Benzoin preparation

The crude benzoin, prepared as above, after being dried and finely powdered, is heated (with frequent shaking) in an open flask on a vigorously boiling water bath for 1-5 to 2-0 hours with twice its weight of pure concentrated nitric acid. The reaction mixture is then diluted with cold water and when the material which separates has solidified the liquid is poured off, and the solid, after being dried on porous plate, is crystallised from alcohol. The crystals, after filtration, are dried in air on filter paper. Melting point 95°. Yield about 90 per cent of the theoretical. 

In this experiment, benzoin, prepared in Chapter 54, is converted to the alkene fraws-stilbene ( -stilbene), which is in turn brominated and dehy-drobrominated to form the alkyne, diphenylacetylene. 

In this experiment, benzil is prepared by the oxidation of an a-hydroxyketone, benzoin. This experiment uses the benzoin prepared in Experiment 32A and is the second step in the multistep synthesis. This oxidation can be done easily with mild oxidizing agents such as Fehling s solution (alkaline cupric tartrate complex) or copper sulfate in pyridine. In this experiment, the oxidation is performed with nitric acid. 

Filter at the pump, and wash well with water. (Yield, about 20 g.) Recrystallise a portion from hot methylated spirit, reserving the remainder for the following preparation. Benzoin is a very pale yellow (almost colourless) crystalline substance, m.p. 137°. 

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). 

The cyclic carbonate of benzoin (4,5-diphenyl-l,3-dioxol-2-one, prepared from benzoin and phosgene) blocks both hydrogen atoms of primary amines after dehydration acid stable, easily crystallizable Sheehan oxazolinones are formed, which are also called Ox derivatives. The amine is quantitatively deblocked by catalytic hydrogenation in the presence of 1 equiv. of aqueous acid (J.C Sheehan, 1972, 1973 M.J. Miller, 1983). An intelligent application to syntheses of acid labile -lactams is given in the previous section (p. 161). 

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. 

Poly(acrylic acid) and Poly(methacrylic acid). Poly(acryHc acid) (8) (PAA) may be prepared by polymerization of the monomer with conventional free-radical initiators using the monomer either undiluted (36) (with cross-linker for superadsorber appHcations) or in aqueous solution. Photochemical polymerization (sensitized by benzoin) of methyl acrylate in ethanol solution at —78° C provides a syndiotactic form (37) that can be hydrolyzed to syndiotactic PAA. From academic studies, alkaline hydrolysis of the methyl ester requires a lower time than acid hydrolysis of the polymeric ester, and can lead to oxidative degradation of the polymer (38). Po1y(meth acrylic acid) (PMAA) (9) is prepared only by the direct polymerization of the acid monomer it is not readily obtained by the hydrolysis of methyl methacrylate. 

Benzoic acid [65-85-0] C H COOH, the simplest member of the aromatic carboxyHc acid family, was first described in 1618 by a French physician, but it was not until 1832 that its stmcture was deterrnined by Wn b1er and Liebig. In the nineteenth century benzoic acid was used extensively as a medicinal substance and was prepared from gum benzoin. Benzoic acid was first produced synthetically by the hydrolysis of benzotrichloride. Various other processes such as the nitric acid oxidation of toluene were used until the 1930s when the decarboxylation of phthaUc acid became the dominant commercial process. During World War II in Germany the batchwise Hquid-phase air oxidation of toluene became an important process. 

Oxazoles, prepared from carboxylic acids (benzoin, DCC NH4OAC, AcOH, BOSS % yield), have been used as carboxylic acid protective groups in a variety of synthetic applications. They are readily cleaved by singlet oxygen followed by hydrolysis (ROH, TsOH, benzene or K2CO3, MeOH ). 

This procedure is representative of a new general method for the preparation of noncyclic acyloins by thiazol ium-catalyzed dimerization of aldehydes in the presence of weak bases (Table I). The advantages of this method over the classical reductive coupling of esters or the modern variation in which the intermediate enediolate is trapped by silylation, are the simplicity of the procedure, the inexpensive materials used, and the purity of the products obtained. For volatile aldehydes such as acetaldehyde and propionaldehyde the reaction Is conducted without solvent in a small, heated autoclave. With the exception of furoin the preparation of benzoins from aromatic aldehydes is best carried out with a different thiazolium catalyst bearing an N-methyl or N-ethyl substituent, instead of the N-benzyl group. Benzoins have usually been prepared by cyanide-catalyzed condensation of aromatic and heterocyclic aldehydes.Unsymnetrical acyloins may be obtained by thiazol1um-catalyzed cross-condensation of two different aldehydes. -1 The thiazolium ion-catalyzed cyclization of 1,5-dialdehydes to cyclic acyloins has been reported. 

Although the catalysis of the dimerization of aldehydes to acyloins by thiazolium ion has been known for some tlrae, the development of procedures using anhydrous solvents which give satisfactory yields of acyloins on a preparative scale was first realized in the submitters laboratories. The mechanism proposed by Breslow - for the thiazolium ion-catalyzed reactions is similar to the Lapworth mechanism for the benzoin condensation with a thiazolium ylide replacing the cyanide ion. Similar mechanisms are involved... 

Triphenylethylene has been prepared by the reaction of phenylmagnesium bromide with benzyl benzoate, with desoxy-benzoin, or with ethyl pheaniylacetate, and by the reaction of diphenylketene-quinoline with benzaldehyde. The above procedure is an adaptation of that described by Hell and Wiegandt. ... 

The DMB carbamate can also be introduced through the 4-nitrophenyl carbonate. " It has been prepared from an isocyanate and 3, 5 -dimethoxybenzoin. The synthesis of a number of other substituted benzoins as possible protective groups has been described. ... 

A new brush-type CSP, the Whelk-0 1, was used by Blum et al. for the analytical and preparative-scale separations of racemic pharmaceutical compounds, including verapamil and ketoprofen. A comparison of LC and SFC revealed the superiority of SFC in terms of efficiency and speed of method development [50]. The Whelk-0 1 selector and its homologues have also been incorporated into polysiloxanes. The resulting polymers were coated on silica and thermally immobilized. Higher efficiencies were observed when these CSPs were used with sub- and supercritical fluids as eluents, and a greater number of compounds were resolved in SFC compared to LC. Compounds such as flurbiprofen, warfarin, and benzoin were enantioresolved with a modified CO, eluent [37]. 

Whereas condensation of a-hydroxy ketones such as benzoin and acetoin on heating with formamide [240] or ureas in acetic acid [239, 242] to form imidazoles such as 769 or 770 is a well known reaction, only two publications have yet discussed the amination of silylated enediols, prepared by Riihlmann-acyloin condensation of diesters [241], by sodium, in toluene, in the presence of TCS 14 [241, 242]. Thus the silylated acyloins 771 and higher homologues, derived from Riihl-... 

Homoenolate Reactivity The ability to generate homoenolates from enals and its application to the preparation of y-butyrolactones 30, through reaction with an aldehyde or aryl trifluoromethyl ketone, was reported independently by Glorius [8], and Bode and Burstein [9] (Scheme 12.4). A sterically demanding NHC catalyst is required to promote reactivity at the d terminus and to prevent competitive benzoin dimerisation. Nair and co-workers have reported a similar spiro-y-lactone formation reaction using cyclic 1,2-diones, including cyclohexane-1,2-dione and substituted isatin derivatives [10]. 

R)-Benzoins and (/ )-2-hydroxypropiophcnonc derivatives are formed on a preparative scale by benzaldehyde lyase (BAL)-catalyzed C-C bond formation from aromatic aldehydes and acetaldehyde in aqueous buffer/DMSO solution with remarkable ease in high chemical yield and high optical purity (Eq. 8.112).303 Less-stable mixed benzoins were also generated via reductive coupling of benzoyl cyanide and carbonyl compounds by aqueous titanium(III) ions.304... 

In the Asia-Pacific region, AItingia excelsa Noronha (Liquidambar altingiana Bl.), Alan-gia gracilipes Hemsl. (Amyris ambrosiaca L.f.), Liquidambar orientalis Mill, and Liquidambar formosana Hance are of medicinal value. The purified basalm obtained from the trunk of Liquidambar orientalis Mill, or prepared storax (British Pharmaceutical Codex, 1969) has been used similarly as Peru basalm in the form of an ointment to treat scabies and other skin diseases, and as an ingredient of Compound Benzoin Tincture. 

Attack by eCN is slow (rate-limiting), while proton transfer from HCN or a protic solvent, e.g. HzO, is rapid. The effect of the structure of the carbonyl compound on the position of equilibrium in cyanohydrin formation has already been referred to (p. 206) it is a preparative proposition with aldehydes, and with simple aliphatic and cyclic ketones, but is poor for ArCOR, and does not take place at all with ArCOAr. With ArCHO the benzoin reaction (p. 231) may compete with cyanohydrin formation with C=C—C=0, 1,4-addition may compete (cf. p. 200). 

A jug containing calcium hypochlorite (probably as moist solid) was used as a disposal receptacle for cyanide wastes from student preparations of benzoin. When a little acetic acid residue was inadvertently added, an explosion occurred, attributed to a cyanide-chlorine redox reaction. 

The procedure described is essentially that of Ballard and Dehn.1 Stilbene has also been prepared by reduction of desoxy-benzoin,20 benzaldehyde,23 and benzil 2o-2c by dehydrogenation of ethyl benzene,30 toluene,30- 33- 3, and bibenzyl 33-3alkaline reduction of phenylnitromethane,40 phenylnitroacetonitrile,40 and desoxybenzoin 43 by distillation of benzyl sulfone,50 benzyl sulfide,60-63 calcium cinnamate,5 cinnamic acid,5d phenyl cinna-mate,6e-6/ and diphenyl fumarate ie by dehydrohalogenation of a,a -dichlorobibenzyl60 and benzyl chloride 63 by dehalogenation of a,a,c/,a -tetrachlorobibenzyl70 and benzal chloride 73 by the coupling of cinnamic acid and phenyldiazonium chloride 8 by de-... 

Tetraphenylcyclopentadienone has been prepared by the action of phenylmagnesium bromide on benzaldiphenylmaleide,1 and by reduction, dehydration, and oxidation of the methylene-desoxybenzoin obtained by condensing formaldehyde with desoxy-benzoin.2 The present procedure is essentially that of Dilthey.3... 

Koenigkramer, R.E. and Zimmer, H., Benzoins via an acyl anion equivalent. Novel one-pot preparation of benzo[b]furans via benzoins using hydriodic acid, J. Org. Chem., 45, 3994, 1980. 

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