1 H NMR ( 400 MHz , DMSO - d 6 ) δ = 12 . 53 ( br s , 1 H ) , 8 . 33 ( s , 1 H ) , 8 . 15 ( d , J = 4 . 3 Hz , 1 H ) , 6 . 37 ( dd , J = 2 . 7 , 19 . 1 Hz , 1 H ) , 5 . 86 ( ddd , J = 2 . 7 , 5 . 1 , 51 . 6 Hz , 1 H ) , 5 . 62 ( ddd , J = 5 . 5 , 7 . 0 , 16 . 0 Hz , 1 H ) , 4 . 49 - 4 . 38 ( m , 2 H ) , 4 . 26 ( dd , J = 4 . 7 , 12 . 1 Hz , 1 H ) , 2 . 19 ( s , 3 H ) , 2 . 04 ( s , 3 H ) .