Compound 21 is prepared using synthesis method 3: intermediate 2e (0.21 g, 1.25 mmol) and intermediate 6a are placed in 5 mL of EtOH in the presence of 0.3 g (3.58 mmol) of sodium bicarbonate, and the mixture is then stirred for 2 h at 80° C. After evaporation under reduced pressure the residue is taken up by water and extracted using AcOEt. After drying of the organic phases and reduction to dryness the residue obtained is triturated in a mixture of EtOH/Et2O and 0.32 g of solid is isolated by filtration (yield: 77%).
