Chemical structure and synthesis

Figure 2.25 illustrates the substitution pattern for a β-amino acid residue. The most commonly used is monosubstituted (β³-) residues. These are derived by Arndt-Eistert homologation of the readily available α-amino acid residues. In α-amino acids both the carboxylic acid group and the amino group are bonded to the same carbon center, the α-carbon (C^α). In β amino acids, the amino group is bonded to the β carbon (C^β). Only glycine lacks a β carbon, which means that β-glycine is not possible.

Figure 2.26: Some examples of crystallographically characterized substituted b -residues.

The chemical synthesis of β-amino acids can be challenging, especially given the diversity of functional groups bonded to the β-carbon and the necessity of maintaining chirality (figure 2.27). In the alanine molecule shown, the β-carbon is achiral; however, most larger amino acids have a chiral C^β atom. A number of synthesis mechanisms have been introduced to efficiently form β-amino acids and their derivatives notably those based on the Arndt-Eistert synthesis. Two main types of β-peptides exist: those with the organic residue (R) next to the amine are called β³-peptides and those with position next to the carbonyl group are called β²-peptides.

Figure 2.27: Various methods for the synthesis of β-amino acids.