1-[(6R,8R,9R)-4-amino-9-{[tert-butyl(dimethyl)silyl]oxy}-6-(hydroxymethyl)-2,2-dioxido-1,7-dioxa-2-thiaspiro[4.4]non-3-en-8-yl]-5-methylpyrimidine-2,4(1H,3H)-dione