Pyrido[3,4‐d]pyridazines 2–5, 21 bearing one, two, or three aryl substituents in the pyridine moiety are shown to be conveniently accessible from 4‐aroyl‐5‐methylpyridazines or 4,5‐diaroylpyridazines, respectively, by condensation reactions with appropriate C‐N fragments. In addition, the novel pyridazine‐annelated pyridones 24, 25 were found to be easily available from ethyl 5‐methyl‐4‐pyridazinecarboxylate.

8‐Phenylpyrido[3,4‐d]pyridazines bearing various amino substituents at C‐5 (7a‐d, 8) were prepared from ethyl 5‐benzyl‐4‐pyridazinecarboxylate 1 via the fused pyridone 5. The isomeric 4‐phenylpyrido[2,3‐d]pyridazines having the amino functions attached to C‐2 (10a‐f) were obtained by a one‐pot cyclization of the amino ketone 1 with appropriate acetamide acetals. These novel triazanaphthalene derivatives are of interest as analogues of diuretic and antithrombotic agents.

Ethyl 5‐benzyl‐4‐pyridazinecarboxylate (I) reacts with the dimethyl acetal (II) to give the enamine (III) which is cyclized to form the bicyclic derivatives (V), (VI), or (VIII).

The chlorocyanopyridazine (I) reacts with the amino esters (II) to form the substitution products (III).

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