Benzil preparation

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 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 ... 

This experiment works best with pure benzil. The benzil prepared in Experiment 32B is usually of sufficient purity after it has been crystallized. 

Chemical Tests. Ketones and aldehydes are often characterized by the preparation of a solid derivative. To assist in the characterization of benzil, prepare its 2,4-dinitrophenylhydrazone or semicarbazone (Chapter 9). The [www]H> melting points of these derivatives are listed in Appendix A, Table A.5. 

Usually prepared by the action of NaCN on benzaldehyde in dilute alcohol. It is oxidized by nitric acid to benzil, and reduced by sodium amalgam to hydrobenzoin PhCHOHCHOHPh by tin amalgam and hydrochloric acid to des-oxybenzoin, PhCH2COPh and by zinc amalgam to stilbene PhCH = CHPh. It gives an oxime, phenylhydrazone and ethanoyl derivative. The a-oxime is used under the name cupron for the estimation of copper and molybdenum. 

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 ... 

Recrystallise 5 0 g. from about 40 ml. of hot rectified (or methylated) spirit upon cooling, 4-5 g. of pure benzoin (a white, crystaUine solid, m.p, 137°) separate. Reserve the remainder of the preparation for benzil and benziUc acid (Sections IV.126 and IV.127 respectively). 

Phenylbenzoyldiazomethane may be prepared by the oxidation of benzil-monohydrazone with mercuric oxide in the presence of dry etber as a solvent Tbe addition of a little alcoholic potassium hydroxide serves to catalyse the reaction ... 

Recently, Koitai et al. (17) have shown that 5,5-diphenyl-2,4-thiazolidinedithione (15) with aluminum chloride in refluxing toluene gives 4,5-diphenyl-A-4-thia2oline-2-thione (16) (Scheme 7). 3-Methyl-4,5-diphenyl (17) and 4,5-diphenyl-A-4-thia2oline-2-thiones (16) are obtained in very low yields (1 to 5%) as by-products of the reaction between deoxybenzoin. benzoin. l,2-diphenyl-1.2-ethanediol. 1.2-diphenylethanol, or benzil, and Sg in hexamethylphosphoamide (18), The transformation of A-4-thiazoline-2-ones to the corresponding thiones by P2S5 (19) is of little synthetic value since the latter are more easily prepared. 

Benzil. Ben il [134-81-6] (diphenylethanedione) is a yellow soHd that crystallines from alcohol in hexagonal prisms. Ben nil can be prepared by the oxidation of bennoin [579-44-2] (2-hydroxy-2-phenylacetophenone) (298,299), which is itself prepared by the self-condensation of bennaldehyde (300). Ben nil is commercially produced in Japan and is used as a uv resin curing sensitizer (301). It has also been suggested as a chigger repeUant (302). 

Diphenylcinnoline can be prepared from benzil monophenylhydrazone in the presence of about 80% sulfuric acid (49MI21200). Synthetically more important, however, is the cyclization of mesoxalyl chloride phenylhydrazones under Frledel-Crafts -conditions (61JCS2828). As outlined in Scheme 67, the starting mesoxalate phenylhydrazones are obtained by coupling dlazotized aromatic amines with diethyl malonate. After conversion... 

In continuance of the policy of including improved directions for preparations published in previous volumes, a new set of directions is given for the preparation of benzil. 

Benzopinacol has been prepared by the action of phenylmag-nesium bromide on benzil 1 or methyl benzilate. Usually it has been obtained by reduction of benzophenone, the reducing agents being zinc and sulfuric acid or acetic acid, aluminum amalgam, and magnesium and magnesium iodide. The present... 

Boujlel and Simonet used an electrochemical method to prepare a group of similar compounds, including compound ]5, shown in Eq. (3.41). In a typical case, benzil was reduced in DMF solution at the dropping mercury electrode in the presence of tetrabutylammonium iodide, used in this case as a supporting electrolyte rather than phase transfer catalyst. In the presence of diethylene glycol ditosylate, compound 15 (mp 77— 78°) was isolated in 10% yield. Using the same approach, acenaphthenedione was reduc-tively cyclized with triethylene glycol ditosylate to afford the product (mp 84—85°, 42% yield) shown in Eq. (3.42). 

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. 

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. 

The reaction of diketosulfides with 1,2-dicarbonyl compounds other than glyoxal is often not efficient for the direct preparation of thiophenes. For example, the reaction of diketothiophene 24 and benzil or biacetyl reportedly gave only glycols as products. The elimination of water from the P-hydroxy ketones was not as efficient as in the case of the glyoxal series. Fortunately, the mixture of diastereomers of compounds 25 and 26 could be converted to their corresponding thiophenes by an additional dehydration step with thionyl chloride and pyridine. 

This reaction is so facile that it is of value both for preparative and characterization purposes, Benzil and phenanthraquinone are convenient reagents for the characterization of o-diamines, and o-phenyl-enediamine is used commonly for the characterization of ff-dicarbonyl compounds. 

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. 

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. 

N-methyl-4-piperidyl benzilate methobromide To a suspension of 0.15 mol of freshly prepared silver bromide in 300 ml of anhydrous methanol is added a solution of 0.1 mol of quaternary iodide obtained as above. The mixture is stirred and refluxed for several hours after which time transhalogenation is complete. The mixture is cooled, the insoluble silver... 

N-ethyl-3-chloropiperidine was prepared according to the method of Fuson and Zirkle described in Volume 70, J. Am. them. Soc., p 2760. 12.0 g (0.0B1 mol) of N-ethyl-3-chloropiperidine was mixed with 1B.6 g (0.0B1 mol) of benzilic acid and 80 cc of anhydrous isopropyl alcohol as a solvent. The mixture was refluxed for 72 hours. The solution was then filtered and concentrated at 30 mm of mercury. The concentrate was dissolved in water, acidified with hydrochloric acid and extracted with ether to remove the unreacted benzilic acid. 

Amino-5-hydrazinopyrazole dihydrochloride 300 is a good source for the synthesis of this type of heterocyclic compound [78JCS(P1)885] and it was prepared by reaction of malononitrile with two equivalents of hydrazine. Reaction of 300 with ethyl pyruvate afforded 301. Unstable hydra-zone 302 formed when 300 was boiled with diacetyl rapidly cyclized to 303. Reaction of 300 with benzil gave 304 directly, which gave an acetyl derivative and resisted reductive deamination. On the other hand, a polymer was isolated from the reaction of 300 with glyoxal (Scheme 65). 

Other miscellaneous imines that underwent photoreaction with chromium alkoxycarbenes include iminodithiocarbonates [33],the mono-N-phenylimine of benzil and the bis-JV-phenyl imine of acetoin [20]. By preparing the chromium carbene complex from 13CO-labeled chromium hexacarbonyl, /J-lactams with two adjacent 13C labels were synthesized [34]. 

Bis(triphenylphosphoniomethyl)-2,3-pyrazinedicarbonitrile dibromide (338), prepared from tbe corresponding bis(bromometbyl) intermediate (337), reacted with benzil to afford 6,7-dipbenyl-2,3-quinoxalinedicarbonitrile (339) (NaH, Me2NCHO, 20°C 125°C, 9 b 58%) " several analogs were made similarly. " ... 

Preparation of the quaternary anticholinergic agent benzilonium bromide (47) is begun by conjugate addition of ethylamine to methylacrylate, giving aminoester 42. Alkylation of 42 with methyl bromo-acetate leads to diester 43, which is transformed into pyrrolidone 44 by Dieckmann cyclization, followed by decarboxylation. Reduction of 44 by lithium aluminum hydride leads to the corresponding amino-alcohol (45). Transesterification of alcohol 45 with methyl benzilate leads to 46. Benzilonium bromide (47) is obtained by alkylation of ester 46 with ethyl bromide. 2... 

In a similar sequence, reaction of ketoester 52 with 2-thienylmagnesium bromide gives a modest yield of the benzilic ester 53. Transesterification of this with aminoalcohol 51, prepared analogously to 45 by starting with methylamine, gives, after quaternization with methyl bromide, heteronium bromide... 

This ring system could be prepared by thionation of benzil monohydrazone 172 with Lawesson s reagent (LR) followed by cyclization. The reaction gives 2,3-diphenyl-7,8,9,9a-tetrahydro-27/,677-pyrido[2,TA][l,3,4]thiadiazine 173 in a diastereoselective way, with the hydrogens at 2- and 9a-position in trans orientation. Benzil monohydrazone 172 was obtained by condensation of benzil 170 with 1 equiv of iV-aminopiperidine 171 (Scheme 23) <1998TA1531>. 

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