Cohalogenation of Alkenes in Ethylene Oxide: Efficient Methodology for the Preparation of Allyl Vinyl Ether Precursors of γ,δ-Unsaturated Aldehydes

An investigation of the cohalogenation-dehydrohalogenation sequence as a method for the preparation of allyl vinyl ethers from simple olefins has been performed. Alkenes 1a-k react with bromine or chlorine in the presence of ethylene oxide at - 80°C to form β,β′-dihalo ethers 2a-k regio- and stereoselectively in high yields. Two of these β,β′-dihalo ether intermediates were selectively monodehydrohalogenated by potassium tert-butoxide to give 2-haloalkyl vinyl ethers in almost quantitative yields. Use of an excess of base produces allyl vinyl ethers 3a-k in good yields. Thermal rearrangement of the latter converts them into the corresponding γ,δ- unsaturated aldehydes 4a-j.