Stereodivergent Assembly of 2,6-<i>cis</i>- and -<i>trans</i>-Tetrahydropyrans via Base-Mediated Oxa-Michael Cyclization: The Key Role of the TMEDA Additive
作者:Uladzimir S. Masiuk、Yaroslav V. Faletrov、Dzmitry G. Kananovich、Iryna V. Mineyeva
DOI:10.1021/acs.joc.2c02382
日期:2023.1.6
The stereodivergent synthesis of cis- and trans-2,6-disubstituted tetrahydropyrans (THPs) via sodium hexamethyldisilazide-promoted oxa-Michael cyclization of (E)-ζ-hydroxy α,β-unsaturated esters is presented. The cyclization affords the kinetically favored trans-THPs with high stereoselectivity (dr up to 93:7) at a low temperature (−78 °C), while the room-temperature reaction does not produce the thermodynamically
介绍了通过六甲基二硅氨基钠促进的 ( E )-ζ-羟基 α,β-不饱和酯的氧杂-迈克尔环化,立体合成顺式和反式-2,6-二取代四氢吡喃 (THP)。环化在低温(-78°C)下提供具有高立体选择性(dr 高达 93:7)的动力学有利的反式-THP,而室温反应不会产生热力学优先的顺式-THP 作为主要产物和发生在立体控制不佳的情况下。四甲基乙二胺 (TMEDA) 的添加显着改善了室温环化的立体化学结果,并允许获得高顺式-选择性(dr 高达 99:1)。TMEDA 的显着效果表明钠阳离子在控制热力学驱动过程的立体选择性方面起着重要作用,即阳离子与环化产物的络合导致选择性降低。DFT 计算支持这一结论,表明与相应的钠螯合物相比,无钠顺式和反式烯醇化物的吉布斯能量差异更大。该方法的合成效用已通过 (+)-Neopeltolide 和 (−)-Diospongin B 的正式合成以及 (−)-Diospongin