8-Hydroxy Entecavir

8-Hydroxy Entecavir is a structurally modified nucleoside analog featuring a hydroxyl substitution at the 8-position of the purine ring system relative to the parent antiviral agent Entecavir. This oxidation introduces a vicinal diol motif between the C8 and adjacent carbon, disrupting the planarity of the aromatic core and altering hydrogen bonding potential. The cyclopentyl substituent remains intact at the 1'-position of the furanoside moiety. This impurity arises via oxidative degradation pathways during synthesis or storage, characterized by selective C8 hydroxylation. It serves as a critical HPLC reference standard for quantifying oxidative impurities in Entecavir drug substance.