(1R,4R)-4-[[(1,1-Dimethylethoxy)carbonyl]amino]-2-cyclopentene-1-carboxylic Acid Methyl Ester is an intermediate in synthesizing trans-Abacavir Dihydrochloride (A104995), which is an impurity of Abacavir. Abacavir (A104990) is a carbocyclic 2′-deoxyguanosine nucleoside reverse transcriptase inhibitor and an anti-HIV drug used to treat HIV infection (1). Intracellular enzymes convert Abacavir to its active form, carbovir-triphosphate (CBV-TP), which then selectively inhibits HIV reverse transcriptase by incorporating into viral DNA (2). Abacavir is metabolized in the liver by uridine diphosphate glucuronyltransferase and alcohol dehydrogenase resulting in inactive glucuronide and carboxylate metabolites, respectively.