Disclosed is a total synthesis of a biologically active β-Lactam—Compound 3, which is related to Salinosporamide A and Omuralide, both structurally and by its activity as a proteasome inhibitor. Also disclosed are proteasome inhibiting compounds having the formula: wherein: R1 is a cyclolower alkyl group; or R1 is a lower alkyl group; and R2 is either hydrogen or a lower alkyl group; R3 is either hydrogen or a lower alkyl group; R4 a halo-lower alkyl group; and R5 is either hydrogen or a lower alkyl group.