rac-N-[(1R)-2-Hydroxy-3-methyl-1-(2-methylpropyl)-3-buten-1-yl]-carbamic Acid 1,1-Dimethylethyl Ester is an intermediate in synthesizing (R)-2-Amino-4-methyl-1-((S)-2-methyloxiran-2-yl)pentan-1-one Trifluoroacetate Salt (A605100), an impurity of carfilzomib (C183460). Carfilzomib is a second-generation proteasome inhibitor that is used as a treatment in relapsed and refractory multiple myeloma.