(9H-Fluoren-9-yl)methoxycarbonyl-protected ethylenediamine hydrochloride (Fmoc-ethylenediamine HCl) is a key building block and intermediate in solid-phase peptide synthesis (SPPS) and bioconjugation. It features an Fmoc-protected secondary amine and a free primary amine (as HCl salt), enabling selective attachment of the free amine to resins, amino acids, or other molecules while the Fmoc group shields the other nitrogen. The Fmoc protection is stable under acidic and basic conditions typical of SPPS and can be selectively removed with mild bases like piperidine in DMF, revealing the amine without disrupting the assembled structure. This orthogonality ensures precise control in synthesizing complex peptides, amino acid derivatives, linkers, and bioconjugates for applications in medicinal chemistry, drug development, and biochemical research, delivering high yields and purity.