4-(Ethylthio)-2-methylpyrimido[4',5':4,5][1,3]thiazolo[3,2-a]benzimidazole AIDS-150601 AIDS150601 Ethyl 2-methylpyrimido[4',5':4,5][1,3]thiazolo[3,2-a]benzimidazol-4-yl sulfide NSC687828