The discovery of tadalafil: a novel and highly selective PDE5 inhibitor. 2: 2,3,6,7,12,12a-hexahydropyrazino[1',2':1,6]pyrido[3,4-b]indole-1,4-dione analogues.
Daugan A, Grondin P, Ruault C, Le Monnier de Gouville AC, Coste H, Linget JM, Kirilovsky J, Hyafil F, Labaudinière R. J Med Chem. 2003 Oct 9; 46(21):4533-42.