Thiourea derived catalysts squaramide-based

As described above, cinchona-based (thio)ureas have proven to be highly efficient H-bond donor catalysts. In 2008, Rawal and coworkers developed a highly promising new family of cinchona-based H-bond donor catalysts such as 157 by replacing the thiourea moiety of cinchona-based thiourea catalysts with the squaramide unit [47]. The squaramide moiety of 157 is able to form two H-bonds to a reactant due to the more accessible reaction site and fixed syn-orientation of the NH-protons. Using only 0.5 mol% of the cinchonine-derived squaramide catalyst 157, various Michael donors 158 and nitroalkenes 130 were smoothly converted to the desired adducts 159 in excellent yield and ee values (up to 99% ee) (Scheme 9.54). 

Song and coworkers further utilized squaramide 33 for methanolytic desym-metrization of meso-glutaric anhydrides (Scheme 10.35) [113, 114]. The authors attributed the high enantioselectivity to the fact that the cinchona-derived dimeric squaramide catalyst does not self-associate. The catalyst can be easily recovered from the reaction mixture after extractive acid/base work up. A thiourea-catalyzed version of this process had been reported by the same authors in 2008 [115]. 

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