ethyl 4,7,7-tricyano-5-imino-1,3-dimethyl-6-oxabicyclo[2.2.1]heptane-2-carboxylate 4,7,7-tricyano-5-imino-1,3-dimethyl-6-oxabicyclo[2.2.1]heptane-2-carboxylic acid ethyl ester AIDS-148796 Oprea1_488764 NCI60_029268 AIDS148796 Ethyl 4,7,7-tricyano-3-imino-1,5-dimethyl-2-oxabicyclo[2.2.1]heptane-6-carboxylate NSC681783