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Syntheses of labeling precursors 17 (A) and 18 (B). Reactions and conditions: (i) NaBH3CN, AcOH, 25 ℃, 5 min; (ii) 2-methoxy-4-(chloromethyl)pyridine, K2CO3, MeCN, r.t., 4 h, 45% yield for two steps; (iii) (2-fluoro-4-hydroxyphenyl)boronic acid, Pd(dppf)Cl2, H2O/1,4-dixone, K2CO3, 100 ℃, 5 h then ammonia 7 M in MeOH, 25 ℃, 4 h, 42% yield for two steps; (iv) NaBH3CN, AcOH, 25 ℃, 5 min; (v) K2CO3, MeCN, r.t., 4 h; (vi) ammonia 7M in MeOH, 25 ℃, 4 h, 70% yield.
