Carbamic acid,[1-[7-(2-butynyl)-1-[2-[2-[2-(ethylamino)-2-oxoethoxy]phenyl]-2-oxoethyl]-2,3,6,7-tetrahydro-3-methyl-2,6-dioxo-1H-purin-8-yl]-3-piperidinyl]-,1,1-dimethylethyl ester