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Platinum- and Acid-Catalyzed Enyne Metathesis Reactions:  Mechanistic Studies and Applications to the Syntheses of Streptorubin B and Metacycloprodigiosin

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Fürstner,  Alois
Research Department Fürstner, Max-Planck-Institut für Kohlenforschung, Max Planck Society;

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Szillat,  Hauke
Research Department Fürstner, Max-Planck-Institut für Kohlenforschung, Max Planck Society;

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Gabor,  Barbara
Service Department Mynott (NMR), Max-Planck-Institut für Kohlenforschung, Max Planck Society;

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Mynott,  Richard
Service Department Mynott (NMR), Max-Planck-Institut für Kohlenforschung, Max Planck Society;

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ja8305.pdf
(Supplementary material), 2MB

ja8305a.pdf
(Supplementary material), 769KB

ja981183g_s.pdf
(Supplementary material), 2MB

ja981183g_sa.pdf
(Supplementary material), 808KB

Citation

Fürstner, A., Szillat, H., Gabor, B., & Mynott, R. (1998). Platinum- and Acid-Catalyzed Enyne Metathesis Reactions:  Mechanistic Studies and Applications to the Syntheses of Streptorubin B and Metacycloprodigiosin. Journal of the American Chemical Society, 120(33), 8305-8314. doi:10.1021/ja981183g.


Cite as: http://hdl.handle.net/11858/00-001M-0000-0024-20E3-3
Abstract
Formal total syntheses of the antibiotics metacycloprodigiosin (2) and streptorubin B (3) are described, which are known to exhibit promising immunomodulating properties. The key step en route to their meta-bridged pyrrole core structures 5 and 7, respectively, consists of a metathesis reaction of electron-deficient enynes catalyzed by either platinum halides, hard Lewis acids, or HBF4. This transformation expands the pre-existing cycloalkene of the substrates by two C atoms, forges the bicyclic pyrrolophane structure of the targets, and simultaneously forms a bridgehead alkene function. The products of this skeletal rearrangement are converted into the targets by a sequence comprising (i) a stepwise reduction of their enone entity to the corresponding saturated alcohols and (ii) an aromatization of the N-tosylated dihydropyrroles 20 and 34 thus obtained via elimination of potassium sulfinate on exposure to KAPA (potassium 3-aminopropylamide). A careful analysis of the minor byproducts formed in the enyne metathesis reactions allows a mechanistic rationale to be proposed for this operationally trivial yet highly attractive transformation which involves “nonclassical” cyclopropylmethyl−homoallyl−cyclobutyl cations as key intermediates. This cationic pathway is distinctly different from mechanistic interpretations of other enyne metathesis reactions previously reported in the literature.