Definition
Fmoc, or 9-fluorenylmethyloxycarbonyl, is a temporary protecting group attached to the alpha-amino group of each incoming amino acid during solid-phase synthesis. It blocks the amine so that only the intended coupling occurs, and it is removed with a mild base, typically piperidine in dimethylformamide, before the next residue is added. Side chains are protected with acid-labile groups such as tert-butyl, Boc and trityl, which survive the basic deprotection steps and are removed only at final cleavage with trifluoroacetic acid. This orthogonal scheme replaced the older Boc/benzyl strategy for most applications because it avoids hazardous hydrogen fluoride. Fmoc release can be monitored by UV absorbance to track deprotection efficiency.