The present invention relates to novel beta-lactone inhibitors of hydrolytic enzymes and their medical use, in particular a compound of formula (I) or a pharmaceutically acceptable salt thereof, for use in therapy. The present invention further relates to a pharmaceutical composition comprising the compound of formula (I) as defined herein and a pharmaceutically acceptable excipient. The instant compounds and compositions are useful in the treatment or prevention of cancer, an infectious disease, an inflammatory disease, or an autoimmune disease, and are useful as proteasome inhibitors. The present invention further relates to a plant protection composition comprising a compound of formula (I) or an agriculturally acceptable salt thereof, and to a method of controlling a plant disease, the method comprising applying a compound or composition according to the invention. The present invention further relates to a disinfectant comprising the compound of formula (I) and to the non-therapeutic use of the instant compounds or disinfectants for disinfecting or sterilizing an inanimate object.