β 2 ‐amino acids—syntheses, occurrence in natural products, and components of β‐peptides 1,2

Although they are less abundant than their α‐analogues, β‐amino acids occur in nature both in free form and bound to peptides. Oligomers composed exclusively of β‐amino acids (so‐called β‐peptides) might be the most thoroughly investigated peptidomimetics. Beside the facts that they are stable to metabolism, exhibit slow microbial degradation, and are inherently stable to proteases and peptidases, they fold into well‐ordered secondary structures consisting of helices, turns, and sheets. In this respect, the most intriguing effects have been observed when β 2 ‐amino acids are present in the β‐peptide backbone. This review gives an overview of the occurrence and importance of β 2 ‐amino acids in nature, placing emphasis on the metabolic pathways of β‐aminoisobutyric acid (β‐Aib) and the appearance of β 2 ‐amino acids as secondary metabolites or as components of more complex natural products, such as peptides, depsipeptides, lactones, and alkaloids. In addition, a compilation of the syntheses of both achiral and chiral β 2 ‐amino acids is presented. While there are numerous routes to achiral β 2 ‐amino acids, their EPC synthesis is currently the subject of many investigations. These include the diastereoselective alkylation and Mannich‐type reactions of cyclic‐ or acyclic β‐homoglycine derivatives containing chiral auxiliaries, the Curtius degradation, the employment of transition‐metal catalyzed reactions such as enantioselective hydrogenations, reductions, C–H insertions, and Michael‐type additions, and the resolution of rac. β 2 ‐amino acids, as well as several miscellaneous methods. In the last part of the review, the importance of β 2 ‐amino acids in the formation of β‐peptide secondary structures is discussed. © 2004 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 2004