General procedure for the synthesis of 7-methoxy-4H-1-benzopyran-4-one and 5-methoxy-4-isodihydrochromanone from 3-(3-methoxyphenoxy)propionic acid: 3-(2-methoxyphenoxy)propionic acid (5.61 mmol) was mixed with polyphosphoric acid (25 mL) and the reaction was heated for 1 hr at 67 °C. After completion of the reaction, the reaction mixture was diluted with ice water (150 mL) and the precipitated solid product was collected by filtration. Subsequently, the solid was washed with water and dried to afford 8-methoxy-2,3-dihydro-4-one (target compound C2) in 67% yield as a brown solid.
