The reaction of phthalazino(2,3-b)phthalazine-5,12(7H, 14H)-diones with nitrous acid.

3,4-Dihydrophthalazin-1(2H)-one (I) was oxidized to phthalazin-1(2H)-one (III) with nitrous acid or with ferric chloride . Phthalazino [2,3-b] phthalazine-5,12(7H, 14H)-diones (IV) did not react with ferric chloride but they were oxidized with nitrous acid to 2-[1(2H)-oxo-2-phthalazinyl] methylbenzoic acids (V) and (VI). The formation of (V) or (VI) depends upon the substituents of compounds (IV). Strucutres (V) and (VI) were established by pKa measurements in methylcellosolve and by mass and N.M.R. spectra.