Asymmetric Syntheses of Unnatural Amino Acids and Hydroxyethylene Peptide Isosteres

Unnatural and naturally occurring nonproteinogenic α-amino acids have become important building blocks for the synthesis of biologically active peptides and peptidomimetic drug molecules. The asymmetric synthesis of α-amino acids has therefore become quite important as an indispensable research tool in academic, government, and industrial laboratories, and methodologies have been reviewed extensively. The established methods for the asymmetric synthesis of amino acids can be divided into roughly six categories (
1
). (1) The highly stereoselective hydrogenation of chiral, nonracemic dehydro amino acid derivatives or the asymmetric hydrogenation of prochiral dehydro amino acid derivatives. Chiral glycine equivalents serve as useful α-amino acid templates undergoing homologation
via
carbon-carbon bond formation at the α-position through nucleophilic carbanion alkylation (2) or electrophilic carbocation substitution (3). In addition both nucleophilic amination (4) and electrophilic amination (5) of optically active carbonyl derivatives has very recently been developed. (6) Enzymatic and whole-cell-based syntheses have recently become more attractive in terms of substrate versatility, cost, and scale. All of these methods have their relative strengths and weaknesses; the optimum method for each individual application must still be considered on a case-by-case basis with respect to functionality, quantity desired, cost, and time. The focus of this chapter will illustrate the utility of chiral, nonracemic glycinates which are commercially available and can be manipulated in a variety of ways to access structurally diverse classes of α-amino acids.