Fmoc‐2‐mercaptobenzothiazole, for the introduction of the Fmoc moiety free of side‐reactions

Abstract

A double side-reaction, consisting in the formation of Fmoc-beta-Ala-OH and Fmoc-beta-Ala-AA-OH, during the preparation of Fmoc protected amino acids (Fmoc-AA-OH) with Fmoc-OSu is discussed. Furthermore, the new Fmoc-2-MBT reagent is proposed for avoiding these side-reactions as well as the formation of the Fmoc-dipeptides (Fmoc-AA-AA-OH) and even tripeptides, which is another important side-reaction when chloroformates such as Fmoc-Cl is used for the protection of the alpha-amino function of the amino acids.

Orthogonal Protecting Groups and Side‐Reactions in Fmoc/tBu Solid‐Phase Peptide Synthesis

“…Multiple addition can also occur if the incoming amino acid building block used in the coupling cycle is partially Fmoc-deprotected either because of impurities or degradation. The formation of dipeptides side-products upon Fmoc protection of amino acids using Fmoc-Cl has also been reported [167]. Regarding the latter case, it should be remembered that Fmoc-amino acids are sensitive to heating and moisture.…”

Oxime Carbonates: Novel Reagents for the Introduction of Fmoc and Alloc Protecting Groups, Free of Side Reactions

“…[13][14][15][16]26,27] Nonetheless, the formation of up to 0.4 % of some amino-acid-based byproducts like Fmoc/Alloc-β-Ala-OH and Fmoc/Alloc-β-Ala-AA-OH can occur when using this reagent for α-amino protection. [28] A detailed discussion of the mechanism of this side reaction has been proposed by Isidro-Llobet et al [25] Azide derivatives, which can be used as solids after isolation from the chloroformates [12,29] or formed in situ before reacting with the amino acid, [30] have also been proposed as an alternative pathway to the N-protection of amino acids, although its explosive nature compromises its use in large-scale synthesis.…”

“…2-MBt (2-mercaptobenzothiazole), [25] although only the hydroxysuccinimido (Su) carbonate found general acceptance. [13][14][15][16]26,27] Nonetheless, the formation of up to 0.4 % of some amino-acid-based byproducts like Fmoc/Alloc-β-Ala-OH and Fmoc/Alloc-β-Ala-AA-OH can occur when using this reagent for α-amino protection.…”

Protection Reactions

“…Rigorous high-performance liquid chromatography (HPLC) analysis has recently shown that the peptides synthesized using Fmoc-amino acids are contaminated with Fmoc-b-Ala-OH and Fmoc-b-Ala-Xaa-OH to the extent of 0.1-0.4%. The origin of these impurities has been explained based on a process that involves nucleophilic ring opening of the HOSu moiety of Fmoc-OSu and Lossen rearrangement of the resultant O-acyl hydroxamate intermediate as key reactions [120]. Consequently, Fmoc-2-mercaptobenzothiazole (MBT) has been proposed as an alternative to FmocOSu [120].…”

“…The origin of these impurities has been explained based on a process that involves nucleophilic ring opening of the HOSu moiety of Fmoc-OSu and Lossen rearrangement of the resultant O-acyl hydroxamate intermediate as key reactions [120]. Consequently, Fmoc-2-mercaptobenzothiazole (MBT) has been proposed as an alternative to FmocOSu [120]. This new Fmoc donor has shown to cause no formation of dipeptide as well as b-Ala impurities.…”