Decyl isocyanate may be used to synthesize the following potential proteasome inhibitors:decyl-urea-Val-NHNHBocdecyl-urea-Val-Leu2-VSdecyl-urea-Val-Leu2-EK(EK = epoxyketone, Val = valine, Boc =tert-butyloxycarbonyl protecting group, Leu = leucine, VS = vinyl sulfones)