Thiourea catalysts quinine-based

Quite recently, we also observed that quinine-based thiourea derivatives showed dramatic concentration and temperature effects on the enantioselectivity in the alcoholytic desymmetrization ofmeso-cyclic anhydrides, which can also be attributed to the self-association of the catalyst [23]. Of course, the possibility that the variation in... 

In addition to chiral PTCs, cinchona-based thioureas have also been proved to serve as catalysts for nitro-Mannich reactions. In 2006, Ricci and coworkers first reported that the quinine-based thiourea 40 (20mol%) can catalyze the aza-Henry reaction between nitromethane and the N-protected imines 93 derived from aromatic aldehydes [40]. N-Boc-, N-Cbz-, and N-Fmoc protected imines gave the best results in terms of the chemical yields and enantioselectivities (up to 94% ee at —40°C) (Scheme 8.30). 

Later on, the same group developed a highly stereoselective Michael-Michael cascade process catalyzed by the quinine-based thiourea 81b [29b]. In the presence of a low catalyst loading (2 mol%), the reaction of trans-3-(2-mercaptophenyl)-2-prope-noic acid ethyl ester (106) with a wide range of nitroalkenes (107) afforded thio-chromanes (108) having three new stereocenters with high stereoselectivity (dr >30 1, up to 99% ee, Scheme 9.36). 

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