N-t-Boc Valacyclovir; N-Tert-Butoxycarbonyl-L-valine-2-[(2-amino-1,6-dihydro-6-oxo-9H-purin-9-yl)methoxy]ethyl Ester; 2-[(2-Amino-6-oxo-1,6-dihydro-9H-purin-9-yl)methoxy]ethyl N-[(1,1-dimethylethoxy)carbonyl]-L-valinate;Valacyclovir Impurity S ;Valaciclovir EP Impurity S;