(2R,5S)-L-Menthol-5-(4-amino-2-oxo-1(2H)-pyrimidinyl-13C)-1,3-oxathiolane-2-carboxylate is a labeled analogue of (2R,5S)-L-Menthol-5-(4-amino-2-oxo-1(2H)-pyrimidinyl)-1,3-oxathiolane-2-carboxylate (M219020), an intermediate in the synthesis of Lamivudine (L172500), a potent nucleoside reverse transcriptase inhibitor. Antiviral. Lamivudine has been used for treatment of chronic hepatitis B.