Cinchona, quaternary ammonium salts ester, alkylation

Five years later, a similar cinchona alkaloid-derived quaternary ammonium salt was applied for the alkylation of N-(diphenylmethylene) glycine tert-butyl ester by O Donnell et al. [17]. By using either 11b or 12a, both enantiomers of the alkylated products, which could be hydrolyzed to afford the chiral a-amino acids, were obtained in high yield with a maximum of 66% ee. Further optimization indicated that, with the corresponding 9-OH-protected catalyst 11c, the enantioselectivity could be enhanced to 81% ee [6bj. 

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