Facile removal of the N-indole-mesitylenesulfonyl protecting group using HF cleavage conditions

Nitrogen indole protection of the β-methyltryptophan side-chain residue is important for avoiding undesired side reactions during peptide synthesis. Of great importance is the choice of a side-chain protecting group for orthogonal peptide synthesis and its stability under a variety of chemical conditions required for synthesis of the four isomers of this unusual amino acid. We report here the successful use of the mesitylenesulfonyl (Mts) protecting group for β-methyltryptophan in the synthesis of melanotropin and CCK peptide analogues and the ready cleavage of this protecting group under HF conditions.