3-((((((3-Amino-2-pyrazinyl)carbonyl)oxy)(dibutyl)stannyl)oxy)carbonyl)-2-pyrazinamine 3-((((((3-Amino-2-pyrazinyl)carbonyl)oxy)(dibutyl)stannyl)oxy)carbonyl)-2-pyrazinylamine AIDS-138034 AIDS138034 NSC643863