(S)-4-Aminotetrahydrofuran hydrochloride is a chiral heterocyclic amine derivative, specifically a hydrochloride salt of an amino-substituted tetrahydrofuran. It serves as a crucial chiral building block in pharmaceutical synthesis. Its primary and most significant application is as a key intermediate in the synthesis of the antiviral drug **Letermovir** (Prevymis®). Letermovir is a non-nucleoside antiviral agent used for the prophylaxis of cytomegalovirus (CMV) infection in adult CMV-seropositive recipients of an allogeneic hematopoietic stem cell transplant. The (S)-4-aminotetrahydrofuran moiety is an essential structural component of the Letermovir molecule, contributing to its biological activity and specificity. This compound is a high-value synthetic intermediate, and its production and use are closely tied to the manufacture of Letermovir. It is typically handled under controlled conditions in research and industrial settings dedicated to active pharmaceutical ingredient (API) development.