(4R,12aS)-N-[(2,4-Difluorophenyl)methyl]-3,4,6,8,12,12a-hexahydro-4-methyl-6,8-dioxo-7-(phenylmethoxy)-2H-pyrido[1’,2’:4,5]pyrazino[2,1-b][1,3]oxazine-9-carboxamide