Cinchona squaramide moiety

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). 

A Study by Xu and coworkers examined a series of cinchona alkaloid derivatives with a benzyl or 1-phenylethyl group at the second nitrogen atom of the squaramide moiety [91]. The catalyst with 1-phenylethyl group and a C6 methoxy group (32) gave the highest yield and enantioselectivity for the conjugate addition of 4-hydroxycoumarins to a-keto esters (Scheme 6.42). 

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