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Synthesis of key intermediate (20). Reagents and conditions: (a) TBDMSCl, imidazole, DMF, 50 °C, 3 d. (b) TFA/H2O/THF (1:1:2), 0 °C, 6 h, 62% (over 2 steps). (c) DMP, pyridine, DCM, rt, on. (d) 37% CHOaq, 2 M NaOHaq, 1,4-dioxane, rt, on. (e) NaBH4, 0 °C to rt, 1.5 h, 70% (over 3 steps). (f) DMTrCl, DCM/pyridine (3:1), rt, 24 h, 56%. (g) TBDMSCl, imidazole, DMF, 50 °C, 24 h, 95%. (h) AcOH 80%aq, THF, rt, 3 h, 59%.
