1: 0.1 M aq. p-toluenesulfonic acid / tetrahydrofuran / 0.3 h / Heating 2: 1.) NaN(SiMe3)2 / 1.) THF, HMPA, RT, 2 h, 2.) from -100 to 0 deg C, 4 h View Scheme

1: 0.1 M aq. p-toluenesulfonic acid / tetrahydrofuran / 0.3 h / Heating 2: 1.) NaN(SiMe3)2 / 1.) THF, HMPA, RT, 2 h, 2.) from -100 to 0 deg C, 4 h 3: diethyl ether / 0 °C View Scheme