Asymmetric Nucleophilic Acylations with Synthetic Equivalents for the Active Aldehydes

3 Asymmetric Nucleophilic Acylations with Synthetic Equivalents for the Active Aldehydes  

When we used a,P-unsaturated sulphones and phosphonates as well as aldehydes (acyloin reaction) as electrophiles to trap the metalated aminonitriles, moderate to good ee-values were obtained and we are still engaged to optimize these variants [58]. 

Although the chiral auxiliary employed in the stoichiometric aminonitrile chemistry described above can be recycled in good chemical yield and maintaining its optical purity, of course, a more elegant and economical solution would be a catalytic process. Therefore we tried to imitate nature again (scheme 17) by developing an enantioselective thiazolium salt catalysis. 

First of all, we had to work out a simple and flexible method for the synthesis of chirally modified thiazolium salts, preferably with the chiral group attached to the nitrogen atom. With a slightly modified procedure already published by Tagaki et al. [59], we were able to prepare a number of novel chiral thiazolium salts. 

By exchange of the gegenion of the hygroscopic chlorides to tetrafluorobo-rates or hexafluorophosphates, crystalline, non hygroscopic salts were obtained. 

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