Imidazolines stereoselective synthesis

Chiral imidazolines such as 4, obtained by condensation of iminoether hydrochlorides with (15,25)-1,2-diaminocyclohexane, may be metalated and alkylated with high stereoselectivity. This process is highly efficient for the stereoselective synthesis of quaternary benzylic stereogenic centers, and has been applied to a total synthesis of mesembrine (eq 6). (15,25)-1,2-Diaminocyclohexane here again gives higher diastereomeric excesses than 1,2-diphenyl ethylenediamine in this reaction. 

YR Lin, XT Zhou, LX Dai, J Sun. Ruthenium Complex-Catalyzed Reaction of Isocyanoacetate and V-Sulfonylimines Stereoselective Synthesis of V-Sulfonyl-2-Imidazolines. J Org Chem 62 1799-1803,1997. 

The first example of the application of pincer complex catalysts for selective synthesis involves aldol type of reactions involving imines and isocyanoacetates. In aldol [13-27] and Michael addition [13, 28-34] reactions, palladium pincer complexes have usually been employed as Lewis acid catalysts, for the first time by Richards and coworkers [13, 28]. This also means that the palladium(II) atom in these processes is not involved in redox reactions, and therefore reduction to palladium(O) does not happen. The aldol and the Michael reactions are useful C-C bond-forming reactions. In these processes, new stereocenters are generated, and therefore the processes can be used for stereoselective synthesis. A typical example is the reaction of suUbnimines (2) with isocyanoacetate (3) to form imidazohne (4) derivatives. The imidazolines can be easily hydrolyzed to diamino acid derivatives in a one-pot process (Figure 4.2) [24]. 

Racemization of Amino Acids. The synthesis of (7 )-alanine was achieved starting from (S)-alanine via formation of the imidazoline with (S)-proline. This result can he explained in terms of epimerization and stereoselective protonation with asymmetric induction hy the chiral center originating from (S)-proline. 

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