Multi-step reaction with 4 steps 1: 86 percent / BF3*Et2O / 4 h / -25 - -18 °C2: 100 percent / n-Bu4NF*3H2O / tetrahydrofuran / 1.5 h / -21 - -17 °C3: 846 mg / CrO3, pyridine / CH2Cl2 / 0.5 h / Ambient temperature4: NH3 / methanol / 20 h / Ambient temperature