boron trifluoride diethyl etherate in dichloromethane T=25°C; 4 h; Yield given;

1: 97 percent / BF3*Et2O / CH2Cl2 / Ambient temperature 2: 84 percent / BBr3 / CH2Cl2 / 15 h / -78 - 0 °C 3: p-toluenesulfonic acid / benzene / 0.5 h / 25 °C 4: BF3*Et2O / CH2Cl2 / 4 h / 25 °C View Scheme

1: 86 percent / tetrahydrofuran / 4 h / -60 °C 2: 79.6 percent / CrO3, aq. H2SO4 / acetone / 0.5 h / Ambient temperature 3: 97 percent / BF3*Et2O / CH2Cl2 / Ambient temperature 4: 84 percent / BBr3 / CH2Cl2 / 15 h / -78 - 0 °C 5: p-toluenesulfonic acid / benzene / 0.5 h / 25 °C 6: BF3*Et2O / CH2Cl2 / 4 h / 25 °C View Scheme

1: 86 percent / tetrahydrofuran / 4 h / -60 °C 2: 79.6 percent / CrO3, aq. H2SO4 / acetone / 0.5 h / Ambient temperature 3: 97 percent / BF3*Et2O / CH2Cl2 / Ambient temperature 4: 84 percent / BBr3 / CH2Cl2 / 15 h / -78 - 0 °C 5: p-toluenesulfonic acid / benzene / 0.5 h / 25 °C 6: BF3*Et2O / CH2Cl2 / 4 h / 25 °C View Scheme

1: 79.6 percent / CrO3, aq. H2SO4 / acetone / 0.5 h / Ambient temperature 2: 97 percent / BF3*Et2O / CH2Cl2 / Ambient temperature 3: 84 percent / BBr3 / CH2Cl2 / 15 h / -78 - 0 °C 4: p-toluenesulfonic acid / benzene / 0.5 h / 25 °C 5: BF3*Et2O / CH2Cl2 / 4 h / 25 °C View Scheme