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