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.