1: 94 percent / HCl / acetic acid / 9 h / Heating 2: tetrahydrofuran; hexamethylphosphoric acid triamide / 18 h / Ambient temperature 3: 56 percent / trimethylsilyl triflate / tetrahydrofuran; hexamethylphosphoric acid triamide; diethyl ether / 1.) -20 deg C, 15 min.; 2.) -40 deg C, 2 h.; 3.) -40 deg C, 1 h. View Scheme