Streptonigrin and related compounds IV. Precursors for the a‐ring

The synthesis of streptonigrin or its tricyclic (ABC) analogues by the Friedlander condensation requires a 2‐nitrobenzaldehyde with all the necessary functionalities. A number of alternative syntheses are examined. It is shown that oxidation of 8‐amino‐5,6‐dimethoxy‐2,2‐pyridylquinoline leads to 8‐amino‐2,2‐pyridylqunoline‐5,6‐dione instead of 6‐methoxy‐2,2‐pyridylquinoline‐5,8‐dione. The synthesis of two useful precursors: 4‐bromo‐3‐hydroxy‐5,6‐dimethoxy‐2‐nitrobenzaldehyde and 4‐bromo‐3‐hydroxy‐5‐methoxy‐2‐nitrobenzaldehyde is described.