Optimized selective N-methylation of peptides on solid support

Peptides containing N^α-methylamino acids exhibit interesting therapeutic profiles and are increasingly recognized as potentially useful therapeutics. Unfortunately, their synthesis is hampered by the high price and nonavailability of many N^α-methylamino acids. An efficient and practical three-step procedure for selective N-methylation of peptides on solid support is described. The procedure was based on the well known solid-phase N-methylation of N^α-arylsulfonyl peptides, which was improved by using dimethylsulfate and the less expensive DBU as base. Every step of the procedure, amine activation by an o-nitrobenzenesulfonyl group, selective N-methylation and removal of the sulfonamide group, was optimized in respect of time and economy. The described optimized three-step procedure is performed in 35 min without solvent changes, instead of 3 h. Tripeptides (Fmoc-Phe-MeXaa-Leu-OH) containing N-methylated common amino acids were also prepared using the optimized procedure to demonstrate its compatibility with these amino acids. The described procedure allows an efficient synthesis of N^α-methylamino acid containing peptides in a very short time using Fmoc solid-phase peptide synthesis.