Journal of the American Chemical Society, Vol.120, No.33, 8305-8314, 1998
Platinum- and acid-catalyzed enyne metathesis reactions : Mechanistic studies and applications to the syntheses of streptorubin B and metacycloprodigiosin
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" cycloprbpylmethyl-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.
Keywords:RING-CLOSING METATHESIS;LOW-VALENT TITANIUM;PRODIGIOSIN 25-C;CONJUGATE REDUCTION;MURINE SPLENOCYTES;OLEFIN METATHESIS;H+-ATPASE;T-CELLS;DEOXYGENATION;ROUTE