8-Methoxy Entecavir

8-Methoxy Entecavir is a purine nucleoside derivative featuring a methoxy group at the 8-position of the guanine-like heterocycle, substituting the hydrogen atom in the parent Entecavir structure. This compound retains the 1-[(2R,5S)-2,5-dihydroxytetrahydrofuran-3-yl]methyl moiety linked to the purine ring, with the methoxy substitution introducing steric and electronic perturbations. Synthetically, it arises as a byproduct during methylation steps in Entecavir production, reflecting regioselectivity challenges in purine functionalization. It serves as a critical HPLC reference standard for impurity profiling in Entecavir bulk drug substance analysis.