Approaches to the asymmetric synthesis of unusual amino acids.

This chapter will focus on critically evaluating some selected new methods to achieve the asymmetric synthesis of a-amino acids. It is not meant to be comprehensive, but rather to reflect some of the possibilities opening up in this area. The study of a-amino acids is one of fundamental importance to many areas of chemistry and its relation to molecular biology. Nature only utilizes 20 amino acids in the production of polypeptides on genes, yet the combinations of these amino acids have provided a wonderful diversity of chemical structures and functions, The number of naturally occurring or synthetically derived nonproteogenic a-amino acids is rapidly increasing and, depending on definitions, might already have exceeded 1000. In addition, because of the significant advances in the technology of synthesizing polypeptides (e.g., the solid-phase Merrifield method), the possibilities in the design and synthesis of new enzymes, hormones, synthetic immunostimulants, drugs, and countless other important biopolymers have been dramatically increased. Certain nonproteogenic amino acids have already proven of considerable experimental value in probing amino acid chemistry and function, as a tool for enhanced understanding of the roles and functions of proteins, in understanding the chemistry and biochemistry of interactions in living systems, and as analogs of naturally occurring hormones, neurotransmitters, growth factors, enzyme inhibitors, neuromodulation, immunomodulators, and many other biologically significant compounds.