Steric Effects in the Tuning of the Diastereoselectivity of the Intramolecular Free-Radical Cyclization to an Olefin As Exemplified through the Synthesis of a Carba-Pentofuranose Scaffold
作者:Mansoureh Karimiahmadabadi、Sayeh Erfan、Andras Földesi、Jyoti Chattopadhyaya
DOI:10.1021/jo300936g
日期:2012.8.17
carba-pentofuranose scaffolds, dimethylbicyclo[2.2.1]heptane 9 vis-a-vis 7-methyl-2-oxabicyclo[2.2.1]heptanes 18a/18b (C7-Me in the former versus 2-O- in the latter), dictates the stereochemical outcome both at the Grignard reaction step as well as in the free-radical ring-closure reaction. The formation of pure 1,8-trans-bicyclo[2.2.1]heptane 9 from 8 suggests that the boat-like transition state is favored
与戊糖的手性C4上的自由基和分子内C1链状烯烃(在自由基前体8和17上)进行的两个自由基环化反应在二甲基双环[2.2.1]庚烷9中产生了一个新的非对映特异性C4-C8键,而在7-甲基-2-氧杂二环[2.2.1]庚烷的新的C4-C7键18A / 18B,得到反式与顺式非对映体,其中所述C4中心的手性得以完全保留作为起始原料的。已经显示了稠合的氨基甲酸酯-戊呋喃糖支架,二甲基双环[2.2.1]庚烷9相对于7-甲基-2-氧杂双环[2.2.1]庚烷18a的化学性质。/ 18b(前者为C7-Me,后者为2- O-)指示了格利雅(Grignard)反应步骤以及自由基闭环反应的立体化学结果。纯1,8-形成反式-双环[2.2.1]庚烷9从8表明,舟状过渡状态由于缺乏膨松1(的空间位阻的青睐小号) - ø - p甲氧基苄基(PMB)和7(R)-Me取代基(均在α面上)与β-面中的8(R)-CH 2 •自由基相同。17