Methyl-2-oxo-2H-pyran-5-carboxylate (R1) was synthesized according to the reported literature procedure. In an oven dried 20 mL 2-necked round-bottom flask, equipped with a reflux condenser, coumalic acid (1.0 g, 1.0 eq.) was mixed with conc. H2SO4 (1.1 mL, 2.8 eq.). To the reaction mixture, MeOH (3 mL) was added, and the resulting solution was refluxed at 100 °C for 1 h and slowly cooled to rt. The mixture was quenched with water (10 ml), extracted with Et2O (3 x 10 mL) and the combined organic phase was dried over anhydrous MgSO4, concentrated under vacuum and the crude mixture was further purified by flash column chromatography.