Fmoc-Asp(CSY)-OH is an Fmoc-protected aspartic acid residue developed in the Bode Lab that contains a cyanosulfurylide (CSY) as a carboxylic-acid protecting group that completely suppresses aspartimide formation in peptide synthesis, a long-standing challenge in peptide chemistry that occurs during Fmoc removal or peptide coupling and affects peptide yield and sequences. Amenable to SPPS, deprotection is achieved under aqueous conditions with electrophilic halogenating agents to convert the ylide to the free acid. Furthermore, the hydrophilic nature of the ylide protecting group improves overall peptide efficiency and solubility.