1-(9-Fluorenylmethyloxycarbonyl-amino)-3,6-dioxa-8-octanamine hydrochloride, with CAS number 868599-73-9, is a specialized chemical compound featuring a polyethylene glycol (PEG) spacer terminated with an amine group and an Fmoc-protected amine group. The Fmoc group is a standard protecting group widely used in solid-phase peptide synthesis (SPPS). The PEG linker imparts hydrophilicity and flexibility to the molecule. This compound is primarily utilized as a heterobifunctional linker or spacer in bioconjugation chemistry and the synthesis of complex biomolecules. Its key applications include the construction of antibody-drug conjugates (ADCs) and, more specifically, as a component in the design of Proteolysis-Targeting Chimeras (PROTACs). In PROTACs, such linkers connect the target protein ligand to the E3 ubiquitin ligase recruiting moiety, with the PEG chain helping to modulate solubility and the distance/orientation between the two binding elements. The amine handles allow for further conjugation to various payloads or functional groups. The Fmoc-protected amine provides orthogonality in synthetic strategies, allowing for selective deprotection and sequential coupling in multi-step assemblies. This makes it a valuable building block in medicinal chemistry and chemical biology research focused on targeted protein degradation and advanced therapeutic modalities.