Synthesis and application of N^α‐Fmoc‐N^π‐4‐methoxybenzyloxymethylhistidine in solid phase peptide synthesis

Abstract

The 4-methoxybenzyloxymethyl (MBom) group was introduced at the Nπ-position of the histidine (His) residue by using a regioselective procedure, and its utility was examined under standard conditions used for the conventional and the microwave (MW)-assisted solid phase peptide synthesis (SPPS) with 9-fluorenylmethyoxycarbonyl (Fmoc) chemistry. The Nπ-MBom group fulfilling the requirements for the Fmoc strategy was found to prevent side-chain-induced racemization during incorporation of the His residue even in the case of MW-assisted SPPS performed at a high temperature. In particular, the MBom group proved to be a suitable protecting group for the convergent synthesis because it remains attached to the imidazole ring during detachment of the protected His-containing peptide segments from acid-sensitive linkers by treatment with a weak acid such as 1% trifluoroacetic acid in dichloromethane. We also demonstrated the facile synthesis of Fmoc-His(π-MBom)-OH with the aid of purification procedure by crystallization to effectively remove the undesired τ-isomer without resorting to silica gel column chromatography. This means that the present synthetic procedure can be used for large-scale production without any obstacles.

Further applications of classical amide coupling reagents: Microwave‐assisted esterification on solid phase

“…We selected the His residue in the present study because π‐nitrogen at the imidazole ring may abstract hydrogen at C α under basic conditions, thereby promoting racemization. We had concern about HBTU/HOBt and COMU/Oxyma microwave‐assisted methods because the ratio of racemization may increase at higher temperatures and also in the presence of a base . Therefore, we employed the linkage of His as the first amino acid and measured racemization based on the RP‐UPLC area of the model peptide Fmoc‐Gly‐Phe‐ d / l ‐His‐OH.…”

“…We had concern about HBTU/HOBt and COMU/Oxyma microwave-assisted methods because the ratio of racemization may increase at higher temperatures and also in the presence of a base. 39,41,42 Therefore, we employed the linkage of His as the first amino acid and measured racemization based on the RP-UPLC area of the model peptide Fmoc-Gly-Phe-D/L-His-OH. The results obtained are summarized in Table 2.…”

Aqueous microwave-assisted solid-phase peptide synthesis using Fmoc strategy. III: Racemization studies and water-based synthesis of histidine-containing peptides

“…The slow reaction rate in aqueous media could easily lead to an increase in the level of racemization at the His residue. Next, we demonstrated our optimized MW-assisted water-based SPPS of NPW30 (10-15), Tyr-His-Thr-ValGly-Arg-NH 2 , using water-dispersible Fmoc-amino acid nanoparticles (Shimomura et al 2002;Hibino et al 2012). The reaction was carried out in an aqueous solution according to the protocol described in Table 2.…”

Evaluation of acid‐labile S‐protecting groups to prevent Cys racemization in Fmoc solid‐phase peptide synthesis

“…Regioselective protection of the π ‐nitrogen of the imidazole ring enables avoidance of racemization during incorporation of the His derivatives, because the unprotected π ‐nitrogen is involved in promoting racemization pathways . Therefore, substitution of the N π ‐t ‐Bum or N π ‐4‐methoxybenzyloxymethyl (MBom) group for the N τ ‐Trt group on Fmoc–His offers a solution for eliminating racemization even in the case of microwave (MW)‐assisted SPPS performed at a high temperature .…”