Synthetically Useful β-Lithioalkoxides from Reductive Lithiation of Epoxides by Aromatic Radical Anions

Article abstract of DOI:10.1021/jo00292a029

Epoxides are reductively cleaved by means of lithium 4,4'-di-tert-butylbiphenylide.Ethylene oxide itself cleaves to lithium 2-lithioethoxide (15) in less than 5 min at -95 deg C.Epoxides possessing one unsubstituted carbon atom reduce in a matter of minutes at -78 deg C.When both carbon atoms are monosubstituted, at least 1 h is required.Epoxides with one or with two geminal saturated substituents open mainly between the oxygen atom and the least substituted carbon atom.Ring opening in the other direction leads to an unstable β-lithioalkoxide which very rapidly forms an olefin.Acyclic 1,2-disubstituted epoxides yield only olefins.Cyclooctene oxide produces, after protonation, a 3:7 ratio of cyclooctanol and cyclooctene.Cyclohexene oxide gives a 3:1 ratio of cyclohexanol and cyclohexene.Vinyloxiranes, on the other hand, open at the most substituted C-O bond to produce an allylic anion associated with an alkoxide.The carbanionic centers of the resulting dianions add to the carbonyl groups of aldehydes and ketones; however, when a hydrogen atom is present on the carbon atom which is attached to both negatively charged atoms, some reduction of the carbonyl group competes with the nucleophilic addition.The allylic anions derived from vinyloxiranes, after treatment with titanium tetraisopropoxide or cerium(III) chloride, add to aldehydes mainly at the most or least substituted terminus, respectively.In the former case, the configuration of the resulting glycols is predominantly anti.A number of adducts of the dianions with conjugated unsaturated aldehydes and ketones can be converted to unsaturated cyclic 6-membered ring ethers in the presence of acid or methanesulfonyl chloride.