α-Amino carboxylic acids are one of five major classes of natural products and play a crucial role in diverse biological functions. Historically, the amino acids have been subdivided into proteinogenic and nonproteinogenic respresentatives. β-Amino acids are similar to alfa-amino acids in that they contain an amino terminus and a carboxyl terminus. However, in β-amino acids two carbon atoms separate these functional termini. β-Amino acids, with a specific side chain, can exist as the R or S isomers at either the α(C2) carbon or the β(C3) carbon. This results in a total of 4 possible diastereoisomers for any given side chain. β-Amino acids are not proteinogenic amino acids, but some of them are constituents of soma natural and biologically active compounds.Unusual β-amino acids are alicyclic β-amino acids and heterocyclic β-amino acids. Alicyclic β-amino acids, in which both the β-amino and the acids functionality are vicinally attached to an aliphatic ring, still represent a demanding challenge to the synthetic chemist. One reason for this lies in the intriguing difficulty associated with controlling the absolute and relative stereochemistry of two adjacent stereocenters. In heterocyclic β-amino acids the nitrogen of amino group is a part of nitrogen heterocycles.Some natural derivatives of alicyclic β-amino acids and heterocyclic β-amino acids, their chemical structures and biological activities are discussed in this mini-review.

Keywords: unusual amino acids; alicyclic β-amino acids; heterocyclic β-amino acids; alkaloids; peptides