Synthesis of 4-azido-3-diazo-3H-pyrazolo[3,4-b]quinoline from 3-amino-4-hydrazino-1H-pyrazolo[3,4-b]quinoline

The reaction of 3-amino-4-hydrazino-1H-pyrazolo[3,4-b]quinoline 8 with nitrous acid did not give 2,4-diazidoquinoline-3-carbonitrile 5 as reported previously, but afforded in 60% yield 4-azido-3-diazopyrazolo[3,4-b]quinoline 10, which gave by azo coupling with 2-naphthol 4-azido-3-(2-hydroxy-1-naphthylazo)-1H-pyrazolo[3,4-b]quinoline 11.