In total and formal syntheses of dictyodendrins B, C, E, and F, the key step involved the direct construction of the pyrrolo[2,3-c]carbazole core by the gold-catalyzed annulation of a conjugated diyne with a pyrrole to form three bonds and two aromatic rings. The subsequent introduction of substituents at the C1 (Suzuki-Miyaura coupling), C2 (addition to an aldehyde), N3 (alkylation), and C5 positions (Ullman coupling) provided divergent access to dictyodendrins.
Tetracyanothiophene and tetracyano-1,4-dithiin react with a leaving group substituted carbon nucleophile such as ethyl benzenesulfonylacetate to afford substituted tetracyanocyclopentadienyl sodium derivatives in moderate to high yields through a putative condensation and desulfurization pathway. Subsequent functional-group transformation reactions on the Cp anion ring provided various C5R(CN)4(-) derivatives.
We achieved direct construction of the common pyrrolo [2,3-d]carbazole core of aspidosperma and malagasy alkaloids by a gold-catalyzed cascade cyclization of ynamide. This reaction involves intramolecular cyclization from indole to ynamide followed by trapping of the resulting iminium intermediate. Through the use of chiral gold complexes, an enantiomerically enriched pyrrolo[2,3-d]carbazole was obtained in up to 74% ee. This methodology was successfully applied to the asymmetric formal synthesis of vindorosine.
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