To DMF (5.93 g) was added POCl3 (12.45 g) dropwise at 0° C. The mixture was warmed to RT, diluted with DCM (40 mL) and cooled again to 0° C. A solution of 1H-pyrrole-2-carboxylic acid benzyl ester (15.00 g, prepared by adopting a procedure from J. Barry, G. Bram and A. Petit, Heterocycles 1985, 23, 875-880) in DCM (40 mL) was added dropwise at such a rate that the temperature of the reaction mixture did not rise above 0° C. The reaction was the heated to reflux for 30 min whereupon it was cooled to 10° C. A solution of sodium acetate (28g) in water (90 mL) was added carefully. After stirring for 15 min the two resulting phases were separated and the aqueous phase was washed with DCM (3×75 mL). The combined organic phases were washed with saturated Na2CO3 solution and brine, dried over MgSO4 and concentrated. The residue was purified by flash column chromatography on silica (ethyl acetate/heptanes 1:2) to provide 5-formyl-1H-pyrrole-2-carboxylic acid benzyl ester (8.0 g) and 4-formyl-1H-pyrrole-2-carboxylic acid benzyl ester (4.0 g).
