N-{[(4R,6R)-6-(6-amino-9H-purin-9-yl)-2,2-dimethyl-tetrahydro-2H-furo[3,4-d] [1,3]dioxol-4-yl]methyl}-N-(2-{[(2R,5R)-2-(6-amino-9H-purin-9-yl)-4-[(tert-butyldimethylsilyl)oxy]-5-{[(tert-butyldimethylsilyl)oxy]methyl}oxolan-3-yl]oxy}ethyl)-4-nitrobenzene-1-sulfonamide (19)