1 H - NMR ( CDCl 3 ) δ 6 . 16 ( s , 1 H ) , 4 . 86 ( dd , 1 H , J = 3 . 6 , 6 . 0 Hz ) , 4 . 66 ( d , 1 H , J = 6 . 0 Hz ) , 4 . 12 ( d , 1 H , J = 6 . 4 Hz ) , 3 . 99 ( dd , 1 H , J = 3 . 6 , 10 . 8 Hz ) , 2 . 05 ( s , 3 H ) , 1 . 48 ( s , 3 H ) , 1 . 33 ( s , 3 H ) .