(3R)-3-[[(1,1-Dimethylethyl)dimethylsilyl]oxy]-1-pyrrolidinecarboxylic Acid 1,1-Dimethylethyl Ester-d4 is an intermediate in synthesizing Monocrotaline-d4 (M526002), a labelled Monocrotaline (M526000) which is a toxic pyrrolizidine alkaloid isolated from Crotalaria spp. It is used for inducing pulmonary diseases in rats.