In contrast to existing methods, we planned to develop a ringopening fluorination of bicyclic azaarenes such as pyrazolopyridines. We hypothesized that treating bicyclic azaarenes with an electrophilic fluorinating agent would result in fluorination at the C3 position, followed by deprotonation at the C2 position and pyrazole ring opening via N-N bond cleavage. Although this can be considered as a simple electrophilic fluorination using an electron-rich heteroaromatic system as a nucleophile, the resulting compound is an sp 3fluorinated compound (C(sp 3 )-F bond) instead of a fluorine-substituted heteroarene (C(sp 2 )-F bond). In other words, we thought that this would be a novel type of fluorination reaction with accompanying skeletal transformation. First, we selected 3-phenylpyrazolopyridine (1A) as the model substrate (which was readily prepared in three steps from a commercially available compound) to examine electrophilic fluorinating agents and reaction conditions (Table ). When N-fluoropyridinium salts (F1-F3) were used in MeCN at 80 °C, ring-opening fluorinated product 2A was successfully obtained, albeit in low yields (Entries 1-3). The use of stronger fluorinating agents such as NFSI and Selectfluor ® gave fluorinated products in high yields (Entries 4 and 5). 9 As for the reaction temperature, the yield of 2A was 68% even at 50 °C. The yield increased as the temperature was increased, and the fluorinated product was obtained quantitatively at 80 °C (Entries 6-8 vs. Entry 5). The reaction proceeded in polar solvents such as acetone and DMF (which is able to dissolve Selectfluor ® ), and gave the fluorinated product 2A (Entries 9-11). Finally, we conformed the optimal conditions: Selectfluor ® (1.0 equiv) at 80 °C in MeCN for 24 h. a Conditions; 1A (0.20 mmol), F + source (1.0 equiv), solvent (1.0 mL),