Stereoselectivity cinchona-PTCs

In 2005 and 2006, Siva and coworkers reported several types of new dimeric and trimeric cinchona-PTCs 23-27 and their application to the asymmetric alkylation of 4b (Scheme 6.8). While, generally, high chemical and optical yields were obtained using dimeric 23 and 24, trimeric PTCs 2S-27 showed slightly lower stereoselectivities [19]. 

The development of polymeric cinchona-derived PTCs was triggered by the group of Jew and Park in 2001 [8]. The group paid particular attention to the fact that the cinchona alkaloids have demonstrated great utility in the Sharpless asymmetric dihydroxylation. Especially, it was noted that the significant improvements in both stereoselectivity and scope of the asymmetric dihydroxylation were achieved when the dimeric ligands of two independent cinchona alkaloid units attached to heterocyclic spacers were used, such as (DHQ)2-PHAL or (DHQD)2-PYR (Figure 4.4) [9]. 

As shown in the next chapters, some of these reactions were also successfully employed to access the chiral skeletons of complex natural products or biologically active molecules. The following chapters will therefore highlight the successful use of the three most prominent chiral ammonium salt PTC classes Cinchona alkaloids, Maruoka s catalysts, and Shibasaki s catalysts) to facilitate demanding stereoselective key steps in complex natural product syntheses and in the synthesis of biologically active (either natural or synthetic) compounds. 

In addition to PTCs derived from Cinchona alkaloids, chiral PTCs derived from (/ )- or (5)-binaphthol (the so-called Maruoka catalysts) have emerged as one of the most powerful catalyst classes in organocatalysis. Without exaggeration it can be said that the Mamoka group has really pioneered a whole field herein by carefully developing these catalysts and by introducing a variety of complex and extraordinarily stereoselective transformations facilitated by these highly potent catalysts [9,20]. These catalysts have also recently... 

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