The synthesis of 5-substituted proline derivatives and of 5-substituted pyrrole-2-carboxylates by 5-endo cyclisations featuring a method for effectively favouring these with respect to 5-exo cyclisations
作者:David W. Knight、Adele L. Redfern、Jeremy Gilmore
DOI:10.1039/b106739g
日期:2001.11.1
Overall 5-endo cyclisations of the C-allylic glycine sulfonamides 5 lead to usually excellent yields of the 2,5-cis- or 2,5-trans-pyrrolidine-2-carboxylates 11 and 12 respectively, depending upon whether base is absent or present. While reductions to the corresponding pyrrolidine-2-methanols 13 proved efficient, subsequent eliminations of the elements of hydrogen iodide gave mixtures of products 14–16. Suitably positioned hydroxy groups compete successfully with the sulfonamide via a 5-exo cyclisation mode. However, when such a substrate contains a furan ring attached to the alkene function (21), then cyclisation does occur at the sulfonamide, presumably
by participation of the furan oxygen, to give an iodopyrrolidine-2-methanol 13a. Finally, base-induced elimination of both hydrogen iodide and toluene-p-sulfinic acid from the initial iodopyrrolidines 11 and 12 leads to 5-substituted pyrrole-2-carboxylates 26. Overall, this sequence is complementary to the Kenner pyrrole synthesis.
C-烯丙基甘氨酸磺酰胺5的五员环化反应通常会得到2,5-顺式或2,5-反式吡咯烷-2-羧酸酯11和12,产率优异,这取决于反应中是否存在碱。虽然还原为相应的吡咯烷-2-甲醇13的反应效率很高,但随后消除氢碘元素会得到混合产物14-16。适当位置的羟基可以通过五元外环化方式成功竞争磺酰胺。然而,当这种底物含有一个呋喃环连接到烯烃功能上(21)时,环化反应则发生在磺酰胺上,推测可能是呋喃氧参与的结果,得到碘吡咯烷-2-甲醇13a。最后,初始碘吡咯烷11和12在碱诱导下同时消除氢碘和甲苯-对磺酸,得到5-取代的吡咯-2-羧酸酯26。总的来说,这一序列是对肯纳吡咯合成法的补充。