Biotin is one of the most useful tags in (bio)analytical science due to its specific interaction with avidin, but is not easy to convert because of its low solubility in most solvents. Friedel-Crafts acylation of biotin acid chloride in triflic acid was examined, and the synthesized derivatives had stronger affinity to avidin than biotin in a binding assay using 2-(4'-hydroxyphenylazo)benzoic acid.