a ammonium salts with benzophenone linkers : enantioselective phase transfer catalysts for the synthesis of a-amino acids †

Since cinchona alkaloids were transformed to asymmetric quaternary quinuclidinium salts with benzyl halides and were used as a phase-transfer catalyst (PTC) (1) by Dolling and O'Donnell, cinchona alkaloids have been widely utilized as chiral templates for phase-transfer catalysis. (Scheme 1) These organic PTCs can be easily prepared from natural and low cost chiral cinchona alkaloids in a few synthetic steps and they are stable and facile under normal reaction conditions in water. Later N-9-anthracenylmethyl quinuclidinium salt PTC (2) was introduced and showed high enantioselectivity for the alkylation of the protected glycine tert-butyl ester. Quinuclidinium PTCs with each pseudo-enantiomeric cinchona alkaloids, such as ( )-cinchonidine and (+)-cinchonine, show enantiomeric selectivity each other, and have been successfully applied in various asymmetric organic synthesis including a-alkyl glycine derivatives synthesis.