Chiral ligands, Betti reaction

On the basis of Betti s initial work, the Betti reaction has been extended to both primary amines and secondary amines. The resulting Betti amines have been applied as ligands or chiral auxiliaries in asymmetric synthesis. In addition, the Betti reaction has also been extended to the reaction among aldehydes, secondary amines, and compounds of active methylene moiety, such as dibenzyl ketones which can tautomerize to enols and mimic naphthol. Moreover, the reaction of aromatic aldoximes and 0-keto esters to afford isoxazolones should also be considered as an extension of the Betti reaction. 

A number of studies have also been reported using enantiopure Betti base derivatives as chiral ligands in asymmetric reactions. For example, Chan et al. reported that Betti base 33 was a useful ligand for the additional of diethyl zinc to aryl aldehydes, with excellent enantiomeric excesses... 

Either ammonia or a variety of amine substrates can be used to prepare the product 2, widely called a Betti base. Various substitution patterns are tolerated on both the naphthol and aryl aldehyde component. While the reaction was classically performed in ethanol, a variety of solvents, and using the substrates neat are also possible. Increased rates have been observed using acid catalysis. The reaction results in a product 2 with a benzylic chiral center, which as such can be resolved into its enantiomers. Alternatively, chiral amines can be used to control the stereoselectivity of the process. Enantiomerically pure Betti bases have shown potential as chiral auxiliaries and as ligands in asymmetric reactions. ... 

In light of the rapid and scalable methods to synthesize enantiomerically pure Betti bases, a number of studies have surveyed their potential as chiral auxiliaries and as ligands in asymmetric reactions. Building on their previous studies to use the Betti base as a chiral auxiliary in the preparation... 

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See also in sourсe #XX -- [ ]

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