Chemistry of acetylenic ethers 65: 4‐Hydroxyalkynyl ethers, their synthesis and conversion into γ‐lactones

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Abstract
4‐Hydroxyalkynyl ethers can be prepared from epoxides and ethoxyethynyllithium in liquid ammonia or bis‐ethoxyethynylmagnesium in ether‐tetrahydrofuran.Under other conditions, notably in the presence of metal halides other products are obtained, including halohydrins and 3‐hydroxyalkynyl ethers, the latter arising from carbonyl compounds formed by isomerisation of the epoxides. For propylene oxide two types of isomerisation into carbonyl compounds have been described in the literature. Lewis acids like magnesium bromide convert the substance into propionaldehyde or into acetone. We found that the same change can also be caused by lithium iodide. For the preparation of pure 4‐hydroxyalkynyl ethers from epoxides it is therefore desirable to employ solutions of metallated acetylenic ethers devoid of salts.The reaction of epichlorohydrin with metallated ethoxyethyne or metallated ethylthioethyne leads to three and four successive products, respectively, depending on the reaction conditions and the solvent. The primary product is a 5‐chloro‐4‐hydroxyalkynyl (thio)ether which under alkaline conditions is converted into a 4,5‐epoxyalkynyl (thio)ether and, when sodamide is present, into a 5‐hydroxy‐Δ3,4 alkynyl (thio)ether. The thio compound (but not the oxygen analogue) finally undergoes ring closure to 2‐(alkylthiomethyl)furan.Hydrolysis and also pyrolysis of 4‐hydroxyalkynyl ethers gives γ‐lactones in good yields.