To a solution of (R)-(1-(5,6-diaminopyridin-2-yl)piperidin-3-yl)(pyrrolidin-1-yl)methanone dihydrochloride (125 mg, 0.3 mmol) in anhydrous N,N-dimethylformamide (2.5 mL) was added 3-(methylsulfonyl)benzaldehyde (76 mg, 0.4 mmol). The solution was stirred at 80° C. for 1 h. Sulfur was added (25 mg, 0.8 mmol) followed by triethylamine (140 μL, 1.0 mmol). The mixture was stirred at 85° C. for 24 h followed by stirring at room temperature for another 24 h. The mixture was concentrated under reduced pressure and the residue was partitioned between dichloromethane and water. The aqueous layer was extracted with dichloromethane and the combined extracts were dried over magnesium sulfate, filtered and concentrated under reduced pressure. The residue was purified via flash chromatography (20-100% ethyl acetate in heptanes gradient followed by 0-10% methanol in dichloromethane). The residue was repurified via flash chromatography (0-10% methanol in dichloromethane) to afford a yellow solid that was stirred in acetonitrile (3 mL) at 40° C. for 2.5 h. The solid was filtered, washed with cold acetonitrile and dried under high vacuum at 40° C. to afford the title compound. 1H NMR (500 MHz, CD3OD) δ 1.60-1.74 (m, 1H) 1.77-1.89 (m, 2H) 1.89-2.11 (m, 4H) 2.81 (br s, 1H) 2.97-3.09 (m, 2H) 3.17-3.26 (m, 3H) 3.41-3.52 (m, 2H) 3.52-3.63 (m, 1H) 3.74 (br s, 1H) 4.37 (br s, 1H) 4.64 (d, 1H) 6.91 (br s, 1H) 7.74-7.89 (m, 2H) 8.05 (d, 1H) 8.35 (br s, 1H) 8.65 (br s, 1H).
