To a solution of 3.1 g of 6-(4-hydroxypiperidino)-3,4-dihydrocarbostyril in 60 ml of acetic acid was added 1.7 g of sodium nitrite. After stirring at room temperature for 1 hr, water and a 17% methanol-chloroform mixture was added to the reaction mixture. The organic layer was washed with water, dried, and evaporated under a reduced pressure. The crude mixture was purified by a silica gel column chromatography and recrystallized from 2-propanol to give 1.1 g of 6-(4-hydroxypiperidino) 7-nitro-3,4-dihydrocarbostyril and 130 mg of 6-(4-hydroxypiperidino)-5-nitro-3,4 dihydrocarbostyril.
