H-Hyp-OMe·HCl, or (2S)-1-(methoxycarbonyl)pyrrolidine-2-carboxylic acid hydrochloride, is a protected derivative of the non-proteinogenic amino acid L-hyp (L-hydroxyproline). It features a pyrrolidine ring structure with a carboxylic acid methyl ester and a free carboxylic acid group, the latter of which is stabilized as its hydrochloride salt. This specific derivative is designed to mask the carboxylic acid functionality, making it a crucial building block in peptide synthesis. Its primary application is in the field of pharmaceutical research and development as a specialized peptide intermediate. The pyrrolidine ring introduces conformational constraints into peptide chains, which is a key strategy for designing peptidomimetics and bioactive peptides with enhanced stability, selectivity, and metabolic resistance. It is extensively used in solid-phase peptide synthesis (SPPS) for constructing peptide-based drug candidates, particularly those targeting various receptors and enzymes. As a protected amino acid derivative, it is a staple reagent in medicinal chemistry for exploring structure-activity relationships (SAR). Its use is critical in the synthesis of complex molecules where the unique ring structure of proline/hydroxyproline analogs is required to achieve specific three-dimensional shapes and biological activities.