Oxidation of alcohols: Synthesis of bicyclic and tricyclic spiral derivatives of tetrahydrofuran
Tertiary alcohols with a delta-hydrogen undergo cyclization reactions when treated with silver salts and bromine in the dark to produce substituted tetrahydrofurans. The reaction preferentially abstracts an unactivated tertiary delta-hydrogen when there is competition between a secondary or a primary hydrogen with a tertiary delta-hydrogen in secondary and tertiary aliphatic alcohols. The mechanism of the oxidative cyclization had been studied and proposed that the reaction progresses via a combination of an ionic and free radical pathway. Cyclic tertiary alcohols with five, six, seven and eight membered rings; a cyclohexanol with a keto group and two bicyclic alcohols were synthesized. All of the alcohols contained a single tertiary delta-hydrogen. Treatment of these alcohols with silver salts (silver acetate, silver carbonate) and bromine in the absence of light yielded the new compounds bicyclic and tricyclic spiral tetrahydrofurans high yields. Optimal reaction conditions for this heterogeneous reaction system were essential to avoid the reaction produced acid from dehydrating the alcohols and producing the undesired corresponding alkene. A fast and simple mass spectral detection of the oxonium ion intermediate that produces the base peak of spiral ethers is proposed. The proton and carbon NMR and mass spectral results of the spiral ethers and applicability of this reaction as a mild and efficient reaction to produce spiral derivatives of tetrahydrofurans are discussed.