期刊
CHEMISTRY-A EUROPEAN JOURNAL
卷 25, 期 19, 页码 4941-4945出版社
WILEY-V C H VERLAG GMBH
DOI: 10.1002/chem.201900564
关键词
asymmetric catalysis; C-C bond activation; cross metathesis; desymmetrization; enyne metathesis; hydrogenation; iridium catalysis
资金
- University of Cologne
A conceptionally new strategy for the asymmetric (2R-selective) synthesis of alpha-tocopherol (vitamin E) was developed. In the stereocontrolled key step, a prochiral spiro[chromane-2,3 '-cyclobutanol] unit is effectively desymmetrized under C-C bond activation in an unprecedented iridium-catalyzed transformation using (S)-DTBM-SegPhos as a chiral ligand (e.r. 97:3). To complete the synthesis, the side chain was attached through Ru-catalyzed cross-metathesis employing an alkene derived from (R,R)-hexahydrofarnesol. To suppress epimerization during the final hydrogenation, PtO2 had to be used as a catalyst instead of Pd/C. In an alternative approach (employing a propargyl-substituted spiro-cyclobutanol), the side chain was constructed prior to the Ir-catalyzed ring fragmentation (>99:1 d.r.) through enyne cross-metathesis (using an alkene derived from (R)-dihydrocitronellal) followed by Cr-catalyzed 1,4-hydrogenation and (diastereoselective) Pfaltz hydrogenation of the resulting triple-substituted olefin. The work demonstrates the potential of iridium catalysis for enantioselective C-C bond activation.
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