Used as an intermediate in the synthesis of antiviral agents, particularly in prodrugs targeting hepatitis B virus (HBV). Its phosphonate group enables mimicry of nucleotide monophosphates, allowing incorporation into viral DNA chains and causing chain termination. The (S)-amino acid moiety supports cellular uptake via amino acid transporters, enhancing delivery to hepatocytes. This compound plays a key role in developing nucleotide analogs with improved metabolic stability and targeted antiviral activity.