General procedure: Example 1 Preparation of 3-methyl-4-oxo-3,4-dihydroimidazo[5,1-d][1,2,3,5]tetrazine-8-carboxylic acid (EP0252682)
1. 3-methyl-4-oxo-3,4-dihydroimidazo[5,1-d][1,2,3,5]tetrazine-8-carboxamide (2.577 mmol, 0.5 g) was mixed with concentrated sulfuric acid (4 mL) under stirring conditions.
2. a solution was prepared by dissolving sodium nitrite (9.4 mmol, 0.65 g) in 2.6 mL of water.
3. Slowly add the aqueous sodium nitrite solution dropwise to the above mixture under cooling in an ice bath (keeping the temperature below 15°C).
4. After the dropwise addition, the reaction mixture was stirred at room temperature overnight.
5. 10 g of ice was added to the reaction mixture and cooling was continued in an ice bath for 1 hour.
6. The solid product was collected by filtration.
7. The collected solid product was dried under vacuum to give 0.493 g of 3-methyl-4-oxo-3,4-dihydroimidazo[5,1-d][1,2,3,5]tetrazine-8-carboxylic acid.
The yield was 98.6%.
