General procedure for the synthesis of 4-isobutyldihydro-2H-pyran-2,6(3H)-dione from 3-isobutylglutaric acid: 3-isobutylglutaric acid (50 g, 266 mmol) was dissolved in propionic anhydride (130 ml), and the reaction mixture was heated and refluxed for 6 h at 140-145 °C. Upon completion of the reaction, propionic acid and unreacted propionic anhydride were removed by distillation under reduced pressure (30-60 °C/15 mm Hg). Subsequently, the product was collected at 100-105 °C/0.5 mm Hg to afford 4-isobutyldihydro-2H-pyran-2,6(3H)-dione as a yellow oil in a yield of 41 g (91% yield) with a GC purity of >98% (HP-1,25 m, 70 kPa, gradient 70-240 °C, 15 °C/min). The structure of the product was confirmed by 1H NMR (CDCl3) and 13C NMR (CDCl3): 1H NMR (CDCl3), δ/ppm: 0.91 (d, 6H, J=6.6Hz), 1.27 (t, 2H, J=7.1Hz), 1.61-1.75 (m, 1H), 2.12-2.50 (m, 3H), 2.85 (dd , 2H, J1=17Hz, J2=4.1Hz); 13C NMR (CDCl3), δ/ppm: 21.86,24.23,25.89,35.43,43.08,166.48.
