21-Hemiacetal Dexamethasone

21-Hemiacetal Dexamethasone is a structurally modified derivative of the corticosteroid dexamethasone, characterized by a hemiacetal linkage at the C21 hydroxyl position. This impurity arises from the reaction of the C21 hydroxyl group with an adjacent carbonyl moiety, forming a five-membered hemiacetal ring. The molecule retains the core cyclopentanoperhydrophenanthrene scaffold with hydroxyl groups at C17 and C11, and a fluorine substituent at C9. The hemiacetal functionality introduces stereochemical complexity, distinguishing it from the parent drug. It serves as an HPLC reference standard for quantifying dexamethasone impurities in pharmaceutical formulations.