Fmoc-3,4-dehydro-L-proline is a non-proteinogenic amino acid derivative where the proline ring contains a double bond between the 3 and 4 positions. It is protected at the N-terminus with a 9-fluorenylmethoxycarbonyl (Fmoc) group, making it a standard building block for solid-phase peptide synthesis (SPPS) using the Fmoc strategy. This constrained proline analogue is primarily used in peptide chemistry and medicinal research to introduce conformational rigidity into peptide backbones. The presence of the double bond restricts the pyrrolidine ring's conformational flexibility, which can significantly alter the peptide's secondary structure, stability against enzymatic degradation, and biological activity. It is a key reagent for synthesizing peptides with enhanced pharmacological properties or for studying structure-activity relationships. As a specialty, non-natural amino acid building block, it finds extensive application in the research and development of peptide-based drugs, peptidomimetics, and chemical biology probes. Its use is almost exclusively tied to advanced peptide synthesis rather than as a general-purpose organic synthesis reagent.