Conjugated elimination versus [1,2]-Wittig rearrangement of unsaturated diox(ol)anes

A set of unsaturated dioxanes and dioxolanes has been prepared in three steps from acetoxy-1-isoprene. When reacted with two equivalents of t-BuLi in THF, these compounds provided, under various conditions, a mixture of two types of 1,3-dienes. The first one, derived from a conjugated elimination reaction resulting in the heterocycles opening, is relatively unstable but could be trapped as an acrylate. The second one resulted probably from a 1,2]-Wittig rearrangement. The competition between these two reactions has been observed for both dioxanes and dioxolanes (but not for acyclic ketals) and is the object of a strong temperature effect, the Wittig rearrangement being favored over the elimination at room temperature. A difference between kinetic and thermodynamic deprotonation sites is proposed to be at the origin of this competition on the base of both experimental and theoretical results.