A Novel and Versatile Silicon-Derived Linkage Agent, 4-[1-Hydroxy-2-(trimethylsilyl)ethyl]benzoic Acid, Compatible with the Fmoc/t-Bu Strategy for Solid Phase Synthesis of C-Terminal Peptide Acids

Abstract

Tryptophan alkylation by resin-derived carbocations and formation of diketopiperazine are two major yield reducing side reactions observed in Fmoc/t-Bu based solid-phase peptide synthesis. With the development of a versatile silicon-based linkage agent, «SAC», 4-[1-hydroxy-2-(trimethylsilyl)ethyl] benzoic acid (1), for the production of peptide C-terminal acids, these problems have been successfully solved. Demonstration of the advantages of using SAC linker was provided by synthesizing a C-terminal tryptophan-containing dodecapeptide and a C-terminal proline-containing undecapeptide. The pure peptides were isolated in 30-40% yields, a dramatic improvement compared to the syntheses using the conventional HMPA linker. SAC linker was prepared in two steps in 70% overall yield. Attachment of the Fmoc-amino acid-SAC linker derivatives to amino functionalized solid supports was best carried out using the performed 2,4-dichlorophenyl ester derivatives. Tha amino acid-SAC linkage was tested and found to be stable under typical peptide synthesis conditions. An additional feature of SAC linker is the ability to generate protected peptide fragments using either fluoride ions or 1% TFA/ CH 2 Cl 2 solution. The versatility of using SAC linker was demonstrated by preparation of several Boc- or Fmoc-protected tetrapeptides and a cyclic heptapeptide by either fluoridolysis methodology or treatment with dilute acid. Furthermore, formation of succinimide, a side reaction resulting from cyclization of β-tert-butyl ester protected aspartyl residues, under fluoridolysis conditions, was also investigated