Fmoc-l-aspartic Acid-alpha-t-butyl Ester  

    Fmoc‑L‑aspartic Acid‑alpha‑t‑butyl Ester (Fmoc‑Asp‑OtBu, CAS: 129460‑09‑9) is a pivotal amino acid building block for Fmoc Solid‑Phase Peptide Synthesis (SPPS), boasting a molecular formula of C₂₃H₂₅NO₆ and a molecular weight of 411.45. Appearing as a white to off‑white powder, it offers a purity of ≥98.0% (HPLC) with enantiomeric purity exceeding 99.0%, a specific rotation of [α]₂₀/D -24±2° (1% in DMF), and a melting point ranging from 90‑98℃ (105‑111℃ for select batches), while being soluble in dichloromethane, DMF, and DMSO and slightly soluble in water. Its molecular structure features two orthogonal protecting groups: the α‑amino acid linked to the fluorenylmethoxycarbonyl (Fmoc) group and the side‑chain carboxyl protected by the t‑butyl ester (OtBu) group. The Fmoc group can be selectively removed under mild alkaline conditions (e.g., piperidine) without impacting the OtBu protecting group, which is alkali‑stable and requires cleavage under acidic conditions (e.g., trifluoroacetic acid); this dual protection strategy effectively inhibits aspartic acid residue cyclization to form aspartimide by‑products, ensuring high‑purity peptide chain synthesis. Widely applied across multiple fields, it serves as a standard module for synthesizing linear peptides, cyclic peptides, and modified peptides containing aspartic acid residues—especially effective for Asp‑Gly sequences prone to side reactions, significantly reducing β‑peptide isomer formation.