aspidospermidine and goniomitine

Aspidospermidine, vincadifformine, 1,2-dehydroaspidospermidine, goniomitine, and quebrachamine, five Aspidosperma alkaloids distributed within three structurally diverse topologies, were synthesized from a single molecular scaffold, namely indole-valerolactam 6. This common intermediate can be divergently manipulated, through the incorporation of conformational and electronic constraints that influence the chemo-selectivity of the iminium ion derived therefrom, between three different reaction paths: N(1) vs. C(3) cyclization (indole numbering) vs. over-reduction. Moreover, a catalytic carbene insertion for direct C(3)-H indole functionalization is reported for the first time in an approach to goniomitine (4), and a following tandem ester reduction/iminium generation/cyclization secured its tetracyclic system. The development of a highly diastereoselective one-pot hemi-reduction/cyclization/deprotection process to obtain a cis-pyridocarbazole directly allowed the synthesis of pentacyclic Aspidosperma alkaloids 1, 2, and 3.

Topics: Alkaloids; Aspidosperma; Chemistry Techniques, Synthetic; Cyclization; Indole Alkaloids; Quinolines; Stereoisomerism

The successful application of dihydropyrido[1,2-a]indolone (DHPI) substrates in Pd-catalyzed asymmetric allylic alkylation chemistry facilitates rapid access to multiple alkaloid frameworks in an enantioselective fashion. Strategic bromination at the indole C3 position greatly improved the allylic alkylation chemistry and enabled a highly efficient Negishi cross-coupling downstream. The first catalytic enantioselective total synthesis of (-)-goniomitine, along with divergent formal syntheses of (+)-aspidospermidine and (-)-quebrachamine, are reported herein.

Topics: Alkylation; Allyl Compounds; Catalysis; Indole Alkaloids; Indoles; Molecular Structure; Palladium; Pyridines; Quinolines; Stereoisomerism

Total syntheses of (±)-goniomitine, (±)-1,2-dehydroaspidospermidine, (±)-aspidospermidine, (±)-vincadifformine, and (±)-kopsihainanine A were achieved featuring two common key steps: (1) a palladium-catalyzed decarboxylative vinylation that provides quick access to cyclopentene intermediates containing all of the carbons present in the natural products and (2) an integrated oxidation/reduction/cyclization (iORC) sequence for skeletal reorganization that converts the cyclopentenes to the pentacyclic structures of the natural products. By incorporation of a geometric constraint to iORC substrates, both the chemoselectivity (C7 vs N1 cyclization) and the stereoselectivity (trans- vs cis-fused ring system) of the cyclization process can be controlled.

Topics: Alkaloids; Biological Products; Chemistry Techniques, Synthetic; Heterocyclic Compounds, 4 or More Rings; Indole Alkaloids; Monoterpenes; Quinolines

A palladium-catalyzed direct 2-alkylation reaction of free N-H indoles was developed based on a norbornene-mediated regioselective cascade C-H activation. The detailed reaction mechanism was investigated by NMR spectroscopic analyses, characterization of the key intermediate, deuterium labeling experiments, and kinetic studies. The results indicate that a catalytic cycle operates, in which an N-norbornene type palladacycle is formed as the key intermediate. Oxidative addition of alkyl bromide to the Pd(II) center in this intermediate is the rate-determining step of the reaction. The synthetic utility of this indole 2-alkylation method was demonstrated by its application in natural product total synthesis. A new and general strategy to synthesize Aspidosperma alkaloids was established employing the indole 2-alkylation reaction as the key step, and two structurally different Aspidosperma alkaloids, aspidospermidine and goniomitine, were synthesized in concise routes.

Topics: Alkylation; Carbon; Catalysis; Hydrogen; Indole Alkaloids; Indoles; Kinetics; Norbornanes; Palladium; Quinolines; Stereoisomerism; Substrate Specificity

Topics: Antineoplastic Agents; Catalysis; Cell Line, Tumor; Cyclization; Cyclopropanes; Drug Screening Assays, Antitumor; Humans; Indole Alkaloids; Quinolines; Stereoisomerism