Stereocontrolled Synthesis and Cycloaddition of 1,4-Dialkoxy 1,3-Dienes

An efficient and stereocontrolled access to 1(Z),3(E)-1,4-dialkoxybutadienes is described that relies on a base-induced conjugated elimination reaction on γ-alkoxy or aryloxy α,β-unsaturated acetals. The dienes have then been applied in [4 + 2] cycloaddition reactions where they demonstrate a good thermal reactivity toward activated dienophiles. Despite the 1,4-competition between oxygenated groups, both regio- and endoselectivities are total. A set of experiments has led to the conclusion that the (1Z,3E) pattern is responsible for these high stereocontrols. Several chiral alkoxy dienes have also been prepared following the same route. Their thermal cycloaddition with N-methylmaleimide leads to corresponding adducts in good yields and with total endoselectivities, but modest diastereoisomeric excess.