On the Stereoselective Opening of Achiral Dioxane Acetals

The stereoselectivity of allylation of achiral dioxane acetals cis- and trans-3 and cis- and trans-5 was found to be highly dependent on the nature of the allylmetal reagent, Lewis acid, and stoichiometry. Using TiCl₂(O-i-Pr)₂ as the Lewis acid in conjunction with allyltrimethylsilane and allyltri-n-butylstannane the selectivity of opening ranged from 1/1 to 18.6/1. In reactions with allyltrimethylsilane, the lack of selectivity for both the cis and trans series (1-2.4/1) was shown to arise from rapid equilibration of ion pairs. Control experiments revealed that the acetals underwent opening faster than isomerization. The reactions with allyltri-n-butylstannane were more selective and dependent on reagent stoichiometry. Moreover, the sense of asymmetric induction for the cis and trans series was opposite. Control experiments again established that isomerization of the acetals occurs slower than reaction with the stannane. These experiments unambiguously rule out the possibility that the opening proceeds via equilibrating ion pairs. The meso dioxane acetal cis-9 reacted with significantly reduced selectivity compared to the 2,4,6-trisubstituted analogue cis-1. On the other hand, the chiral acetal (±)-13 reacted much more selectively than the 2,4,6-trisubstituted analogue (±)-11. These reactions illustrate the sensitivity of stereochemical outcome to structural and experimental variables and demonstrate the ability to intercept reactive ion pairs under conditions of kinetic control.