3-((2R,4S,5S)-4-Azido-5-hydroxymethyl-tetrahydro-furan-2-yl)-2,7-dioxo-1,3,5,7-tetrahydro-2H-pyrano[4,3--d]pyrimidine-8-carboxylic acid ethyl ester AIDS-059322 AIDS059322 Bicyclic pyrimidine