1: 2,4-dihydroxy-benzoic acid With sodium hydride in DMF (N,N-dimethyl-formamide) T=20°C; 0.5 h; 2: With boric acid trimethyl ester in DMF (N,N-dimethyl-formamide) T=20°C; 1 h; 3: 2,2,3,3,3-pentafluoropropyl trifluoromethanesulfonate in DMF (N,N-dimethyl-formamide) T=100°C;