Synthesis starting with 6-aminoquinoline: Sch"onh"ofer, Henecka, DE 583207 (1933 to I. G. Farben); Haskelberg, J. Org. Chem. 12, 434 (1947); Reuter, Aust. Chem. Inst. J. Proc. 16, 164 (1949).