piperidine in pyridine 1) room temperature, 16 h; 2) 60 deg C, 4 h; 3) 100 deg C, 3 h;

1: conc. HCl, conc. H2SO4 / 3 h / 70 °C 2: 54 percent / hexamethylenetetramine, aq. HOAc / 2 h / Heating 3: 96.6 percent / piperidine / pyridine / 1) room temperature, 16 h; 2) 60 deg C, 4 h; 3) 100 deg C, 3 h View Scheme

1: 54 percent / hexamethylenetetramine, aq. HOAc / 2 h / Heating 2: 96.6 percent / piperidine / pyridine / 1) room temperature, 16 h; 2) 60 deg C, 4 h; 3) 100 deg C, 3 h View Scheme