Other Primary Amino Acid Catalysts

Some other catalysts based on primary amino acid structures, such as 3-amino acids, dipeptides, and primary amine-amides/sulfonamides (19-22), have also been applied to asymmetric synthesis. 

Applications oT Non-Proline Primary Amino Acid Catalysts 

In the following sections, applications of the above non-proline amino acid catalysts in asymmetric organocatalytic reactions will be described in detail the discussion is organized according to the reaction type. 

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Chiral amines (both primary and secondary amines) and amino acids have been used as catalysts for aldol reactions, Mannich-type reactions, and other reactions that proceed through enamine intermediates. An enamine-based catalytic cycle is shown in Scheme 2.1. The catalytic cycle includes formation of an iminium intermediate between a donor carbonyl compound and the amine-containing catalyst, the formation of an enamine intermediate from the iminium, C-C bond forma-... 

The name protein comes from the Greek proteios, meaning primary. Although many other important biomolecules exist, the emphasis on protein as fundamental is appropriate. Proteins serve as important stmctural components of cells. More importantly, almost all the catalysts, or enzymes, in biological systems are composed of proteins. Proteins are linear chains of amino acids joined by peptide bonds. Twenty amino acids are incorporated into a protein by translation (for a reminder of these concepts, see Chapter 4). In some proteins, the amino acids are modified by subsequent post-translational events. The sequence of amino acids of a protein is termed its primary structure. 

The new Monsanto process makes use of a copper catalyst, which is used to reduce diethanolamine to DSIDA, the intermediate to Roundup (see Fig. 9.27). This process totally eliminates the use of ammonia, hydrogen cyanide, and formaldehyde, is free of contaminants and byproducts, and hence does not require further purification steps. The stream can be recycled after filtration of catalyst. Monsanto s process can also be used in the production of other amino acids such as glycine through reduction and is a general method for conversion of primary alcohols to carboxylic acid salts. The development of this technology for processes pertaining to the agricultural, commodity, specialty, and pharmaceutical sectors would have a pronounced impact on the environment. 

We have also prepared a number of other derivatives in which the pyridoxamine system is attached to binding groups. In one case we attached it to C-3 of)8-cyclodex-trin, and found that this material also shows a preference for the formation of aromatic amino acids in which the substrate can use the cyclodextrin binding site [30]. More interestingly, the amino acid tryptophan is synthesized with a reversed stereochemical preference from this catalyst relative to the stereochemistry obtained with the previously described cyclodextrin derivative. That is, a derivative attached to the primary carbon of cyclodextrin shows preferential formation of the L-amino acid while that attached to the other face of the cyclodextrin unit shows preferential formation of the D-amino acid. 

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