An aerobic oxidation/homolytic substitution-cascade for stereoselective methylsulfanyl-cyclization of 4-pentenols

Abstract

4-Pentenols (dihomoallylic alcohols) are oxidized by cobalt(II)-activated dioxygen in solutions of dimethyl disulfide and cyclohexa-1,4-diene to afford methylsulfanyl (CH₃S)-functionalized tetrahydrofurans in up to 74% yield. The reaction is a cascade, composed of oxidative alkenol cyclization providing tetrahydrofuryl-2-methyl radicals, which are trapped in dimethyl disulfide. Homolytic methylsulfanyl substitution by carbon radicals is a slow reaction, as exemplified by the rate constant of k^SCH₃ = 3 × 10⁴ M⁻¹ s⁻¹ (70 °C) derived from competition kinetics for the reaction between dimethyl disulfide and the trans-2-phenyltetrahydrofuryl-5-methyl radical. Methylsulfanyl-cyclizations therefore are experimentally performed in neat dimethyl disulfide, containing the minimum amount of cyclohexa-1,4-diene necessary for attaining almost quantitative alkenol conversion. The oxidative tetrahydrofuran synthesis occurs with noteworthy (>99%) 2,5-trans-stereoselectivity, as shown by the synthesis of diastereomerically pure 2,3- and 2,3,3-substituted 5-(methylsulfanyl)methyltetrahydrofurans from stereodefined 1,2-di- and 1,2,2-trisubstituted 4-pentenols. Changing the chemical nature of the disulfide reagent or the alkenol extends the scope of alkylsulfanyl-cyclization to ethylsulfanyl-cyclization, allylsulfanyl-transfer, or tetrahydropyran synthesis.