Development of Two Complementary Epimerization-Resistant Fragment Coupling Reactions for Highly Convergent Synthesis of <i>N</i> -Alkyl-Rich Peptides

Cyclic peptides containing N -alkylated amino acids represent a promising therapeutic modality, offering access to intracellular targets previously considered “undruggable” and potential for oral administration. The N -alkyl groups (e.g., N -methyl groups) play an important role in enhancing the pharmacological properties of these peptides by improving cell membrane permeability and metabolic stability; however, the efficient synthesis of N -alkyl-rich peptides has remained an underdeveloped area of research. Herein, we report two types of peptide fragment coupling reactions developed toward the realization of highly convergent synthesis of N -alkyl-rich peptides. These two reactions enable fragment coupling of diverse N -methylated substrates with exceptional resistance to epimer formation─a challenge that has proven difficult with established methods. The first reaction utilizing pivaloyl mixed anhydride under liquid–liquid biphasic conditions addresses the substrate combinations with N -methylation at the amino terminus. The second reaction utilizing 2-hydroxypyridine N -oxide (HOPO) and Oxyma or Oxyma-B addresses the substrate combinations with N -methylation at both amino and carboxyl termini. Together, these two complementary reactions represent an advancement in the synthesis of N -alkyl-rich peptides.