Derivatives of belactosin and their synthesis are disclosed. In certain embodiments, compounds of the present invention exhibit anti-cancer, antiviral, antibiotic, and/or auto-immune therapeutic abilities. In general, methods of synthesis disclosed herein allow for introduction of a variety of substituents at numerous positions as well as the facile introduction of a beta-lactone ring moiety. The synthetic steps comprise, in preferred embodiments, a tandem Mukaiyama aldol lactonization reaction. Data demonstrating the utility of some of the derivatives as proteasome inhibitors is also disclosed.