Asp-based lactam cyclic peptides are considered promising drug candidates. However, using Fmoc solid-phase peptidesynthesis (Fmoc-SPPS) for these peptides also causes aspartimide formation, resulting in low yields or even failure to obtain the target peptides. Here, we developed a diaminodiacid containing an amide bond as a β-carboxyl-protecting group for Asp to avoid aspartimide formation. The practicality