Used in solid-phase peptide synthesis to introduce aspartic acid in a protected form, allowing selective deprotection and minimizing side reactions. The Fmoc group enables mild base removal for stepwise chain elongation, while the tert-butyl (OtBu) ester protects the aspartic acid side chain from unwanted coupling or cyclization. The N-hydroxysuccinimide (OSu) ester enhances reactivity, enabling efficient amide bond formation with minimal racemization. Commonly applied in the synthesis of complex peptides and proteins, especially where aspartic acid residues are prone to side reactions like aspartimide formation.