1: 95 percent / pyridine / tetrahydrofuran / 48 h / 25 - 30 °C 2: 89.6 percent / H2SO4 / 20 - 25 °C 3: chlorosulfonic acid / 12 h / 50 °C 4: conc. NH4OH / tetrahydrofuran / 1 h / 0 - 5 °C 5: 1) borane-dimethyl sulfide, 2) H2SO4 / 1) THF, r.t. View Scheme