Disclosed herein are analogs of Salinosporamide A, having the Formulae Ia-IVa as follows: Like Salinosporamide A, the compounds of the present invention will inhibit the proteasome, an intracellular enzyme complex that destroys proteins the cell no longer needs. Without the proteasome, proteins would build up and clog cellular machinery. Fast-growing cancer cells make especially heavy use of the proteasome, so thwarting its action is a compelling drug strategy.