Fmoc-D-Glu(OtBu)-OPfp is a protected derivative of D-glutamic acid used primarily in solid-phase peptide synthesis. The Fmoc group protects the α-amino group, providing stability under basic conditions and allowing selective removal with base. The OtBu group protects the side-chain carboxylic acid, while the OPfp serves as an active ester for efficient coupling to the growing peptide chain. This building block facilitates the stepwise assembly of complex peptides by ensuring selective reactions at specific sites, making it valuable for synthesizing peptide-based pharmaceuticals and biomolecules.