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 : r 1 is a cyclolower alkyl group ; or r 1 is a lower alkyl group ; and r 2 is either hydrogen or a lower alkyl group ; r 3 is either hydrogen or a lower alkyl group ; r 4 a halo - lower alkyl group ; and r 5 is either hydrogen or a lower alkyl group .