The unusual sugars, α-aculosides, which appear in certain antibiotics and have an α,β-unsaturated ketone structure, were found to be novel and selective Michael acceptors for the thiol function of cysteine residues. A coumarin derivative possessing α-aculoside as a Michael acceptor effectively and irreversibly operated as a fluorescent probe in cells. Furthermore, α-aculosides exhibited cytotoxic activity against several cancer cell lines.