The present invention relates to a compound of formula (I), wherein X is C=0, C=S or B-OH Y is an electrophile and Z is a leaving group, or Y--Z is an electrophile R1 comprises or consists of (a) (i) a first group binding to a proteolytic site of a proteasome, said first group being bound to X and (ii) optionally a second group enhancing delivery or (b) a group binding between subunits ß1 and ß2 of a proteasome R2 and R3 are independently selected from H, methyl, methoxy, ethyl, ethenyl, ethinyl and cyano, wherein methyl and ethyl may be substituted with OH or halogen.
