Fmoc-S-tert-butylthio-D-cysteine is a protected derivative of the amino acid D-cysteine. It features an Fmoc (9-fluorenylmethoxycarbonyl) group protecting the amino terminus and a tert-butylthio (S-tBu) group protecting the thiol side chain of cysteine. This specific combination of protecting groups makes it a crucial building block in solid-phase peptide synthesis (SPPS). Its primary application is in the synthesis of peptides containing D-cysteine residues or peptides where the cysteine thiol needs to be orthogonally protected. The S-tBu group is stable under the basic conditions used for Fmoc deprotection but can be selectively removed under reductive conditions (e.g., using dithiothreitol or phosphines) to generate the free thiol. This allows for the subsequent formation of disulfide bonds or other site-specific modifications, which are critical for the structure and activity of many therapeutic peptides and proteins. This compound is a specialty reagent essential for research and development in peptide chemistry, particularly for creating peptides with non-natural configurations or complex architectures like cyclic peptides and peptide-drug conjugates.