Diiodomethane → 2-[1-(Cyclopentylmethyl)-5-fluoro-1H-pyrazolo[3,4-b]pyridin-3-yl]-4-iodo-5,5-dimethyl-5,7-dihydro-6H-pyrrolo[2,3-d]pyrimidin-6-one

40.549 g (151.398 mmol) of diiodomethane and 4.420 g (37.738 mmol) of isopentyl nitrite were added to 711 mg (1.799 mmol) of 4-amino-2-[1-(cyclopentylmethyl)-5-fluoro-1H-pyrazolo[3,4-b]pyridin-3-yl]-5,5-dimethyl-5,7-dihydro-6H-pyrrolo[2,3-d]pyrimidin-6-one. The mixture was stirred at 85° C. for 8 h. After cooling, the mixture was filtered, and the filtrate was diluted with cyclohexane and sucked through silica gel. The silica gel was washed with cyclohexane and the product was eluted with dichloromethane/methanol (v/v=50:2). The collected fractions were concentrated on a rotary evaporator, taken up in 20 ml of ethyl acetate and 5 ml of isopropanol, filtered and concentrated again. The residue was triturated with 5 ml of isopropanol. The solid was filtered off with suction and dried under high vacuum. This gave 230 mg (purity 92%, 23% of theory) of the target compound.