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- Title
Totalsynthese von natülichem α-Tocopherol. 5. Mitteilung. Asymmetrische Alkylierung und asymmetrische Epoxidierung als Methoden zur Einführung der ( R)-Konfiguration an C(2) des Chroman-Systems.
- Authors
Hübscher, Josef; Barrier, Richard
- Abstract
Total Synthesis of Naturally Occurring α-Tocopherol. Asymmetric Alkylation and Asymmetric Epoxidation as Means to Introduce ( R)-Configuration at C(2) of the Chroman Moiety Based on the reductive, stereospecific ring closure of (2 R,4′ R,8′ R)-α-Tocophcrylquinone′ or corresponding analogues with a short, functionalized side chain ( B, Scheme 1) to 1 resp. the chroman system of 1 (C), two different approaches for the introduction of the required tertiary methyl-substituted alcohol structure in the side chain of the aromatic precursors ( A, Scheme 1) were developed. The first approach uses asymmetric alkylation in three different versions featuring (a) diastereoselective steering with chiral auxiliaries I-IV ( Scheme 2) attached as esters to a-keto acids, (b) intermediate transfer of chirality in an ester enolate (from 18, Scheme 4) derived from an optically active α-hydroxy acid, ( c) enantioselective alkylation of phytenal ( 20) and subsequent ring closure with chirality transfer ( Schemes 5-7). The second approach is based on the asymmetric epoxidation of β-metallylalcohol ( Sharpless epoxidation), the corresponding epoxyalcohol being converted in situ to the ( S)-or ( R)-chlorodiol ( S)-and ( R)- 29, respectively, for isolation ( Schemes 8 and 9). Nucleophilic epoxide opening with a (3 R 7 R)-3,7,11-trimethyldodecyl (C15**) and an ArCH2 unit in appropriate sequence is used to assemble the C-framework of the target molecule via corresponding epoxide intermediates from either chlorodiol. Combined with the use of the methoxymethyl-ether function for protection of the hydroquinone system, the epoxide approach provides a short route to 1 ( Scheme 10).
- Publication
Helvetica Chimica Acta, 1990, Vol 73, Issue 4, p1068
- ISSN
0018-019X
- Publication type
Article
- DOI
10.1002/hlca.19900730431