1-[(6R,8R,9R)-4-amino-9-[tert-butyl(dimethyl)silyl]oxy-6-[[tert-butyl(dimethyl)silyl]oxymethyl]-2,2-dioxo-1,7-dioxa-2λ⁶-thiaspiro[4.4]non-3-en-8-yl]-3-ethyl-5-methylpyrimidine-2,4-dione