1-(3-chloro-2,3-dideoxy-β-D-erythro-pentofuranosyl)-5-methylpyrimidine-2,4(1H,3H)-dione; 1-[(2R,4S,5R)-4-chloro-5-(hydroxymethyl)oxolan-2-yl]-5-methylpyrimidine-2,4-dione