Used in solid-phase peptide synthesis to introduce a keto-functionalized proline residue, enabling site-specific modifications such as oxime or hydrazone ligation for peptide conjugation, labeling, or macrocyclization. The Fmoc group allows for orthogonal protection, compatible with standard peptide synthesis protocols, while the 4-keto moiety provides a reactive handle for bioorthogonal chemistry in the development of peptide-based therapeutics, probes, and biomaterials.