Cross-benzoin condensation enantioselectivity

Unfortunately, the chiral bicyclic triazolium salt that had been found to be an excellent catalyst for the enantioselective intermolecular benzoin condensation proved to be ineffective in the intramolecular reaction. In searching for alternative catalysts, we synthesized the novel triazolium salts 19 and 20, starting from easily accessible enantiopure polycyclic y-lactams (Schemes 9.4 and 9.5) that finally delivered good results in the enantioselective intramolecular cross-benzoin condensation [35]. 

The precatalyst 20 led to excellent results in the enantioselective intramolecular crossed benzoin condensation of the aldehyde ketones 24, as shown in Scheme 9.6. The quaternary stereocenter of the acyloins 25 was created with good to very good yields and excellent ee-values. (For experimental details see Chapter 14.20.1). The precatalyst 19 proved to be even more active, and the yields were consistently excellent, albeit accompanied by lower ee-values (63-84%). 

The substrates of the enantioselective intramolecular crossed benzoin condensation were varied to widen the scope of the reaction. Promising results were... 

Melchione s team reported the asymmetric catalysis of Diels-Alder reactions of IQDs (Scheme 11, equation 1) [69, 70], In addition to other nitro-substituted arylethenes, methyleneindolinones were employed as dienophiles. A limited selection of the compounds synthesized is shown in Scheme 11 (30-32). The third compound (32) is the result of a final cross-benzoin condensation. Chen and colleagues effected an asymmetric Diels-Alder reaction of IQDs (33) generated under mild acidic conditions from 2-methyl-3-indolemethanols and a,p-unsaturated aldehydes (equation 2) [71], Three representative indoles that were prepared in this fashion are 34 to 36. The IQD 33 is presumed to be in equilibrium with the 3-vinylindolenium species. A wide range of substituted trani-cinnamalde-hydes was successfully employed. Although other acids (HOAc, TFA, PhCO H, silica gel) effected the reaction, Montmorillonite KIO clay was superior in terms of yield, enantioselectivity, and diastereoselectivity. 

A -Heterocyclic carbenes represent valuable organocatalysts for the enantioselective cross-benzoin type condensation between aldehydes and ketones, giving a-hydroxyketones derivatives (Scheme 7.24) [40],... 

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