1 H - NMR ( CDCl 3 ) δ 5 . 39 ( s , 1 H ) , 4 . 82 ( dd , 1 H , J = 3 . 6 , 6 . 0 Hz ) , 4 . 55 ( d , 1 H , J = 6 . 0 Hz ) , 4 . 05 ( dd , 1 H , J = 3 . 6 , 10 . 2 Hz ) , 4 . 00 ( d , 1 H , J = 10 . 0 Hz ) , 1 . 45 ( s , 3 H ) , 1 . 30 ( s , 3 H ) .