5 g of (7R,8R,9R)-7,8-dihydroxy-2,3-dimethyl-9-phenyl-7,8,9,10-tetrahydroimidazo[1,2-h][1,7]-naphthyridine are dissolved in 40 ml of 2-(2-methoxyethoxy)ethanol, 3.2 g of sulfuric acid (98% strength) are added and the mixture is warmed at 50° C. for 16 hours. It is then poured onto ice, 100 ml of methylene chloride are added and the mixture is adjusted to pH 7 using aqueous 8 N sodium hydroxide solution. After separation of the organic phase, the aqueous phase is extracted a further two times using 50 ml of methylene chloride each time, the combined organic phases are washed with 100 ml of water, dried over sodium sulfate and the solvent is stripped off in vacuo. The residue is purified on silica gel (eluent: diethyl ether/2-propanol=10/1). 105 mg of the title compound are obtained. 1H-NMR (200 MHz, DMSO): δ=2.25 (s, 3H), 2.33 (s, 3H), 3.23 (s, 3H), 3.32–3.47 (m, 6H), 3.59–3.69 (m, 2H), 3.97–4.07 (q, 1H), 4.44–4.47 (m, 2H), 5.18–5.21 (d, 1 OH), 5.85–5.86 (d, 1 NH), 6.74–6.78 (d, 1H), 7.19–7.45 (m, 6H).
