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

A novel and convenient procedure for the preparation of conjugated vinyl ke tene thioacetals is described: it involves addition of CBr4 to acrolein, fo llowed by thioacetalization of the corresponding aldehyde 2 and double dehy drobromination of 3 under basic conditions For comparison, three O-acetals 4-6 were also prepared. In contrast to the reaction of dithioacetal 3, thes e acetals undergo either a single or double HBr elimination with formation of mono-olefins or cumulated diene, respectively. The mechanism of formatio n of the reaction products is discussed, and the reactivity of the novel pe r-functionalized tetrasubstituted 1,3-butadiene 1 with the strongly electro philic dienophile PTAD is presented, (C) 1999 Elsevier Science Ltd. All rig hts reserved.