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The Use of Crown Ethers in Peptide Chemistry – V. Solid‐phase Synthesis of Peptides by the Fragment Condensation Approach using Crown Ethers as Non‐covalent Protecting Groups

We have previously described the conditions by which peptide synthesis by the solid‐phase fragment condensation approach can be carried out using crown ethers as non‐covalent protection for the Nα ‐amino group. Here we demonstrate that the procedure can be extended to large, partially protected peptide fragments possessing free Lys and/or Arg residues. The first step was to ensure that complex formation on the side chain of amino acids was not detrimental to the methodology and exhibited the same solubility and coupling properties as Nα ‐complexed peptides. Thus, a model hexapeptide was synthesized using Fmoc chemistry containing Lys and Arg residues, which, when complexed with 18‐Crown‐6, was readily soluble in DCM and coupled quantitatively to a resin‐bound tetrapeptide. Two tripeptides were then prepared, one containing a free Ser residue, the other free Tyr, to examine the possible occurrence of side reactions. After coupling using standard conditions only the former tripeptide exhibited the formation of the O‐acylation by‐product (5%). Another model hexapeptide containing Lys, Tyr, Ser and Asp protected with a TFA‐stable adamantyl group was complexed with 18‐Crown‐6 and coupled to the resin‐bound tetrapeptide with near quantative yield. Extending the length of the peptide to 21 and 40 residues, which represent sequences Gly52 to Leu72 (21‐mer) and Pro33 to Leu72 (40‐mer) from Rattus norvegicus chaperonin 10 protein, respectively, resulted in partially protected fragments that were readily soluble in water, thus enabling purification by RP‐HPLC. Complexation with 18‐Crown‐6 gave two highly soluble products that coupled to resin‐board tetramer with 68% and 50% coupling efficiencies for the 21‐mer and 40‐mer, respectively. Treatment with 1% DIEA solutions followed by acidolytic cleavage and purification of the major product confirmed that the correct product had been formed, when analysed by amino acid analysis and ESI‐MS. These results served to extend the methodology of non‐covalent protection of large partially protected peptide fragments for the stepwise fragment condensation of polypeptides.