The ubiquitin–proteasome pathway (UPP) influences essential cellular functions including cell growth, differentiation, apoptosis, signal transduction, antigen processing and inflammatory responses. The main proteolytic component of the UPP is the 26S proteasome, which is responsible for the turnover of many cellular proteins and represents an attractive target for the treatment of pathologies such as cancer, as well as inflammatory, immune and neurodegenerative diseases. Natural and synthetic proteasome inhibitors having different chemical structures and potency have been discovered. We report herein the synthesis, proteasome inhibition and modelling studies of novel C-terminal isoxazoline vinyl ester pseudopeptides. Some new compounds that contain a C-terminal extended conjugation inhibit β1 and especially β5 proteasomal catalytic subunits with IC50 values ranging from 10 to 100 µm. These results will permit further optimization based on these structural moieties to develop more active and selective molecules.