An expedient route for the stereoselective construction of bridged polyheterocyclic ring systems using the tandem ''pincer'' Diels-Alder reaction

The tandem ''pincer'' Diels-Alder reaction, consisting of two consecutive [4 + 2] cycloadditions between two dienes and an acetylenic bis-dienophile, has been applied toward the rapid construction of bridged polyoxacyclic ring systems when furan derivatives are used as the diene components. The study has demonstrated the stereoselectivity (exo-exo adduct), the chemoselectivity (''pincer'' vs ''domino''), as well as the regioselectivty of the reaction. The reaction has been successfully applied to a variety of 2-substituted furans and tethered bis-furans in combination with monoactivated and diactivated dienophiles. The synthesis of unsymmetrical cycloadducts starting from the aza- and oxanorbornadiene-type intermediate has also been realized.