Benzilic acid, preparation reactions

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. 

A mixture containing 8 g (0.06 mol) of N-methyl-3-chloro-piperidine and 13.6 g (0.06 mol) of benzilic acid in 50 cc of anhydrous isopropyl alcohol was refluxed for 3 days the isopropyl alcohol was removed by distillation in vacuo, the residue treated with dilute aqueous hydrochloric acid and the aqueous acid mixture extracted repeatedly with ether. The aqueous phase was separated, made strongly alkaline with 20% aqueous sodium hydroxide and extracted with ether. The ether extracts were dried with potassium carbonate and distilled the product was collected at 175° to 176°C (0.03 mm), yield 11.5 g (59%). The ester base thus prepared was then dissolved in 75 cc of isopropyl alcohol and 3.4 g (0.037 mol) methyl bromide added. The reaction mixture was allowed to stand at 30°C for 2 days and the product isolated by filtration, yield, 13 g (87%), MP 228° to 229°C dec. 

Due to excellent yields, mild reaction conditions, and a fast reaction rate, the azolide method is well suited to the synthesis of isotopically labeled esters, even ones with very short half-lives, just as it is always useful for the esterification of sensitive carboxylic acids, alcohols, and phenols under mild conditions. An example is provided by the synthesis of [nC]-quinuclidinyl benzilate prepared from benzilic acid, CDI, and nC-labeled quinuclidinol.[147]... 

In the preparation of certain benzilic acid derivatives froir substituted benzils, it has been found that simply dissolving the substituted benzil in alcoholic alkali is sufficient to complete the conversion and no heating is required. With unsubstituted benzil, however, a much poorer yield of benzilic acid is obtained if the reaction mixture is not heated. Longer heating than that given in the directions is likely to decompose some of the product to benzoic acid. 

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. 

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... 

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... 

In 1910, Hinsberg described the reaction between benzil and diethylthiodiacetate, resulting in the preparation of the thiophene ring system. The reaction was run under Claisen condensation conditions, and after hydrolysis with aqueous acid at reflux, the free dicarboxylic acid 1 was produced. 

Benzil monohydrazone (610) and urea are reported to form 5,6-diphenyl-l,2,4-triazin-3-one (611) (71MI21903), and a-hydrazonocarboxylic acids (612) can be cyclized with imidates to l,2,4-triazin-5-ones (613) (74T3171). Both these reactions can also be treated as [2+2 + 2] syntheses, since the hydrazones (610) and (612) are prepared from the appropriate ketones and hydrazine. 

Preparative Methods the most convenient preparation of (/ ,/ )-stilbenediamine is described in Organic Syntheses." Condensation of benzil and cyclohexanone in the presence of ammonium acetate and acetic acid (eq 1) produces a spirocyclic 2//-imidazole (mp 105-106 °C). Reduction with Lithium in THF/NH3 followed by an ethanol quench and hydrolysis with aqueous HCl (eq 2) affords the racemic diamine as a pale yellow solid (mp 81-82 °C). Resolution is achieved by multiple recrystallizations of the tartaric acid salts ifom water/ethanol. The sulfonamides are prepared by reaction of the enantiomeri-cally pure diamine with the appropriate anhydride or sulfonyl chloride in CH2CI2 in the presence of Triethylamine and a catalytic amount of 4-Dimethylaminopyridine (eq 3). 

Preparative Methods (i) preparation of racemic DPEN and its optical resolution Reaction of benzil and cyclohexanone in the presence of ammonium acetate and acetic acid at reflux temperature gives a cyclic bis-imine (1) (eq 1). Stereoselective reduction of the bis-imine with lithium in THF-liquid ammonia at —78 °C followed by addition of ethanol, then hydrolysis with hydrochloric acid and neutralization with sodium hydroxide produces the racemic diamine (2). Recrystallization of the l-tartaric acid salt from a 1 1 water-ethanol mixture followed by neutralization with sodium hydroxide, recrystallization from hexane results in (5,5)-DPEN (3) as colorless crystals. 

Oligomeric benzil ketals (DTGPA), prepared by reaction of 2,2-dimethoxy-2-phenyl-acetophenone (DMPA) with triethyleneglycol, in the presence of p-toluen-sulphonic acid, are reported [88] to be efficient in the UV curing of styrene/ unsaturated polyester formulations giving rise to coatings with very low odour. 

Either antipode of bromoborane 332 can be prepared in a six-step sequence from benzil (330, Scheme 11-19) [127, 254, 258J. Reaction of benzil with cyclohexanone in the presence of ammonium acetate and acetic acid generates a cyclic bis-imine which is subsequently reduced with lithium in ammonia. The resulting racemic fra .s-imidazolidine is subsequently hydrolyzed to the diamine 331. Resolution of 331 is accomplished by crystallization with either antipode of tartaric acid. The enantiomerically enriched stein ligand 331 is then sulfonylated and condensed with boron tribromide, giving the chiral bromoborane 332. Transmetalla-tion of allyltri-n-butylstannane with bromoborane (R,/ )-332 then affords the allyl-boron reagent (R,/ )-198. 

Diarylquinoxalines are similarly prepared by reaction of o-phenylenediamine with benzils. The required benzils have been prepared traditionally by oxidation of the corresponding benzoins, which in turn have been prepared by treatment of the appropriate aldehydes with potassium cyanide. In a more recent procedure, benzils have been obtained in good yield by oxidation of para-substituted diphenyloxazoles with bromine in acetic acid. The oxazoles themselves are readily prepared by cyclization of the benzoin acetate with urea (Scheme 3). ... 

When benzoin phenylhydrazone is heated for 2 hours in glacial acetic acid, a 77% yield of benzil is obtained,611 the phenylhydrazine formed acting as dehydrogenating agent on the secondary hydroxyl group. Oximes can be cleaved in a very mild reaction by nitrous acid or isopentyl nitrite in the cold, these methods being especially indicated for isolation of sensitive aldehydes for instance, Mannich and Budde prepared succindialdehyde as follows 612... 

Oxalyl chloride affords diketones (benzene gives benzil) but under the reaction conditions oxalyl chloride decomposes partly to phosgene, so that the acid chloride or monoketone may also be formed.606 The method thus has value also for preparation of carboxylic acids as examples, carboxylation of mesitylene in 65-70% yield by oxalyl chloride is recorded in Organic Syntheses,601 and l,l-di-(p-tolyl)ethylene gives 3,3-di-(/ -tolyl)acrylic acid in 80% yield.608... 

The second general route to 1,3-dioxepanes (1) involves acid-catalyzed alcohol exchange of an acyclic acetal (6) with a 1,4-diol (transacetalization) <8iJOC298i>, as demonstrated by the synthesis of (12 X = C1, Br) by reaction of 2-halo-1,1-dialkoxyethane with 1,4-butanediol <79MI 911-01, 95JOC5729) or the preparation of (13) from benzil dimethyl acetal <90UKZlll6>. Transacetalization is preferred for the synthesis of 1,3-dioxepanes derived from ketones <61JOC4762> and the stereo-... 

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