Facile synthesis of oxa- and azacyclic dienes via cycloalkenation of alkynyltungsten compounds. Stereoselective construction of tricyclic furan and pyran derivatives via intramolecular Diels-Alder reaction

Abstract

A convenient and short synthesis of functionalized oxacyclic and azacyclic dienes is developed on the basis of organotungsten chemistry. Alkynyltungsten compounds bearing a tethered alcohol and amine are treated with aldehydes and BF3·Et2O in cold diethyl ether to give tungsten- heterocyclic carbenium salts, further leading to tungsten-heterocyclic dienes via deprotonation with Et3N. Hydrodemetalation of these tungsten- heterocyclic dienes is performed by the action of anhydrous Me3NO in CH3CN. This method is applicable to the synthesis of a number of oxa- and azacyclic dienes, including those tethered with an electron-deficient olefin. The oxacyclic 1,3,8-nonatrienes and 1,3,9-decatrienes undergo intramolecular Dieis-Alder reactions upon heating in toluene, yielding tricyclic tetrahydropyran and -furan derivatives with excellent diastereoselectivities.