Larsen Scott D., Fisher Peter V., Libby Brian E., Jensen Randy M., Mizsak Stephen A., Watt William, Ronk Warren R., Hill Scott T.
Department of Chemistry, Alma College, Alma, Michigan 48801.
J Org Chem. 1996 Jul 12;61(14):4725-4738. doi: 10.1021/jo960537o.
An investigation of the scope and mechanism of a new synthesis of cyclopentenes from 3,6-dihydro-2H-thiopyrans is described. Alkyl halides substituted with an electron-withdrawing group in the alpha-position were reacted with sodium thiosulfate, yielding the corresponding Bunte salts, which could be transformed to reactive thiocarbonyl compounds by elimination of the elements of bisulfite with mild base treatment. In situ trapping by 1,3-dienes afforded in good yields a variety of 3,6-dihydro-2H-thiopyrans substituted with electron-withdrawing groups at the 2-position. Exposure of these cycloadducts to strong base at low temperature effected a novel ring contraction, affording 2-(methylthio)-3-cyclopentenes after quenching with methyl iodide. The level of diastereoselectivity exhibited during the generation of these cyclopentenes was found to be dependent on the nature of the electron-withdrawing group at the 2-position of the dihydrothiopyran as well as the substitution pattern originally present in the diene component. In some cases, reducing the temperature during the ring contraction resulted in the isolation of good yields of vinyl cyclopropanes of high isomeric purity. With one substrate, highly diastereoselective rearrangement of a vinyl cyclopropane to a cyclopentene was unambiguously demonstrated, suggesting that this might be a key feature of the overall ring contraction mechanism.
本文描述了对由3,6-二氢-2H-硫代吡喃合成环戊烯的新方法的范围和机理的研究。在α-位被吸电子基团取代的卤代烃与硫代硫酸钠反应，生成相应的邦特盐，通过用温和碱处理消除亚硫酸氢根元素，可将其转化为活性硫羰基化合物。1,3-二烯原位捕获以良好产率得到了多种在2-位被吸电子基团取代的3,6-二氢-2H-硫代吡喃。将这些环加成物在低温下暴露于强碱会发生一种新型的环收缩反应，用碘甲烷淬灭后得到2-(甲硫基)-3-环戊烯。发现这些环戊烯生成过程中表现出的非对映选择性水平取决于二氢硫代吡喃2-位吸电子基团的性质以及二烯组分中最初存在的取代模式。在某些情况下，环收缩过程中降低温度可分离出高产率的高异构体纯度的乙烯基环丙烷。对于一种底物，明确证明了乙烯基环丙烷向环戊烯的高度非对映选择性重排，这表明这可能是整个环收缩机理的一个关键特征。
