prepared. In contrast to the reaction of dithioacetal 3, these acetals undergo either a single or double HBr elimination with formation of mono-olefins or cumulated diene, respectively. The mechanism of formation of the reaction products is discussed, and the reactivity of the novel per-functionalized tetrasubstituted 1,3-butadiene 1 with the strongly electrophilic dienophile PTAD is presented.