9AH,10aH-3a,6a,2-[1,1,3]propanetriyl-4H-difuro[2,3-b:3',2'-g]oxocin-3-carboxylic acid, 8-(3-furanyl)hexahydro-6-methyl-, methyl ester AIDS-140753 AIDS140753 Crovatin NSC653859