1: 98 percent / diethyl ether / 1.) - 78 deg C, 2 h, 2.) room temperature, 5 h 2: 90 percent / oxalic acid, silica gel / CH2Cl2 / 4 h / Ambient temperature 3: 72 percent / copper(II) bromide, benzoyl peroxide / tetrahydrofuran / 24 h 4: 95 percent / 24 h / Heating 5: 98 percent / 550 °C / 0 Torr View Scheme

1: 98 percent / diethyl ether / 1.) - 78 deg C, 2 h, 2.) room temperature, 5 h 2: 90 percent / oxalic acid, silica gel / CH2Cl2 / 4 h / Ambient temperature 3: 72 percent / copper(II) bromide, benzoyl peroxide / tetrahydrofuran / 24 h 4: 95 percent / 24 h / Heating View Scheme