1: 44 percent / potassium carbonate / dimethylsulfoxide / 1 h / 20 °C 2: 70 percent / conc. H2SO4, boric acid / 0.5 h / 80 °C 3: 72 percent / conc. H2SO4, boric acid / 0.5 h / 80 °C 4: 91 percent / K2CO3 / acetone / 4 h / Heating 5: 74 percent / KOH, sulfamic acid / 9 h / Heating 6: 93 percent / conc. H2SO4, boric acid / 2 h / 130 - 140 °C View Scheme

1: 44 percent / potassium carbonate / dimethylsulfoxide / 1 h / 20 °C 2: 70 percent / conc. H2SO4, boric acid / 0.5 h / 80 °C 3: 72 percent / conc. H2SO4, boric acid / 0.5 h / 80 °C 4: 91 percent / K2CO3 / acetone / 4 h / Heating 5: 74 percent / KOH, sulfamic acid / 9 h / Heating 6: 83 percent / boron trifluoride etherate / toluene / 4 h / Heating View Scheme