Tandem cycloadditions of functionalized bis-dienes. An orthogonal route to new dipseudoglycals

A direct and stereocontrolled access to a new class of conjugated bis-dienes is presented that relies on a double conjugated-elimination reaction induced by the action of strong bases such as n-BuLi or KHMDS on bis-alpha,beta-unsaturated acetals. These bis-dienes undergo double [4 + 2] cycloadditions with activated dienophiles under thermal conditions. With methyl acrylate, the reaction is totally endo and regiocontrolled with respect to both dienes and exhibits up to 60% inter-ring diastereocontrol when performed under high pressure. Mixed adducts can also be obtained by adding successively two different dienophiles. Again, the endo and regiocontrols are total while the ring-to-ring diastereocontrol depends on reaction conditions but remain, most of the time, relatively low. Double hetero-Diels-Alder reactions have also been performed, yielding bicyclic skeletons of novel 4<->4' disaccharidic structures. Finally, the corresponding nonconjugated bis-dienes have also been evaluated in cycloadditions. They react at room temperature with NMM or diethyl ketomalonate to provide highly functionalized polycyclic structures in high yields.