conjugate additions to aliphatic (E)-nitro alkenes 2a–j were carried out in good yields (35-87%) and excellent diastereomeric excesses (de = 94–≥98%). After reduction of the nitro group and protection of the amino function (11a–h, 73-87%, both steps), the cleavage of the auxiliary occurred without epimerisation (69-99%) using Na/NH3. The Boc-protected 2-amino alcohols 12a–h could be obtained in good overall
报告了第一个分子间不对称 oxa Michael 加成物,在氢氧化物源中具有可去除的手性信息。由于使用对映纯氧亲核试剂作为手性氢氧化物等价物 N-甲酰基去甲麻黄碱 (7) 并且以良好的产率 (35-87%) 和优异的非对映体过量 (de = 94–≥98%)。在还原硝基和保护氨基官能团(11a-h,73-87%,两个步骤)后,使用 Na/NH3 进行辅助裂解,没有差向异构化(69-99%)。Boc 保护的 2-氨基醇 12a-h 可以以良好的总产率(30-58%,四步)和优异的非对映体和对映体过量(de,ee = 94-≥98%)获得。
A Novel Synthesis of Bicyclic Isoxazolines<i>via</i>Sequential Michael and Intramolecular 1,3-Dipolar Additions
Bicyclic isoxazolines are obtained in good yields by the titanium tetrachloride-mediated reaction of allylic stannanes with 1-nitroalkadienes. Titanium tetrachloride converts stannyl nitronates generated in the Michael addition step to nitrile oxide equivalents which, on adding triethylamine, undergo intramolecular 1,3-dipolar cycloaddition to give the isoxazolines.
diastereoselective inter- and intramolecular 1,3-dipolarcycloaddition reaction of optically active nitrile oxides as a key step. The nitro alkane starting materials were obtained by diastereoselective oxa Michael addition of (1R,2S)-(–)-N-formylnorephedrine (1) to aliphatic (E)-nitro alkenes 2, 6a, b (de = 96– ≥ 98%). Subsequent diastereo- and regioselective cycloaddition reactions to highly substituted 4,5-isoxazolines