Extended Solution-phase Peptide Synthesis Strategy Using Isostearyl-Mixed Anhydride Coupling and a New C-Terminal Silyl Ester-Protecting Group for N-Methylated Cyclic Peptide Production

Abstract

Herein, we present a new and efficient convergent solution-phase synthetic strategy for producing peptides containing N-methyl amino acids. Specifically, we have synthesized a model cyclic octapeptide with two N-methyl amino acids, utilizing an isostearyl-mixed anhydride coupling methodology and a novel silyl ester-protecting group, cyclohexyl di-tert-butyl silyl (cHBS). This newly developed method uses an isostearic acid chloride (ISTA-Cl) and silylation reagent that allows coupling between N-and Cterminally unprotected amino acids with sterically hindered N-methyl amino acids. High yields of four dipeptide fragments are efficiently synthesized by omitting the traditional C-terminal deprotection step. The silyl ester-protecting group at the C-terminus is stable during general peptide synthesis, and is selectively cleaved by fluoride ions. This group further suppresses diketopiperazine formation during the deprotection of the N α -amino-protecting group. The linear octapeptide precursor is convergently synthesized utilizing protection and selective deprotection of the silyl ester-protecting group, and the cyclic octapeptide can be obtained with high purity using this novel methodology, via a route shorter than conventional solution-phase peptide synthesis strategies.

Silicon-based hydrophobic tags applied in liquid-phase peptide synthesis: protected DRGN-1 and poly alanine chain synthesis

“…12 In general, silyl groups are rarely utilized as protecting groups directly for carboxylic acids due to the labile Si–O bonds except super silyl groups 15 and some other silyl groups with special structures. 14 To make the silyl group containing tag stable and also recyclable after deprotection, we decided to use a short alkyl chain as a bridge between the C-terminal and the silyl group. As is shown in Scheme 2c, the octadecyldiisobutylsilyl group with a short alkyl chain ( TAG3 ) was developed.…”

“…When we were preparing this manuscript, the Nishizawa group reported a silyl ester tag, using a di- tert -butyl silyl group attached to the C-terminal directly. 14 Although this publication demonstrated the use of a silyl group for hydrophobicity, the silyl group cannot be easily recycled after deprotection by fluoride ions because of the formation of silyl fluoride.…”

Recent Advances in the Solid- and Solution-Phase Synthesis of Peptides and Proteins Using Microflow Technology

“…α-Peptides comprising proteinogenic amino acids usually suffer from poor pharmacological properties, such as low metabolic stability and cell membrane permeability. Specialty peptides, including N-methylated peptides, ,− β-peptides, ,− and cyclic peptides, ,− generally have higher metabolic stability. In particular, it is believed that cyclic N-methylated peptides have better affinity and specificity against their biological targets because of their constrained chemical structure.…”

Super silyl-based stable protecting groups for both the C- and N-terminals of peptides: applied as effective hydrophobic tags in liquid-phase peptide synthesis

“… 13 We found that when the silyl groups were introduced, the hydrophobicity of the tags could be increased and the resulting products became difficult to solidify. Some relevant simple silyl-based tags were also reported by Nishizawa 14 and Kubota. 15 Kubota has proved that the number of silyl groups could affect the peptide solubility.…”