Protocol for Facile Synthesis of Fmoc-N-Me-AA-OH Using 2-CTC Resin as Temporary and Reusable Protecting Group

One approach to enhance the bioavailability and half-life of peptides in vivo is through N-methylation of one or more of the amino acids within the peptide sequence. However, commercially available Fmoc-N-Me-AA-OHs are limited and often expensive. In this study, a solid-phase synthesis method for Fmoc-N-Me-AA-OH was developed using a 2-chlorotrityl chloride (2-CTC) resin as a temporary protective group for the carboxylic acid strategy. Two strategies for the alkylation step were compared, employing either dimethyl sulfate or methyl iodide in the Biron-Kessler method. In this work we tested the protocol with two amino acids: Fmoc-Thr(tBu)-OH and Fmoc-βAla-OH. The first one is an alpha amino acid, very hindered and with the amine group directly influenced by the electronic effects of the carboxy group, whereas in Fmoc-βAla-OH, the presence of a methylene group weakens this influence due to the intervening carbon atoms. The desired amino acids, Fmoc-N-Me-Thr(tBu)-OH and Fmoc-N-Me-βAla-OH, were synthesized by both strategies with high yield and purity.