4,7-Diazaspiro[2.5]octane-4-carboxylic acid, 7-[2-(2,8-dimethylimidazo[1,2-b]pyridazin-6-yl)-4-oxo-4H-pyrido[1,2-a]pyrimidin-7-yl]-, 1,1-dimethylethyl ester