycyclopentane-l-carboxylic acid 35 as a single product in quantitative yield. This transformation has been explained by sequential participation of intermediates 33 and 34. 

Ba(OH)2 is known to cattdyze several base-catalyzed organic reactions in the solid form, Of the reactions, aldol condensation is the most common. In recent years, several organic reactions besides aldol condensation have been found to be effectively catalyzed by Ba(OH)2. These reactions are the Claisen-Schmidt reaction, esterification of acid chlorides, Williamson s ether synthesis, benzil-benzilic acid rearrangement, the synthesis of A -pyrazolines by the reaction of a,/3-unsaturated ketone with PhNHNHz Wittig-Homer reaction, and Michael addition. For these reactions, the Ba(OH)2 catalyst prepared from Ba(0H)2-8H20 by cidcination at 473 K shows the highest activity. 

Cannizzaro Reaction Preparation of Furoic Acid 9-8. Benzilic Acid Rearrangement... 

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