1-[2-deoxy-5-O-[[(2S,5R)-5-[5-methyl-2,4-dioxo-3,4- dihydropyrimidine-1(2H)-yl]-2,5-dihydrofuran2-yl]methyl]-β-D-threo-pentofuranosyl]-5- methylpyrimidine-2,4(1H,3H)-dione