A new and effective proteasome inhibitor, beta-lactam 3, has been accessed enantioselectively by multistep synthesis from the readily prepared intermediates 7 and 8 which were joined by a [2 + 2]-cycloaddition reaction to form the spiro beta-lactam 9 stereoselectively. The intermediate 9 was converted to 3 in seven steps and 30% overall yield. The beta-lactam 3 is stable for many days in water at pH 7, in contrast to the natural beta-lactones salinosporamide A (1) and omuralide (2). In common with 1 and 2, the beta-lactam 3 effectively inhibits the mammalian proteasome.