1 H - NMR ( CDCl 3 ) δ 8 . 15 ( s , 1 H ) , 6 . 07 ( s , 1 H ) , 5 . 41 ( d , 1 H , J = 6 . 0 Hz ) , 5 . 26 - 5 . 29 ( m , 1 H ) , 4 . 25 - 4 . 31 ( m , 2 H ) , 1 . 57 ( s , 3 H ) , 1 . 41 ( s , 3 H ) ;