1: pyridine / 18 h / 10 - 20 °C 2: caesium carbonate / 1-methyl-pyrrolidin-2-one / 16 h / 60 °C / |Large scale 3: trifluoroacetic acid / dichloromethane / 4.17 h / -2 - 20 °C / |Large scale 4: N-ethyl-N,N-diisopropylamine; 1-propanephosphonic acid cyclic anhydride / dichloromethane / 18 h / 12 - 44 °C / |Large scale View Scheme