During the course of a mechanism-based screening program designed to identify new microtubule-disrupting agents from natural products, we identified a crude extract from Tacca chantrieri that initiated Taxol-like microtubule bundling. Bioassay-directed purification of the extract yielded the highly oxygenated steroids taccalonolides E and A. The taccalonolides caused an increased density of cellular microtubules in interphase cells and the formation of thick bundles of microtubules similar to the effects of Taxol. Mitotic cells exhibited abnormal mitotic spindles containing three or more spindle poles. The taccalonolides were evaluated for antiproliferative effects in drug-sensitive and multidrug-resistant cell lines. The data indicate that taccalonolide E is slightly more potent than taccalonolide A in drug-sensitive cell lines and that both taccalonolides are effective inhibitors of cell proliferation. Both taccalonolides are poorer substrates for transport by P-glycoprotein than Taxol. The ability of the taccalonolides to circumvent mutations in the Taxol-binding region of beta-tubulin was examined using the PTX 10, PTX 22, and 1A9/A8 cell lines. The data suggest little cross-resistance of taccalonolide A as compared with Taxol, however, the data from the PTX 22 cell line indicate a 12-fold resistance to taccalonolide E, suggesting a potential overlap of binding sites. Characteristic of agents that disrupt microtubules, the taccalonolides caused G(2)-M accumulation, Bcl-2 phosphorylation, and initiation of apoptosis. The taccalonolides represent a novel class of plant-derived microtubule-stabilizers that differ structurally and biologically from other classes of microtubule-stabilizers.

https://cancerres.aacrjournals.org/content/canres/63/12/3211.full.pdf

Taccalonolides E and A: Plant-derived steroids with microtubule-stabilizing activity.

The monoterpene indole alkaloids represent the largest family of alkaloid natural products whose more than 2,000 members display a broad range of chemical diversity and potent biological activity.1 The structural challenges presented by this family have long been a source of interest, resulting in the development of a variety of inventive synthetic strategies to access various family members. The biogenetically related natural alkaloids N-methylaspidospermidine (1), N-methylquebrachamine (2), and tabernaebovine (3) represent the aspidosperma subfamily of monoterpene indole alkaloids (Figure 1).2-5 The dimeric alkaloid 3, isolated from Tabernaemontana bovina in 1998,2e has a fascinating molecular constitution that exhibits a unique C2–C15′ linkage between two pentacyclic aspidosperma skeletons. While elegant strategies for synthesis of other dimeric monoterpene indole alkaloids have been reported,5 no synthetic solution to the distinctive C2–C15′ union present in 3 exists. As an outgrowth of our laboratory's studies concerning electrophilic amide activation,6 we report a concise and convergent strategy for the enantioselective synthesis of alkaloids (–)-1, (+)-2, and dimeric (+)-dideepoxytabernaebovine (4).

A Concise and Versatile Double‐Cyclization Strategy for the Highly Stereoselective Synthesis and Arylative Dimerization of Aspidosperma Alkaloids

We report an efficient and highly stereoselective synthetic strategy for the synthesis of aspidosperma alkaloids based on the transannular cyclization of a chiral lactam precursor. Three new stereocenters are formed in this key step with excellent level of diastereoselection due to the conformational bias of the cyclization precursor, leading to a versatile pentacyclic intermediate. A subsequent stereoselective epoxidation followed by a mild formamide reduction enabled the first total synthesis of the aspidosperma alkaloids (−)-mehranine and (+)-(6S,7S)-dihydroxy-N-methylaspidospermidine. A late-stage scandium trifluoromethanesulfonate-mediated dimerization of (−)-mehranine enabled the first total synthesis of (−)-methylenebismehranine.

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Concise and enantioselective total synthesis of (-)-mehranine, (-)-methylenebismehranine, and related Aspidosperma alkaloids.

The first syntheses of the alkaloids (−)-mehranine (3), (+)-voaphylline/conoflorine (4), (+)-Na-methylvoaphylline/hecubine (5), and (−)-lochnericine (2) were achieved by stereoselective epoxidation starting from (−)-tabersonine (1), through intermediates with the aspidospermane and quebrachamine skeleton.

Synthesis of vinca alkaloids and related compounds. 100. Stereoselective oxidation reactions of compounds with the aspidospermane and quebrachamine ring system. First synthesis of some alkaloids containing the epoxy ring.

This first part of a planned review on developments in the field of total and formal total synthesis of the post-secodine indole alkaloids concentrates on primary alkaloid types. It reviews the synthesis of secodine, aspidospermane, pseudoaspidospermane and ibogane alkaloids; andranginine is also included. It covers the literature from 1992-1993 up to approximately May 2004. A review with 179 references.

A Review on Recent Developments in Syntheses of the Post-Secodine Indole Alkaloids. Part I: The Primary Alkaloid Types

The structure including the absolute configuration of the indole alkaloid (-)-mehranine hydrobromide (1·HBr) has been assigned by X-ray analysis. This result gave the possibility to assign the absolute configurations of some related alkaloids

Absolute Configuration of the Indole Alkaloid (‐)‐Mehranine Hydrobromide

The invention relates to natural compounds of formula (I) some of which are to processes for obtaining and preparing them, to derivatives of these natural compounds, and to their use for controlling animal pests in which R?6 and R7? together represent the group -OCOC(OH)(CH?3?)-, where the oxygen is linked to the carbon atom carrying the R?6? radical, or R?7 and R8? together represent the group -C(OH)(CH?3?)COO-, where the carbon atom is linked to the carbon atom carrying the R?7? radical.

Van Sung Tran

Papers

Absolute Configuration of the Indole Alkaloid (‐)‐Mehranine Hydrobromide

Taccalonolides and their use