1: 1.5 M tetrafluoroboric acid, H2O / tetrahydrofuran / 2 h / Ambient temperature 2: 95 percent / trifluoroacetic anhydride, trifluoroacetic acid / 16 h / Ambient temperature View Scheme

With  tetrafluoroboric acid; water in  tetrahydrofuran 2 h;  Ambient temperature;  Yield given;