To a mixed solution of 3,4-dihydro-1H-benzopyran (3 g, 22.4 mmol, 1.0 eq.) and N,N-dimethylformamide (3.3 g, 45.2 mmol, 2 eq.) in 1,2-dichloroethane (20 mL) was added phosphorus trichloride (3.4 g, 45.2 mmol, 2 eq.) dropwise, slowly at 50 °C or below, with dropwise addition time controlled at 30 minutes. The reaction mixture was then stirred continuously at 85 °C for 12 hours. Upon completion of the reaction, the reaction was quenched with ice water and extracted with ethyl acetate (3 x 300 mL). The organic phases were combined, dried over anhydrous sodium sulfate, filtered and concentrated under reduced pressure. The residue was purified by silica gel column chromatography (petroleum ether: ethyl acetate = 8:1) to afford the yellow oily product benzodihydropyran-6-carbaldehyde (2.0 g, 12.3 mmol, 55.1% yield). The structure of the product was confirmed by 1H NMR (CDCl3, 400 MHz) and electrospray ionization mass spectrometry (ESIMS): 1H NMR δ 2.01 (q, J = 45.8 Hz, 2H), 2.82 (t, J = 6.4 Hz, 2H), 4.24 (t, J = 5.2 Hz, 2H), 6.85 (d, J = 8.4 Hz, 1H), 7.52- 7.64 (m, 2H), 9.79 (s, 1H); ESIMS m/z 163.1 ([M + H]+).
