1 H NMR ( 400 MHz , DMSO - d 6 ) : δ ppm 11 . 57 ( s , 1 H ) , 10 . 72 ( s , 1 H ) , 8 . 17 ( s , 1 H ) , 8 . 13 ( s , 1 H ) , 7 . 92 ( d , J = 8 . 6 Hz , 1 H ) , 7 . 81 ( s , 1 H ) , 7 . 73 - 7 . 70 ( m , 1 H ) , 7 . 56 ( d , J = 8 . 6 Hz , 1 H ) , 7 . 45 - 7 . 42 ( m , 1 H ) , 6 . 63 ( s , 1 H ) , 4 . 21 ( s , 3 H ) . MS ( ESI ) : m / z 420 . 0 [ M + H ] +.