Thiourea derivatives, organocatalysis with

Scheme 12.19 Organocatalysis with an activated thiourea derivative (Acat/Auncat = 8 at 1 mole % in CHC13).

A pioneer in the field of the asymmetrie (thio)urea organocatalysis was Eric Jacohsen, who first reported a chiral (polymer-hound) Schiff base thiourea derivative for asymmetric Strecker reactions optimised from parallel synthetic libraries/ These catalysts can be used either in solution or immobilised to a polystyrene resin, with the latter retaining efficiency, after repeated recycling/ The key factors responsible for high enantioselectivities were the presence of bullqr substituents at both the amino acid position and at the 3-position of the aromatic ring (Scheme 19.3). 

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. 

This gives chapter an overview of natural cinchona alkaloids and synthetic derivatives together with examples of their use in asymmetric organocatalysis. In recent years, the emphasis has been on the development of cinchona-based bifunctional catalysts, in particular species with a thiourea moiety. The search for new cinchona-based organocatalysts continues and new derivatives are relentlessly being prepared and applied for specific enantioselective reactions. The design of these new... 

A quantitative approach to nucleophilic organocatalysis 12BJ01458. Selective photocatalytic reactions with organic photocatalysts 13CS561. Terpene-derived bifunctional thioureas in asymmetric organocatalysis 13CAC2756. 

New cinchona derivatives are continuously being developed for appUcation in asymmetric organocatalysis. Examples of catalysts obtained by further modification of existing derivatives include an N-oxide of dihydrocupreidine [127], a C6 -N-Boc-glycine-P-isocupreine (53) [128], a C9 thiourea substituted at a remote site with a sulfonamide group (54) [129, 130], and catalysts with an amine as well as a thiourea group (55) [131] (Figure 6.16). 

Recently, a dual organocatalysis approach, namely, the combination of the achiral nucleophilic Lewis base catalyst DMAP (23) and the chiral anion-binding thiourea catalyst 27, was applied to the Steglich rearrangement to provide a,a-disubstituted amino acid derivatives 24 in a highly enantioselective fashion (Scheme 43.5) [14]. Notably, replacement of the nucleophilic codiamino acid derivatives with excellent enantiomeric excesses (88-93% ee). 

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