Tetrabutylammonium Fluoride → 4-Amino-5-(quinolin-3-yl)-6,7,8,9-tetrahydropyrimido[5,4-b]indolizin-8-ol

A solution of 1 M tetrabutylammonium fluoride in THF (2.09 ml) was added to a solution of 8-(tert-butyldimethylsilyloxy)-5-(quinolin-3-yl)-6,7,8,9-tetrahydropyrimido[5,4-b]indolizin-4-amine (778 mg) obtained in Step 7 in THF (20 ml) at room temperature. The mixture was stirred at the same temperature for 1 hour, and the solvent was distilled off under reduced pressure. The resulting residue was treated with a saturated aqueous ammonium chloride solution, followed by extraction with ethyl acetate. The organic layer was dried over anhydrous sodium sulfate, and the solvent was then distilled off under reduced pressure. The resulting residue was purified by silica gel column chromatography (developing solvent: chloroform/methanol) to obtain the title compound as a light-yellow solid (580 mg).