The highly reactive precursor molecule, presaccharothriolide X, was successfully isolated from the rare actinomycete Saccharothrix sp. A1506. The comparable biological activity of presaccharothriolide X and its Michael addition product saccharothriolide B unveils a unique masking/activating property of 2-aminophenol. Unexpectedly, 2-aminophenol in saccharothriolide B was eliminated through a retro-Michael reaction, to yield presaccharothriolide X under physiological conditions. 2-Aminophenol might be developed into a useful protecting group for bioactive small molecules with an α,β-unsaturated ketone.