Scheme 4.
Reagents and conditions: (a) TMSOTf, NIS, 4 Å MS, CH2Cl2, −20 °C to rt, 3 h; (b) NaOMe, MeOH, rt, 6 h: (c) H2S, Py/H2O, 40 °C, 16 h; (d) (HCO)2O, MeOH, −20 °C, 3 h; (e) H2, Pd(OH)2, MeOH/H2O, rt, 16 h.
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