Structure-Activity Relationships of Thiophene Carboxamide Annonaceous Acetogenin Analogs: Shortening the Alkyl Chain in the Tail Part Significantly Affects Their Growth Inhibitory Activity against Human Cancer Cell Lines.

In a previous study, we found that the thiophene carboxamide solamin analog, which is a mono-tetrahydrofuran annonaceous acetogenin, showed potent antitumor activity through the inhibition of mitochondrial complex I. In this study, we synthesized analogs with short alkyl chains instead of the n-dodecyl group in the tail part. We evaluated their growth inhibitory activities against human cancer cell lines. We found that the alkyl chain in the tail part plays an essential role in their activity.

Muricatacin, a Gateway Molecule to Higher Acetogenin Synthesis.

In this review, we have abstracted the various syntheses of acetogenins where the start point has been muricatacin. The latter can best be synthesized in either enantiomer form by the Sharpless method in three steps and can be admired as a gateway molecule for a quick assembly of many higher acetogenins. Muricatacin with orthogonally differentiated hydroxy groups and the available lactone carbonyl for chain elongation/modification can enable the concise synthesis of higher acetogenins. Many closely related synthetic intermediates possible from muricatacin are also abstracted here. The review should give impetus for future synthetic endeavours where the start point could be muricatacin and at least to the new molecules that are yet to be synthesized.

Synthesis, Characterization, and Reactivity of Complex Tricyclic Oxonium Ions, Proposed Intermediates in Natural Product Biosynthesis.

Reactive intermediates frequently play significant roles in the biosynthesis of numerous classes of natural products although the direct observation of these biosynthetically relevant species is rare. We present here direct evidence for the existence of complex, thermally unstable, tricyclic oxonium ions that have been postulated as key reactive intermediates in the biosynthesis of numerous halogenated natural products from Laurencia species. Evidence for their existence comes from full characterization of these oxonium ions by low-temperature NMR spectroscopy supported by density functional theory (DFT) calculations, coupled with the direct generation of 10 natural products on exposure of the oxonium ions to various nucleophiles.

Total Synthesis of Two Possible Diastereomers of Natural 6-Chlorotetrahydrofuran Acetogenin and Its Stereostructural Elucidation.

The first total synthesis of two possible diastereomers of natural 6-chlorotetrahydrofuran acetogenin 1 has been achieved. The synthetic route features 5-exo-tet cyclization, Z selective Wittig reaction and Julia olefination for the construction of conjugated diene and enyne moieties, and stereoselective chlorination. Comparison of their 1 H and 13 C NMR data and specific rotation with those of the natural product elucidated the absolute configuration of natural (-)-6-chlorotetrahydrofuran acetogenin 1.

Simplifying nature: Towards the design of broad spectrum kinetoplastid inhibitors, inspired by acetogenins.

The need for new treatments for the neglected tropical diseases African sleeping sickness, Chagas disease and Leishmaniasis remains urgent with the diseases widespread in tropical regions, affecting the world's very poorest. We have previously reported bis-tetrahydropyran 1,4-triazole analogues designed as mimics of the annonaceous acetogenin natural product chamuvarinin, which maintained trypanocidal activity. Building upon these studies, we here report related triazole compounds with pendant heterocycles, mimicking the original butenolide of the natural product. Analogues were active against T. brucei, with a nitrofuran compound displaying nanomolar trypanocidal activity. Several analogues also showed strong activity against T. cruzi and L. major. Importantly, select compounds gave excellent selectivity over mammalian cells with a furan-based analogue highly selective while remaining active against all three cell lines, thus representing a potential lead for a new broad spectrum kinetoplastid inhibitor.

Total Synthesis of (+)-Goniodenin.

Goniodenin is a lipophilic polyketide originating from plant sources and which possesses a potent cytotoxic activity against cancer cell lines. The first total synthesis of (+)-goniodenin has been achieved in 23 steps from (R)-glycidol. The synthetic sequence featured a cross metathesis for the formation of the C8-C9 bond and installation of the terminal γ-butenolactone ring unit by the alkylation of α-phenylthio-γ-butyrolactone with the corresponding C3-O-triflate. The stereogenic center at C18 carbon was created by Hiyama-Fujita reduction of the corresponding ketone with high diastereoselectivity.

Total Synthesis of the Proposed Structure for Aromin and Its Structural Revision.

This paper describes the first total synthesis of the proposed structure for aromin, an annonaceous acetogenin possessing an unusual bis-THF ring system, and its 4S,7R-isomer. The key steps involve an oxidative cyclization of a couple of terminal-diene alcohols and an intermolecular metathesis of an alkenyl tetrahydrofuran with an enone carrying a tetrahydrofuranyl lactone. The spectral data of both samples did not match those of aromin. Re-examination of the NMR data using the CAST/CNMR Structure Elucidator and chemical derivations suggested that the real structure of aromin should be revised to be a tetrahydropyran acetogenin, montanacin D. Cytotoxicities in human solid tumor cell lines for synthetic samples were also evaluated.

De novo protecting-group-free total synthesis of (+)-muricadienin, (+)-ancepsenolide and (+)-3-hexadecyl-5-methylfuran-2(5H)-one.

A de novo protecting-group-free total synthesis of (+)-muricadienin, (+)-ancepsenolide and (+)-3-hexadecyl-5-methylfuran-2(5H)-one has been achieved. Ring-closing-metathesis has been the key step in the synthesis. In (+)-muricadienin synthesis, a long chain alkyl group has been installed by an sp-sp3 Sonogashira type reaction followed by a cis-selective Lindlar reduction. The total synthesis is achieved in 7 steps and in excellent 43.5% overall yield. Similarly, (+)-ancepsenolide and (+)-3-hexadecyl-5-methylfuran-2(5H)-one synthesis is completed in 5 steps each and in 48 and 68% overall yields, respectively.

Short Route to the Total Synthesis of Natural Muricadienin and Investigation of Its Cytotoxic Properties.

An original synthesis of the acetogenin muricadienin, the bioprecursor of solamin, has been developed. The key step in the five-step 41% overall yield synthesis is the catalytic cross-cyclomagnesiation reaction of functionally substituted 1,2-dienes with EtMgBr in the presence of Cp2TiCl2 and magnesium metal. It has been demonstrated for the first time that muricadienin exhibits a moderate in vitro inhibitory activity against topoisomerases I and IIα, key cell cycle enzymes. Using flow cytometry, muricadienin was shown to have high cytotoxicity toward the HEK293 kidney cancer cells (IC50 0.39 µM).

Development of Simplified Heterocyclic Acetogenin Analogues as Potent and Selective Trypanosoma brucei Inhibitors.

Neglected tropical diseases caused by parasitic infections are an ongoing and increasing concern. They are a burden to human and animal health, having the most devastating effect on the world's poorest countries. Building upon our previously reported triazole analogues, in this study we describe the synthesis and biological testing of other novel heterocyclic acetogenin-inspired derivatives, namely 3,5-isoxazoles, furoxans, and furazans. Several of these compounds maintain low-micromolar levels of inhibition against Trypanosoma brucei, whilst having no observable inhibitory effect on mammalian cells, leading to the possibility of novel lead compounds for selective treatment.

Synthesis of dansyl-labeled probe of thiophene analogue of annonaceous acetogenins for visualization of cell distribution and growth inhibitory activity toward human cancer cell lines.

The convergent synthesis of the dansyl-labeled probe of the thiophene-3-carboxamide analogue of annonaceous acetogenins, which shows potent antitumor activity, was accomplished by two asymmetric alkynylations of the 2,5-diformyl THF equivalent with an alkyne having a thiophene moiety and another alkyne tagged with a dansyl group. The growth inhibitory profiles toward 39 human cancer cell lines revealed that the probe retained the biological function of its mother compound, and would be useful for studying cellular activity.

Total synthesis of muricadienin, the putative key precursor in the solamin biosynthesis.

The first total synthesis of muricadienin, the unsaturated putative precursor in the biosynthesis of trans- and cis-solamin is described. Key steps in the synthesis are a chemoselective hydroboration, a Z-selective Wittig reaction, and a Fries rearrangement for introducing the terminal α-substituted butenolide. Thus, muricadienin can be synthesized in 11 steps from commercially available starting materials in 42% overall yield.

Thiophene-3-carboxamide analogue of annonaceous acetogenins as antitumor drug lead.

Five novel acetogenin analogues with a furan, thiophene, or thiazole ring were synthesized, and their inhibitory activities toward human cancer cell lines were evaluated. The analogues showed more potent activities than natural acetogenin. One of them, the thiophene-3-carboxamide analogue, strongly inhibited the growth of human lung cancer cell line NCI-H23 in the xenograft mouse assay without critical toxicity.

Non-natural acetogenin analogues as potent Trypanosoma brucei inhibitors.

Neglected tropical diseases remain a serious global health concern. Here, a series of novel bis-tetrahydropyran 1,4-triazole analogues based on the framework of chamuvarinin, a polyketide natural product isolated from the annonaceae plant species are detailed. The analogues synthesized display low micromolar trypanocidal activities towards both bloodstream and insect forms of Trypanosoma brucei, the causative agent of African sleeping sickness, also known as Human African Trypanosomiasis (HAT). A divergent synthetic strategy was adopted for the synthesis of the key tetrahydropyran intermediates to enable rapid access to diastereochemical variation either side of the 1,4-triazole core. The resulting diastereomeric analogues displayed varying degrees of trypanocidal activity and selectivity in structure-activity relationship studies. Together, the biological potency and calculated lipophilicity values indicate that while there is room for improvement, these derivatives may represent a promising novel class of anti-HAT agents.

Identification of novel bivalent mimetics of annonaceous acetogenins via a scaffold-hopping strategy.

A series of novel bivalent mimetics of annonaceous acetogenins have been designed, synthesized, and evaluated. Among these, compound 7 bearing a homopiperazine ring in the middle region exhibited more potent growth inhibitory activity and higher selectivity against cancer cells over normal cells by comparison with AA005. This work indicates that modification of the middle piperazine ring is a useful optimizing tool for the simplified acetogenin mimetics.

Synthesis and tumor cell growth inhibitory activity of biotinylated annonaceous acetogenins.

Nineteen biotinylated squamocin/bullatacin derivatives have been synthesized for targeted delivery to biotin receptor overexpressed tumor cells. Most biotinylated squamocin and bullatacin derivatives show similar in vitro cytotoxicity against the biotin receptor non-overexpressed L1210 cells as squamocin and bullatacin, respectively, while against biotin receptor overexpressed 4T1 and P815 tumor cells, several derivatives show significantly higher potency and better selectivity. Among all the synthesized compounds, 15,28-di-O-(6-biotinylamidohexanoyl)squamocin (16) is the most potent, which is 10 and 26 times more active than squamocin against 4T1 and P815 cells, respectively. Compound 16 also appears to be six and fifteen times more selective than squamocin towards 4T1 and P815 cells, respectively, against L1210 cells. The structure activity relationship analysis has revealed that the preferred site for biotinylation is different for squamocin and bullatacin, and it also depends on whether a linking spacer is present.

Synthesis and anti-tumor activity of carbohydrate analogues of the tetrahydrofuran containing acetogenins.

The tetrahydrofuran (THF) containing annonaceous acetogenins (AAs) are attractive candidates for drug development because of their potent cytotoxicity against a wide range of tumors and their relatively simple and robust structures. Replacement of the THF segment with a sugar residue may deliver analogues with improved tumor selectivity and pharmacokinetics and are therefore attractive for drug development. As a first test to the feasibility of such structures, a set of such monosaccharide analogues was synthesized and assayed against four human tumor cell lines, cervical (HeLa), breast (MDA-MB231), T-cell leukemia (Jurkat) and prostate (PC-3). Certain analogues showed low micromolar activity that was comparable to a structurally similar, naturally occurring mono-THF acetogenin. A preliminary examination of the structure-activity profile of these carbohydrate analogues suggests that they have a similar mechanism of action as their THF congeners.

Palladium-catalyzed medium-ring formation for construction of the core structure of laurencia oxacycles: synthetic study of laurendecumallene B.

Palladium-catalyzed medium-ring formation from a cyclic propargyl carbonate via a ring-opening and -closing cascade proceeded at the central carbon atom of the propargyl unit to provide a tetrahydro-2H-oxocine derivative bearing the core structure of laurencia oxacycle. The synthetic application of this reaction to a possible laurendecumallene B precursor is also presented.

Total synthesis, stereochemical assignment, and biological activity of chamuvarinin and structural analogues.

A highly stereocontrolled synthesis of (+)-chamuvarinin has been completed in 1.5% overall yield over 20 steps. The key fragment coupling reactions were the addition of alkyne 8 to aldehyde 7 (under Felkin-Anh control), followed by the two step activation/cyclization to close the C20-C23 2,5-cis-substituted tetrahydrofuran ring and a Julia-Kocienski olefination at C8-C9 to introduce the terminal butenolide. The inherent flexibility of our coupling strategy led to a streamlined synthesis with 17 steps in the longest sequence (2.2% overall yield), in which the key bond couplings are reversed. In addition, a series of structural analogues of chamuvarinin have been prepared and screened for activity against HeLa cancer cell lines and both the bloodstream and insect forms of Trypanosoma brucei, the parasitic agent responsible for African sleeping sickness.

Chemoenzymatic synthesis of trans-tetrahydrofuran cores of annonaceous acetogenins from bromobenzene.

Two types of trans-THF cores, present in acetogenins, have been synthesized by an intramolecular iodoetherification reaction. The starting alkenol was obtained in a few steps from a chiral cis-diol resulting from microbial oxidation of bromobenzene. The cyclization gave complete stereoselectivity for trans-THF cores with either (S,S) or (R,R) configurations at the THF chiral carbons.