Asymmetric reductions, cinchona-based

In contrast to the aforementioned fullerenes, C76 is a chiral molecule containing 30 different types of carbon-carbon bond. In this molecule five different pyracylene-type carbon-carbon bonds repeat to form chrysene-shaped units. Kinetic resolution of this fullerene has been achieved via asymmetric osmylation in the presence of a cinchona based chiral ligand (see Section 4.4.4.1.1., ligand 1 d/2 d, Table 5). The calculated enantiomeric excess of the recovered material (after 95% conversion) is >97%, whereas the regenerated C76, formed by tin(II) chloride reduction of the osmylated material (after 33 % conversion), is enriched in the opposite enantiomer. Analysis of the local curvature of the C76 molecule indicates that Os04 should selectively add to two of the 30 types of bonds86. 

Cinchona-Based Organocatalysts for Asymmetric Oxidations and Reductions... 

As mentioned briefly in Chapter 2, very few publications describing cinchona alkaloid-based asymmetric reduction systems have appeared, despite the importance of this reaction, and they are restricted to the reduction of aromatic ketones. 

Consequently, Dehmlow and coworkers modified the cinchona alkaloid structure to elucidate the role of each ofthe structural motifs of cinchona alkaloid-derived chiral phase-transfer catalysts in asymmetric reactions. Thus, the quinoline nucleus of cinchona alkaloid was replaced with various simple or sterically bulky substituents, and the resulting catalysts were screened in asymmetric reactions (Scheme 7.2). The initial results using catalysts 8-11 in the asymmetric borohydride reduction of pivalophenone, the hydroxylation of 2-ethyl-l-tetralone and the alkylation of SchifF s base each exhibited lower enantiomeric excesses than the corresponding cinchona alkaloid-derived chiral phase-transfer catalysts [14]. 

In 2003, Rawal reported the use of TADDOLs 177 as chiral H-bonding catalysts to facilitate highly enantioselec-tive hetero-Diels-Alder reactions between dienes 181 and different aldehydes 86 (Scheme 6.29A) [82], and also BINOL-based catalysts 178 were found to facilitate this reaction with excellent selectivities [83]. TADDOLs were also successfully used as organocatalysts for other asymmetric transformations like Mukaiyama aldol reactions, nitroso aldol reactions, or Strecker reactions to mention a few examples only [84]. In addition, also BINOL derivatives have been employed as efficient chiral H-bonding activators as exemplified in the Morita-Baylis-Hilhnan reaction of enone 184 with different carbaldehydes 86 [85]. The use of chiral squaramides for asymmetric reactions dates back to 2005 when Xie et al. first used camphor-derived squaric amino alcohols as ligands in borane reductions [86]. The first truly organocatalytic application was described by Rawal et al. in 2008 who found that minute amounts of the bifunctional cinchona alkaloid-based squaramide 180 are... 

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