1H NMR (400 MHz, CDCl3): δ 7.69 (d, J = 7.9 Hz, 2H), 7.41 (d, J = 8.0 Hz, 2H), 5.92 (s, 1H), 5.60 (s, 1H), 2.07 (s, 3H). 13C NMR (100 MHz, CDCl3): δ 197.0, 143.6, 138.4, 135.9, 130.8, 128.5, 127.1, 18.6.