(1R,7aR)-2,3,5,7a-Tetrahydro-1-hydroxy-1H-pyrrolizine-7-carboxylic Acid Methyl Ester 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.