1: With titanium(IV) isopropylate in tetrahydrofuran; dichloromethane T=20°C; 0.75 h; 2: With boron trifluoride diethyl etherate in tetrahydrofuran; dichloromethane T=20°C; 2 h; 3: With hydrogenchloride; water; ammonium chloride in tetrahydrofuran; dichloromethane T=-25°C;