POLYMER-SUPPORTED SYNTHESIS OF CYCLIC ETHERS - ELECTROPHILIC CYCLIZATION OF TETRAHYDROFUROISOXAZOLINES

The polymer-supported synthesis of cyclic ethers has been achieved using an intramolecular 1,3-dipolar cycloaddition/electrophilic cyclization sequence. The five-step reaction scheme converts heptadienols to cyclic ethers in 21-49% overall yield from polymer-supported nitroolefin. The heptadienols were varied with a Ph, CH3, and H group in the allylic position to give the accordingly substituted cyclic ethers. The 1,3-dipolar cycloaddition sets three stereocenters in the tetrahydrofuroisoxazoline, and the electrophilic cyclization establishes a fourth stereocenter in the cyclic ether. The degree of functionalization at the nitroolefin stage was determined by reduction to the primary amine and quantification with ninhydrin. The cis cyclic ether is the predominant stereoisomer from the electrophilic cyclization process. This latter step is specific for removal of only the desired cyclic ether from the polymer support.