(R)-2-((2-amino-6-oxo-3,6-dihydro-9H-purin-9-yl)methoxy)-3-hydroxypropyl L-valylvalinate ; (2R)-2-((2-amino-6-oxo-3H-purin-9(6H)-yl)methoxy)-3-hydroxypropyl 2-((S)-2-amino-3-methylbutanamido)-3-methylbutanoate;