Orthogonal protecting groups forNα-amino andC-terminal carboxyl functions in solid-phase peptide synthesis

Abstract

For the controlled synthesis of even the simplest dipeptide, the Nα‐amino group of one of the amino acids and the C‐terminal carboxyl group of the other should both be blocked with suitable protecting groups. Formation of the desired amide bond can now occur upon activation of the free carboxyl group. After coupling, peptide synthesis can be continued by removal of either of the two protecting groups and coupling with the free C‐terminus or Nα‐amino group of another protected amino acid. When three functional amino acids are present in the sequence, the side chain of these residues also has to be protected. It is important that there is a high degree of compatibility between the different types of protecting groups such that one type may be removed selectively in the presence of the others. At the end of the synthesis, the protecting groups must be removed to give the desired peptide. Thus, it is clear that the protection scheme adopted is of the utmost importance and makes the difference between success and failure in a given synthesis. Since R. B. Merrifield introduced the solid‐phase strategy for the synthesis of peptides, this prerequisite has been readily accepted. This strategy is usually carried out using two main protection schemes: the tert‐butoxycarbonyl/benzyl and the 9‐flourenylmethoxycarbonyl/tert‐butyl methods. However, for the solid‐phase preparation of complex or fragile peptides, as well as for the construction of libraries of peptides or small molecules using a combinatorial approach, a range of other protecting groups is also needed. This review summarizes other protecting groups for both the Nα‐amino and C‐terminal carboxyl functions. © 2000 John Wiley & Sons, Inc. Biopoly 55: 123–139, 2000

Amino Acid-Protecting Groups

“…40 This is a common characteristic of several acid labileprotecting groups. 41 Most Bpoc-amino acids are oils and are unstable because the free α-carboxylic acid is acidic enough to remove the Bpoc group. Thus, these amino acids are usually stored either as DCHA salts or as pentafluorophenyl esters.…”

6.11 N-Acylation Reactions of Amines

“…This approach enables large excesses of reagents and activating agents to be used to drive peptide bond-forming reactions to completion and avoids handling problems associated with the poor solubility and aggregation of medium-length peptides. 6,207 A significant amount of research has been carried out in improving the efficiency of solid-phase protocols, including studies on the use of different protecting groups (e.g., N-FMoc vs. N-Boc, O-Bn vs. O-t Bu), 189,194,208 optimization of the performance of different polymer supports (e.g., Merrifield vs. PEGA), 209-216 the use of different coupling reagents (e.g., phosphonium vs. uronium salts), 154 variation in the point of attachment of the peptide (e.g., N-terminus vs. side chain vs. backbone amide), 217 and optimization of the linker/cleavage conditions employed to attach/release peptides to/from solid support. 218 Manual solid-phase peptide synthesis may be performed using inexpensive protocols that employ syringes fitted with filters, 6 or using variations of the polypropylene 'Teabag' technology developed by Houghten and coworkers.…”

Protection Reactions

“…The Trt group is stable to bases. It is cleaved by mild acids such as 1% TFA or 3% trichloroacetic acid (TCA) in DCM, 0.1 M 1-hydroxy-1 H-benzotriazole (HOBt) in trifluoroethanol (TFE), or moist 0.2% TFA in DCM [26][27][28]. The latter two conditions are compatible with acid-labile linkers in SPPS.…”