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Synthesis of precursor 203 (A) and of PD1 (B), as reported by Ogawa and Kobayashi [105]. Reagents and conditions: (A). i. LiNH2, NH3/THF; ii. Br (CH2)3OTBS, 81%; iii.H2,Lindlar cat., quinoline; iv. I2, PPh3, imidazole; v. PPh3, MeCN, 71% over the last three steps; vi. NaHMDS, 94%; vii. PPTS, CH2Cl2/EtOH (1:1), quant.; viii. SO3•Py; ix. NaClO2; x. CH2N2, 73%; xi. TBAF, 90%. (B). i. Pd (PPh3)4 (20 mol%) 2 N NaOH (10 equiv.) THF, rt, 1 h; ii. TBAF; iii. 2 N L iOH THF, H2O, 42%.
