An aerobic oxidation/homolytic substitution-cascade for stereoselective methylsulfanyl-cyclization of 4-pentenols
作者:Patrick Fries、Melanie Kim Müller、Jens Hartung
DOI:10.1039/c3ob26590k
日期:——
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 kSCH3 = 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.
在二甲基二硫醚和环己-1,4-二烯的溶液中,4-戊烯醇(二高烯丙醇)被钴 (II) 活化的分子氧氧化,得到甲硫基 (CH3S) 官能化的四氢呋喃,产率高达 74%。该反应是一个级联反应,由氧化烯醇环化提供四氢呋喃基-2-甲基自由基组成,该自由基被捕获在二甲基二硫醚中。碳自由基的均解甲硫基取代是一个缓慢的反应,例如,源自二甲基二硫醚和反式-2-反应的竞争动力学的速率常数 kSCH3 = 3 × 104 M−1 s−1 (70 °C)苯基四氢呋喃基-5-甲基基团。因此,甲硫基环化反应在纯二甲基二硫醚中进行实验,其中含有实现几乎定量的烯醇转化所需的最小量的环己-1,4-二烯。氧化四氢呋喃合成具有显着的 (>99%) 2,5-反式立体选择性,如由立体定义的 1 合成非对映体纯的 2,3- 和 2,3,3-取代的 5-(甲硫基)甲基四氢呋喃所示, 2-二-和1,2,2-三取代的4-戊烯醇。改变二硫化物试剂或烯醇的化学性质将烷基硫基环化的范围扩展到乙基硫基环化、烯丙基硫基转移或四氢吡喃合成。