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 BF(3).Et(2)O in cold diethyl ether to give tungsten-heterocyclic carbenium salts, further leading to tungsten-heterocyclic dienes via deprotonation with Et(3)N. Hydrodemetalation of these tungsten-heterocyclic dienes is performed by the action of anhydrous Me(3)NO in CH(3)CN. 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 Diels-Alder reactions upon heating in toluene, yielding tricyclic tetrahydropyran and -furan derivatives with excellent diastereoselectivities.