Catalysis with C9 and C6 Thiourea Derivatives

The C9 thiourea derivative 23 [67] was appUed in a plethora of organic reactions. Much of this research has been reviewed elsewhere (see, for example. References [3e,g,i,j]) and demonstrates the outstanding potential and versatility of thiourea-derived cinchona alkaloids for organocatalysis. Recently, several studies were concerned with conjugate additions catalyzed by C9 thiourea derivatives. For example. 

The conjugate addition of aromatic and aliphatic a-nitro-ketones to a,P- 

Recently, 27 was appHed in the asymmetric conjugate addition of thiols to N-acylated oxazoHdin-2-one derivatives of a,P-unsaturated amino acids [80]. The products were formed in good yields, with a low to moderate diastereoselectivity 

Catalysis with C9 Sulfonamide, Squaramide, and Guanidine Derivatives 

Comparable results were obtained for asymmetric conjugate addition of aromatic and aliphatic thiols to chalcones [88]. For this reaction it was also observed that 31 was a more suitable catalyst than derivatives with, for example, a 4-CH3OQH4, a 3,5-(CH3)2QH3, or a cydohexyl group, which is in line with the expectation that the acidity of the squaramide N-H groups is related to the stereoselectivity and yield of the catalytic reaction. 

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