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