Hydrobenzoin and

The stereoselective synthesis of hexacoordinated phosphate anions was also reported by the same group. A general one-pot process was developed for the preparation of C2-symmetric anions 15,16 and 17 containing enantiopure BINOL, hydrobenzoin, and tartrate-derived ligands respectively [38-40] Cpsymmetric anion 18 being prepared similarly in two steps from methyl-a-... 

The use of tartrate esters was an obvious place to start, especially since both enantiomers are readily available commercially and had already found widespread application in asymmetric synthesis (Figure 11) (e.g.. Sharpless asymmetric epoxidation).23.24 Reagents 36-38 are easily prepared and are reasonably enantioselective in reactions with achiral, unhindered aliphatic aldehydes (82-86% ee) typical results are given in Figure 12.3c,h Aromatic and a,p-unsaturated aldehydes, unfortunately, give lower levels of enantioselection (55-70% e.e.). It is also interesting to note that all other C2 symmetric diols that we have examined (2,3-butanediol, 2,4-pentanediol, 1,2-diisopropylethanediol, hydrobenzoin, and mannitol diacetonide, among others) are relatively ineffective in comparison to the tartrate esters (see Table ll).25... 

Benzaldehyde.—Kauffmann,3 by electrolyzing benzaldehyde in a 12-15% solution of potassium bisulphite, obtained at the cathode a mixture of hydrobenzoin and isohydrobenzion. According to his statements,4 an alcoholic solution of sodium hydroxide is more suitable for the reaction than the aqueous solution of bisulphite. Other aldehydes and ketones show a behavior similar to that of benzaldehyde, as will be explained under the individual substances. 

Resolution by entrainment is best illustrated through the use of an example, and the laboratory scale resolution of hydrobenzoin [38] is an appropriate example. Initially, 1100 mg of racemic material is dissolved along with 370 mg of (—)-hydrobenzoin in 85 g of 95% ethanol, and then the solution is cooled down to 15 °C. Ten milligrams of the (—)-isomer is added in the form of seed crystals, and a crop of additional crystalline material is allowed to form. After 20 minutes, 870 mg of (—)-hydroben-zoin was recovered. Then, 870 mg of racemic hydrobenzoin was dissolved with heating. The resulting solution was cooled to 15 °C and seeded with 10 mg of the (+)-isomer. The quantity of (+)-hydrobenzoin recovered at this time was 900 mg. The process was cycled 15 times, and ultimately yielded 6.5 g of (—)-hydrobenzoin and 5.7 g of ( )-hydrobenzoin. Each isomer was obtained as approximately 97% enantiomerically pure. 

Reactions of organosamarium(II) halides with aldehydes are sranewhat more complicated and synthetically less useful than those with ketones. The ability of Sm species to serve as strong reducing agents introduces a number of alternative reaction pathways. For example, reaction of EtSmI with benzalde-hyde provides a mixture of benzyl alcohol, benzoin, hydrobenzoin, and benzyl benzoate in low yields. Die first three products presumably arise from benzaldehyde ketyl, generated by single-electron transfer from the Sm reagent to benzaldehyde. The benzyl benzoate apparently is derived frmn a Tischenko-type condensation reaction between a samarium alkoxide species and benzaldehyde. 

The first stage of the process consists of refluxing the glycol with 1,1 -thiocarbonyldi-imidazole for 30 min 48 the resulting cyclic thiocarbonate is converted into the olefin by 70-80 hours refluxing with an excess of trimethyl phosphite. In this way, for example, cis-stilbene was obtained in 92% yield from meso-hydrobenzoin, and trans-cyclodecene in 81 % yield from trans-1,2-cyclodecanediol. 

Reductive couplings The low-valent titanium species fonned by interaction of TiCl4 with EtiN is capable of transforming ArCHO and ArCH=NAr to hydrobenzoins and 1,2-diarylethylenediamines, respectively. 

Thus from the sodium salt of glycolic acid (CH,OH. COOH), ethyl-glycolic acid (CH,OC,Hj.COOH), and a-lactic acid (CH3.CH(OH)COOH) the chief product obtained is acetic aldehyde, and it is quite possible that the production of hydrobenzoin and isohydrobenzoin from mandelic acid is due to the electrolytic reduction of the benzaldehyde originally formed. 

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. 

Benzoin yields hydrobenzoin on reduction with sodium amalgam, and clesoxybenzoin, C H,CO.CHj.C,Hj, when reduced with zinc and hydrochloric acid. 

Cr(II) reduction of benzaldehyde in aqueous acidic ethanol also yields largely hydrobenzoin although other products were characterised, e.g. hydrobenzoin monoethyl ether, and a 1 1 stoichiometry is preserved. The rate law in ethanol acidified both with HCIO4 and HCl is complex for example, in HCl-ethanor°°,... 

Rovis and co-workers have extended the application of redox transformations to generate chiral acylazolium species from a-haloaldehydes 245 and the NHC derived from pre-catalyst 247, allowing the desymmetrisation of mei o-hydrobenzoin 246 to give ester 248 in good yield and enantioselectivity (Scheme 12.54) [30]. 

The enantioselective synthesis of an allenic ester using chiral proton sources was performed by dynamic kinetic protonation of racemic allenylsamarium(III) species 237 and 238, which were derived from propargylic phosphate 236 by the metalation (Scheme 4.61) [97]. Protonation with (R,R)-(+)-hydrobcnzoin and R-(-)-pantolactone provided an allenic ester 239 with high enantiomeric purity. The selective protonation with (R,R)-(+)-hydrobenzoin giving R-(-)-allcnic ester 239 is in agreement with the... 

It oxidizes thiophenol to diphenyl disulphide, hydroquinone to p-benzoquinone, benzoin to benzyl, benzylic alcohols to the corresponding carbonyl compounds and cleaves hydrobenzoin to benzaldehyde. 

In the context of our work in the area of chiral nucleophilic carbenes and their utility in organic synthesis, we have developed a conceptually distinct approach to catalyzed acylation using a-haloaldehydes as acylation precursors. The use of a chiral triazolium salt in the presence of base allows an enantioselective desymme-trization of meio-hydrobenzoin to proceed in 83% ee and good yield ... 

A Raney nickel surface is also suitable for electrocatalytic hydrogenation [205]. This surface is prepared by electrodepositing nickel from a solution containing suspended Raney nickel alloy (Ni 50% A1 50%). Some alloy particles stick to the surface, which is then activated by leaching the aluminium using hot aqueous sodium hydroxide. Cyclohexanone, acetophenone and benzil have been converted to the corresponding alcohol and there is no stereoselectivity for the formation of hydrobenzoin from benzil. 

Figure 14. Chiral 1,2-diols that have been incorporated into monocyclic crown compounds. Listed under (o) are piecursors with C2 symmetry obtained from L-taitaric acid and L-thieitol, as well as from (5S)-hydrobenzoin as its p-methoxy analog, under (b) are precursors with Cj symmetiy obtained from D-mannitol and L-iditol, and under (c) are asymmetric precursors obtained from (5)-lactic acid, (5)-mandelic acid, and L-glyceraldehyde dithioethylacetal.

Both the (RR)- and (5S)-enantiomers of hydrobenzoin (Figure I4a) have been made to react (110, 117, 118) successfully in a (2 + 2) cyclization with ethylene glycol ditosylate in the presence of bases (e.g., NaOH in dioxane or NaH in MA -dimethylformamide) to give the enantiomeric tetiaphenyl-18-crown-6 derivatives (RRRR)-73 and (SSSS)-73. The corresponding optically pure tetra-anisyl, tetra-a-naphthyl, and tetra-/3-naphthyl-18-crown-6 derivatives 74, 75, and 76, respectively, have also been prepared in similar fashion (119). It should be noted that (RRRR)- and (,SSSS)-73 have also been obtained as a result of a base-promoted (1 + 1) cyclization (120) between the chiral extended diol and... 

The first method is successful on a large scale with methadone (3) and l,2-diphenyl-l,2-ethanediol (hydrobenzoin, 4). 

Table 7 shows a series of alcohols analyzed with the Alexakis reagent. In the 31P-NMR spectra, alcohol derivatives appear as two singlets (P—H decoupled) corresponding to each diastereomer. Shift differences are about 10 times larger in the primary, nonthionated product. No racemization could be observed during the derivatization step as demonstrated with ethyl lactate and hydrobenzoin. 

The only good method for the preparation of benzoin1 appearing in the literature is by the action of potassium cyanide upon an alcoholic solution of benzaldehyde, a method described first by Wohler and later modified slightly by Zinin and Zincke. The other methods are of theoretical importance only and have, therefore, not been studied. Thus, by the reduction of benzil,2 and by the oxidation of hydrobenzoin,3 benzoin can be formed it happens, however, that both benzil and hydrobenzoin are commonly made from benzoin. 

Both benzil and benzoin are reduced by TiCl3 in an acidic medium to hydrobenzoin (95% yield) but in this case the meso-isomer is strongly favored over the d/-isomer (—80 20). 

Di(arenesulfonyl) peroxides (20, R = R1 = aryl) react with aromatic solvents to form aryl arenesulfonates. These peroxides also form 1 1 adducts with styrene and form hydrobenzoin diarenesulfonates with slilbcncs. Di(bcn/cncsulfonyl) peroxide decomposes in water to phenol and sulfuric acid. 

Chiral Ti complex, derived from hydrobenzoin dilithium salt and TiCl4, can be used for the asymmetric Diels-Alder reaction of several dienes with fumarate [53] (Eq. 8A.30). However, attempted use of acrylate as dienophile resulted in low enantioselectivity. 

Electron-withdrawing groups close to the alcohol functionality may likewise destabilize intermediate carbocations and result in very slow oxidations. For instance, sterol 88 is oxidized with DDQ at the allylic alcohol two hundred times slower than the corresponding compound lacking the fluorine atom,94 and the treatment of hydrobenzoin (89) with DDQ results in the oxidation of a single alcohol because a second oxidation would involve a carbocation highly destabilized by the presence of a carbonyl group.95f... 

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