Cycloadditions of Bifunctional Vinyl Ethers with Electrophilic Alkenes through Tetramethylene Zwitterion Intermediates

Bifunctional vinyl ethers react with electron-poor alkenes to cyclobutanes in good yields. The second CC bond reacted with neither the cyclobutane nor its zwitterion intermediate, even on heating. Cyclobutanes formed from ‘tetracyanoethylene’ (8) were transformed into tetrahydropyridines by reaction of the corresponding zwitterion with MeCN as the solvent. In contrast, cyclobutanes formed from dimethyl (dicyanomethylidene)propanedioate (9) did not react with MeCN, which is ascribed to diminished stabilization of the zwitterion intermediate, and increased steric effects. These results extend the classical studies of Huisgen and his co-workers.