Fmoc-L-threonine tert-butyl ester is a protected derivative of the proteinogenic amino acid L-threonine. It features two orthogonal protecting groups: the base-labile 9-fluorenylmethoxycarbonyl (Fmoc) group on the α-amino function and the acid-labile tert-butyl ester on the carboxylic acid. This specific combination makes it a crucial chiral building block. Its primary and most significant application is in solid-phase peptide synthesis (SPPS) using the Fmoc strategy. As a protected amino acid, it is directly incorporated into peptide chains during automated synthesis. The tert-butyl side-chain protection of the threonine hydroxyl group is essential to prevent side reactions during the coupling and deprotection cycles. It is therefore a standard reagent for introducing threonine residues into synthetic peptides, which are key intermediates in pharmaceutical research for drug discovery and development. The compound is commercially available from major suppliers of peptide synthesis reagents and fine chemicals. It must be stored under inert conditions and at low temperature to maintain stability, as the Fmoc group can be susceptible to premature deprotection under basic conditions.