(S)-2-(Methoxymethyl)morpholine is a chiral morpholine derivative characterized by a methoxymethyl substituent at the 2-position of the morpholine ring. It is a colorless to pale yellow liquid and serves as a versatile chiral building block in organic synthesis. This compound is primarily employed as a key chiral intermediate in the pharmaceutical industry. Its most notable application is in the synthesis of Gefapixant (formerly known as AF-219 or MK-7264), a P2X3 receptor antagonist developed for the treatment of refractory chronic cough. The (S)-enantiomer is crucial for imparting the desired biological activity to the final drug molecule. The morpholine core and the chiral center make it a valuable scaffold for constructing more complex bioactive molecules. As a chiral amine, it is also used in asymmetric synthesis and as a ligand precursor in catalysis. Its dual functionality (basic nitrogen and ether oxygen) allows it to participate in various chemical transformations, making it a useful reagent in medicinal chemistry research beyond its specific role in Gefapixant synthesis.