Brominated vinyl ketene thioacetal, ene- and diene acetals; potential four-carbon synthons

A novel and convenient procedure for the preparation of conjugated vinyl ketene thioacetals is described: it involves addition of CBr₄ to acrolein, followed by thioacetalization of the corresponding aldehyde 2 and double dehydrobromination of 3 under basic conditions for comparison, three O- acetals 4-6 were also 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.