1 H NMR ( 300 MHz , DMSO - d 6 ) δ 9 . 12 ( s , 1 H ) , 8 . 73 ( s , 1 H ) , 7 . 71 ( d , 2 H , J = 8 . 9 Hz ) , 7 . 60 ( d , 2 H , J = 9 . 1 Hz ) , 7 . 18 ( d , 1 H , J = 2 . 1 Hz ) , 6 . 79 ( m , 2 H ) , 5 . 97 ( s , 2 H ) .