Solvolysis and aminolysis on peptidyl‐Kaiser oxime resin assisted by Ca2+and Eu3+: a mild procedure to prepare α‐methyl and ‐ethyl esters of protected peptides

Abstract: Ca²⁺ and Eu³⁺ were able to assist solvolysis on peptidyl‐Kaiser oxime resins generating α‐methyl and ‐ethyl esters of protected peptides. The methanolysis assistance was at least twice as effective as that of acetic acid, the common catalyst used in aminolysis of the ester oxime linkage. No molar excess of Ca²⁺ or Eu³⁺ was needed to enhance this reaction efficiency. Ca²⁺ also assisted aminolysis on peptidyl‐Kaiser oxime resins. Solvolysis and aminolysis rates depended on the nature of theC‐terminal residue attached to the resin and on the alcohol used. Both reactions were selective to the ester oxime linkage since no significant amount of secondary products, resulting from rearrangements or simultaneous transesterification of the β‐benzyl or cyclohexyl esters, was detected in the reaction media. The α‐methyl and ‐ethyl esters of Ac‐Ala‐Gly‐X where, X = Gly, Ala, Phe or Lys (2‐Cl‐Z)] and of Ac‐Ile‐Ser (Bzl)‐Asp(OZ) (where, Z = Bzl or cHex) were essentially the only products formed in the solvolyses performed. Ac‐Ile‐Ser(Bzl)‐Asp(OcHex)Arg(HCl)‐OMe and Ac‐Ile‐Ser(Bzl)‐Asp(OcHex)Arg (HCl)‐OEt were the major products formed in the aminolysis reactions. In the presence of the metal ions, the resin‐cleavage yields were > 50%. In their absence, they were < 15%.