AbstractThe determination of the rates for peptide coupling in tetrahydrofuran between Z‐Aaa‐OPcp (Aaa = Ala, (NMe)Ala, Pro, Thz, Pip and (S)Pip) and H‐Bbb‐OMe (Bbb = (NMe)Ala, Pro, Thz, Pip and (S)Pip) allowed us to evaluate the relative reactivities between these members, either as active components or as amino components. The reactivity of Pro (either as the active species or the amino component) equals that of (NMe)Ala. The reactivity of Pip and (S)Pip as imino components is low while the activation energy is raised by an amount that roughly equals the energy increment needed for bringing the carboxylate grouping from the equatorial to the axial position. The reactivities of these six‐membered residues are also appreciably low when being the active components during peptide coupling. The presence of a ring‐sulfur atom at γ‐position of nitrogen additionally decreases the rate for peptidation, especially if (S)Pip is the imino component.