{[(4aS,13aR)-8-({[(2,4-Difluorophenyl)methyl]amino}carbonyl)-9,11-dioxo-2,3,4a,5,9,11,13,13a-octahydro-1H-pyrido[1,2-a]pyrrolo[1',2':3,4]imidazo[1,2-d]pyrazin-10-yl]oxy}methyl 2-(methyl)ethyl carbonate