Glycinate Schiff Base Asymmetric Synthesis of a-Amino Acids

A significant advance in this field was made recently by two independent research groups through the development of a new class of cinchona alkaloid- 

We designed and prepared the structurally rigid, chiral spiro ammonium salts 32 as a new C2-symmetric chiral phase-transfer catalyst (Fig. 4.1), and successfully applied this to the highly efficient, catalytic enantioselective alkylation of 28 under mild conditions [24]. A key finding was the significant effect of an aromatic substituent at the 3,3 -position of one binaphthyl subunit of 32 (Ar) on the enantiofacial discrimination, and (S,S)-32e was revealed as the catalyst of choice for preparing a variety of essentially enantiopure a-amino acids by this transformation (Table 4.3). 

Generally, 1 mol% of 32e is sufficient for the smooth alkylation. In the reaction with simple alkyl halides such as ethyl iodide, the use of aqueous cesium hydrox- 

These reports have accelerated research into improvements of the asymmetric alkylation of 28, and have resulted in the emergence of a series of appropriately modified cinchona-alkaloid-based catalysts, as well as the elaboration of purely 

Catalytic asymmetric alkylations of 28 have also been carried out with polymer-bound glycine substrates [43], or in the presence of polymer-supported cinchona alkaloid-derived ammonium salts as immobilized chiral phase-transfer catalysts [44], both of which feature their practical advantages especially for large-scale synthesis. 

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