Benzoins mixed

By the use of 1 mol each of two different aldehydes, an unsymmetrical or mixed benzoin is obtained, for example ... 

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... 

Regio-control of the formation of benzoic esters of mixed benzoin derivatives results from the initial formation of the cyanhydrin benzoate (see Chapter 3). Subsequent base-catalysed hydrolysis of the esters produces the mixed benzoins (Scheme 6.20) [66]. 

Scheme 2.2.7.23 Asymmetric synthesis of mixed benzoins 41a-41c by use of BFD H281A.

Remarkably, the 2,2 -disubstituted benzoin or the mixed benzoin substituted in the 2 position was not generated in any of these reactions, revealing that the ortho-substituted benzaldehydes 40a-40c react selectively as acceptors, as expected. 

The selective donor-acceptor concept can be transferred to other ThDP-dependent enzymes. For example, enantiopure mixed benzoins were obtained when 2-chlorobenzaldehyde reacted with a variety of selective donor aldehydes in the presence of BAL [67]. By performing various cross-benzoin condensation reactions with this enzyme, not only new selective donors but also additional aldehydes reacting selectively as acceptors, such as 2-iodobenzaldehyde or 2,6-difluorobenzaldehyde, could be identified. Again all the mixed benzoins generated exhibited an R-configuration and were obtained with high to excellent enantiomeric excesses [69]. 

Unsymmetrical or mixed benzoins may be obtained in good yield from two different aldehydes ... 

Crossed condensation of two aldehydes has been accomplished as in the preparation of p-methoxybenzoin. The condensation is reversible, as has been demonstrated by the preparation of mixed benzoins from benzoin and an aromatic aldehyde. ... 

This reaction was first described as a new synthesis for mixed benzoins. A solution of the aryl glyoxal in the aromatic hydrocarbon is stirred at 0° for 3-20 hours with aluminum chloride. Carbon disulfide may be used as a solvent if necessary. The yields vary from 35% to 90%. The reaction has been extended to the preparation of a-hydroxy ketones of the types RCOCHOHAr and CHjCOCOHlCHjlAr by substituting r-butylglyoxal and biacetyl, respectively, for the aryl glyoxal. 

The fact that treatment of a benzoin with potassium cyanide in the presence of a second aldehyde leads to mixed benzoins indicates that all the steps are reversible (equation 1). 

Upon treating certain (but not all) aromatic aldehydes or glyoxals (a-keto aldehydes) with cyanide ion (CN ), benzoins (a-hydroxy-ketones or acyloins) are produced in a reaction called the benzoin condensation. The reverse process is called the retro-benzoin condensation, and it is frequently used for the preparation of ketones. The condensation involves the addition of one molecule of aldehyde to the C=0 group of another. One of the aldehydes serves as the donor and the other serves as the acceptor. Some aldehydes can only be donors (e.g. p-dimethylaminobenzaldehyde) or acceptors, so they are not able to self-condense, while other aldehydes (benzaldehyde) can perform both functions and are capable of self-condensation. Certain thiazolium salts can also catalyze the reaction in the presence of a mild base. This version of the benzoin condensation is more synthetically useful than the original procedure because it works with enolizable and non-enolizable aldehydes and asymmetric catalysts may be used. Aliphatic aldehydes can also be used and mixtures of aliphatic and aromatic aldehydes give mixed benzoins. Recently, it was also shown that thiazolium-ion based organic ionic liquids (Oils) promote the benzoin condensation in the presence of small amounts of triethylamine. The stereoselective synthesis of benzoins has been achieved using chiral thiazolium salts as catalysts. 

One of the most interesting aspects of the formation of mixed benzoins is the structure of the product obtained. With p-dimethylaminobenzal-... 

Finally, product formation is reversible. Thus, treatment of 2-hydroxy-l,2-diphenyl-ethanone (benzoin) with other aromatic aldehydes in the presence of cyanide results in the formation of mixed benzoins (Equation 9.50). ... 

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