1: With triethylamine in tetrahydrofuran Robinson annulation; 2: With toluene-4-sulfonic acid in benzene T=100°C; 40 h;

toluene-4-sulfonic acid in benzene 40 h; Heating;

1: Et3N / tetrahydrofuran / 0.5 h / 0 °C 2: 211 g / p-TsOH / benzene / 40 h / Heating View Scheme

1.1: bromine / H2O / 0 - 23 °C 2.1: 31.86 g / hydrobromic acid / H2O / 12 h / 100 °C 3.1: triethylamine / tetrahydrofuran 3.2: 96 percent / p-toluenesulfonic acid / benzene / 40 h / 100 °C View Scheme

1.1: 31.86 g / hydrobromic acid / H2O / 12 h / 100 °C 2.1: triethylamine / tetrahydrofuran 2.2: 96 percent / p-toluenesulfonic acid / benzene / 40 h / 100 °C View Scheme