Solution and Solid Phase Synthesis of Unusual a-Amino Acids From

and separation of diastereomers or through enzymatic methods. Amino acids have aiso
played a pivotal role in the field of stereoselective synthesis, either as targets or reagents
(for some reviews of early developments in asymmetric synthesis and the importance of
amino acids as targets and reagents see Kagan and Fiaud," Apsimon and Séguin,lg
Halpern," Mosher and Hamison2' and Bosnich and Fryzuky. A number of elegant
approaches for the asymrnetric synthesis of various a-amino acids in their optically pure
forms have ken developed and are summarized below.
1.3.1 Enanfioselective Introduction of the a-Hydrogen
The principal transformations leading to a-amino acids 1.6 by the
enantioselective introduction of the a-hydmgen are shown in Scheme 1.6." The chiral a-
carbon of 1.6 is generated either by hydrogenation of 1.7 or via 1,4-addition of
nucleophiles (via 1.8). protonation of enolates 1.8, hydration of a-amino-ketenes 1.9 or
by reductive amination of a-keto acids 1.10.
Scheme 1.6
1.8 1.9
1.3.1.1 Asymemk Hydroge~tion of a;gDidehydro Amino Acids
The utility of this method in the generation of optically active a-amino acids is
twofold; the ease of synthesis of the appropnately functionaiized starting material via
6