Hydrogen fluoride catalyzed migration of side chain protecting groups onto Fmoc during solid phase peptide synthesis

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Abstract
The solid‐phase synthesis of the Nα‐Fmoc analog of protein kinase C substrate (PKCS, Lys‐Arg‐Ala‐Lys‐Ala‐Lys‐Thr‐Thr‐Lys‐Lys‐Arg) was characterized by low recovery from the resin and the concomitant appearance of four impurities. FAB‐MS revealed molecular weights for two of these impurities that corresponded to the desired peptide plus Tos or Bzl. The other two were justified by invoking a CO2 elimination of the Clz protecting group to yield: 1) peptide plus 2‐chlorobenzyl (CIBzl) and 2) peptide plus ClBzl and Tos. A CF‐FAB analysis of carboxypeptidase digestions allowed observation of peptide cleavage down to an ion corresponding to lysine, Fmoc, and the corresponding protecting group(s). These data revealed that the impurities were not the result of incomplete deprotection but the result of migration of the protecting groups to the N‐terminal end of the peptide. NMR experiments were subsequently performed and revealed the exact site of substitution: the meta positions of the N‐terminal Fmoc. These impurities are presumed to arise by electrophilic aromatic substitution of the fluorene group during HF treatment. The desired Fmoc analog served as a convenient, albeit low‐yielding, intermediate for purification of the highly charged PKCS by preparative self‐displacement HPLC.