The Effect of Counterion and Tertiary Amine on the Efficiency of N‐Triazinylammonium Sulfonates in Solution and Solid‐Phase Peptide Synthesis

Abstract A collection of N ‐triazinylammonium sulfonates, designed according to the concept of “superactive esters”, was obtained by treatment of ammonium sulfonates with 2‐chloro‐4,6‐dimethoxy‐1,3,5‐triazine. The structure of the tertiary amine as well as sulfonate anion influenced their reactivity and stability in N , N ‐dimethylformamide (DMF) solution. The reagents were successfully used in solution‐ and solid‐phase synthesis of Z‐, Boc‐, and Fmoc‐protected peptides containing natural and unnatural sterically hindered amino acids as well as in [2+1] fragment condensation approaches, yielding the final products in 80–100 % yield and high optical purity. In manual SPPS of the [Aib] 2 [Aib] 4 ‐enkephalin analogue and the ACP(65–74) peptide fragment VQAAIDYINEG, several sulfonates yielded peptides significantly faster than TBTU or HATU. Comparative analyses demonstrated that 4‐(4,6‐dimethoxy‐1,3,5‐triazin‐2‐yl)‐4‐methylmorpholinium 4‐toluenesulfonate was the most versatile reagent in a wide range of coupling procedures.