Stereocontrolled Total Synthesis of Amphidinolide X via a Silicon-Tethered Metathesis Reaction

Actin-Interacting Amphidinolides: Syntheses and Mechanisms of Action of Amphidinolides X, J, and K

Amphidinolides are a family of more than forty macrolides of varying sizes and complex structures isolated from dinoflagellates of the genus Amphidinium. Although all of them display potent-to-moderate cytotoxicity, their full bioactivity profile and mode of action have not been fully investigated. Access to enough material is needed for these studies, but samples of these compounds are limited due to the minute amounts that can only be obtained by either large-scale cultivation of the organism that produces them or by total synthesis. Of all the amphidinolides known to date, only the targets of five of them (B1, H1, J, K, and X) have been examined and all have been found to interact with actin, a crucial cytoskeletal protein. This paper reviews what is currently known about actin-interacting amphidinolides, with a focus on the research of our group. Amphidinolides J and X are F-actin destabilizers, whereas Amphidinolides H1 and K stabilize actin filaments, likely via different mechanisms. More precise details of the interaction between amphidinolides and actin are missing.

Diastereoselective synthesis of trisubstituted olefins using a silicon-tether ring-closing metathesis strategy

The diastereoselective synthesis of trisubstituted olefins with concomitant C-C bond formation is still a difficult challenge, and olefin metathesis reactions for the formation of such alkenes are usually not high yielding or/and diastereoselective. Herein we report an efficient and diastereoselective synthesis of trisubstituted olefins flanked by an allylic alcohol, by a silicon-tether ring-closing metathesis strategy. Both E- and Z-trisubstituted alkenes were synthesised, depending on the method employed to cleave the silicon tether. Furthermore, this methodology features a novel Peterson olefination for the synthesis of allyldimethylsilanes. These versatile intermediates were also converted into the corresponding allylchlorodimethylsilanes, which are not easily accessible in high yields by other methods.

Further Insight into the Interactions of the Cytotoxic Macrolides Laulimalide and Peloruside A with Their Common Binding Site

The binding site of the macrolides laulimalide and peloruside A, which is different from that of the clinically useful drugs paclitaxel/taxol and ixabepilone (tax site), is known to be between two adjacent β-tubulin units (ext site). Here, we report our study of the binding of these molecules to an α1β1/α2β2-tubulin "tetramer" model. AutoDock 4.2.6//AutoDock Vina dockings predicted that the affinities of laulimalide and peloruside A for the tax site are quite similar to those for the ext site. However, molecular dynamics (MD) simulations indicated that only when these two ligands are located at the ext site, there are contacts that help stabilize the system, favoring the β1/β2 interactions. The binding affinity of laulimalide for this site is stronger than that of peloruside A, but this is compensated for by additional β1/β2 contacts that are induced by peloruside A. MD studies also suggested that epothilones at the tax site and either laulimalide or peloruside A at the ext site cause similar stabilizing effects (mainly linking the M-loop of β1 and loop H1-B2 of β2). In a "hexamer" model (3 units of αβ-tubulin), the effects are confirmed. Metadynamics simulations of laulimalide and peloruside A, which are reported for the first time, suggest that peloruside A produces a stronger change in the M-loop, which explains the stabilization of the β1/β2 interaction.

The Performance of Several Docking Programs at Reproducing Protein-Macrolide-Like Crystal Structures

The accuracy of five docking programs at reproducing crystallographic structures of complexes of 8 macrolides and 12 related macrocyclic structures, all with their corresponding receptors, was evaluated. Self-docking calculations indicated excellent performance in all cases (mean RMSD values ≤ 1.0) and confirmed the speed of AutoDock Vina. Afterwards, the lowest-energy conformer of each molecule and all the conformers lying 0–10 kcal/mol above it (as given by Macrocycle, from MacroModel 10.0) were subjected to standard docking calculations. While each docking method has its own merits, the observed speed of the programs was as follows: Glide 6.6 > AutoDock Vina 1.1.2 > DOCK 6.5 >> AutoDock 4.2.6 > AutoDock 3.0.5. For most of the complexes, the five methods predicted quite correct poses of ligands at the binding sites, but the lower RMSD values for the poses of highest affinity were in the order: Glide 6.6 ≈ AutoDock Vina ≈ DOCK 6.5 > AutoDock 4.2.6 >> AutoDock 3.0.5. By choosing the poses closest to the crystal structure the order was: AutoDock Vina > Glide 6.6 ≈ DOCK 6.5 ≥ AutoDock 4.2.6 >> AutoDock 3.0.5. Re-scoring (AutoDock 4.2.6//AutoDock Vina, Amber Score and MM-GBSA) improved the agreement between the calculated and experimental data. For all intents and purposes, these three methods are equally reliable.

Diastereoselective addition of monoorganocuprates to a chiral fumarate: reaction development and synthesis of (-)-dihydroprotolichesterinic acid

Recent studies of diastereoselective conjugate additions of monoorganocuprates, Li[RCuI], to chiral γ-alkoxycrotonates and fumarates are disclosed. This methodology was applied to the shortest total synthesis of (-)-dihydroprotolichesterinic acid to date, but several attempts to prepare other succinate-derived natural products, such as pilocarpine and antrodin E, were unsuccessful.

Electrophilic derivatives antagonise pheromone attraction in Cydia pomonella

BACKGROUND

Pheromone antagonists are good disruptants of the pheromone communication in insects and, as such, have been used in mating disruption experiments. In this study, new non-fluorinated electrophilic keto derivatives structurally related to the pheromone of Cydia pomonella (codlemone) have been synthesised and tested as putative pheromone antagonists.

RESULTS

Codlemone (1) was prepared in excellent stereoselectivity in a new, iterative approach involving two Horner-Wadsworth-Emmons reactions. Methyl ketone (2), keto ester (3) and diketone (4) were obtained from codlemone in straightforward approaches in good overall yields and excellent stereochemical purity (≥98% E,E). In electrophysiology, only compound 2 displayed inhibition of the antennal response to the pheromone after presaturation of the antennal receptors. Compounds 2 to 4 did not inhibit the pheromone-degrading enzyme responsible for codlemone metabolism, but mixtures of ketone 2 and diketone 4 with codlemone elicited erratic flights on males in a wind tunnel. In the field, blends of either compound (2 or 4) with the pheromone caught significantly fewer males than codlemone alone.

CONCLUSION

Codlemone and the potential antagonists 2 to 4 have been synthesised in good yields and excellent stereoselectivity. These chemicals behave as pheromone antagonists of the codling moth both in the laboratory and in the field.

Exploiting hidden symmetry in natural products: total syntheses of amphidinolides C and F

The total synthesis of amphidinolide C and a second-generation synthesis of amphidinolide F have been accomplished through the use of a common intermediate to access both the C1-C8 and the C18-C25 sections. The development of a Ag-catalyzed cyclization of a propargyl benzoate diol is described to access both trans-tetrahydrofuran rings. The evolution of a Felkin-controlled, 2-lithio-1,3-dienyl addition strategy to incorporate C9-C11 diene as well as C8 stereocenter is detailed. Key controlling aspects in the sulfone alkylation/oxidative desulfurization to join the major subunits, including the exploration of the optimum masking group for the C18 carbonyl motif, are discussed. A Trost asymmetric alkynylation and a stereoselective cuprate addition to an alkynoate have been developed for the rapid construction of the C26-C34 subunit. A Tamura/Vedejs olefination to introduce the C26 side arm of amphidnolides C and F is employed. The late-stage incorporation of the C15, C18 diketone motif proved critical to the successful competition of the total syntheses.

Concise synthesis and biological assessment of (+)-neopeltolide and a 16-member stereoisomer library of 8,9-dehydroneopeltolide: identification of pharmacophoric elements

We describe herein a concise synthesis of (+)-neopeltolide, a marine macrolide natural product that elicits a highly potent antiproliferative activity against several human cancer cell lines. Our synthesis exploited the powerful bond-forming ability and high functional group compatibility of olefin metathesis and esterification reactions to minimize manipulations of oxygen functionalities and to maximize synthetic convergency. Our findings include a chemoselective olefin cross-metathesis reaction directed by H-bonding, and a ring-closing metathesis conducted under non-high dilution conditions. Moreover, we developed a 16-member stereoisomer library of 8,9-dehydroneopeltolide to systematically explore the stereostructure-activity relationships. Assessment of the antiproliferative activity of the stereoisomers against A549 human lung adenocarcinoma, MCF-7 human breast adenocarcinoma, HT-1080 human fibrosarcoma, and P388 murine leukemia cell lines has revealed marked differences in potency between the stereoisomers. This study provides comprehensive insights into the structure-activity relationship of this important antiproliferative agent, leading to the identification of the pharmacophoric structural elements and the development of truncated analogues with nanomolar potency.

Amphidinolide B: total synthesis, structural investigation, and biological evaluation

The total syntheses of amphidinolide B1 and the proposed structure of amphidinolide B2 have been accomplished. Key aspects of this work include the development of a practical, non-transition-metal-mediated method for the construction of the C13-C15 diene, the identification of α-chelation and dipole minimization models for diastereoselective methyl ketone aldol reactions, the discovery of a spontaneous Horner-Wadsworth-Emmons macrocyclization strategy, and the development of a novel late stage method for construction of an allylic epoxide moiety. The originally proposed structure for amphidinolide B2 and diastereomers thereof display potent antitumor activities with IC50 values ranging from 3.3 to 94.5 nM against human solid and blood tumor cells. Of the different stereoisomers, the proposed structure of amphidinolide B2 is over 12-fold more potent than the C8,9-epimer and C18-epimer in human DU145 prostate cancer cells. These data suggest that the epoxide stereochemistry is a significant factor for anticancer activity.

Iododesilylation of TIPS-, TBDPS-, and TBS-substituted alkenes in connection with the synthesis of amphidinolides B/D

The C-Si bonds of triisopropylsilyl-substituted alkenes, 1,3-dienes, and related multifunctional substrates, as well as analogous C-TBDPS and C-TBS bonds, are readily and chemoselectively cleaved with NIS (or other sources of I(+), such as N-iodosaccharin, 1,3-diodohydantoin, and Ipy(2)BF(4)). The desired iodoalkenes are obtained stereospecifically without byproducts, provided that the reactions are carried out in CF(3)CHOHCF(3) and, in general, with 30 mol % of Ag(2)CO(3) (or AgOAc/2,6-lutidine) as an additive. Fragment C10-C18 of cytotoxic amphidinolides B1-B3 and D has been synthesized using this improved procedure.

Total synthesis and biological evaluation of amphidinolide V and analogues

The awesome power of metathesis is illustrated by a concise synthesis of the extremely scarce marine natural product amphidinolide V, which hinges on a sequence of ring-closing alkyne metathesis followed by intermolecular enyne metathesis with ethylene (see scheme). As a complete set of conceivable stereoisomers was prepared, the constitution and absolute configuration of this macrolide could be established and first insights into structure-activity relationships governing its cytotoxicity were obtained.A sequence of ring-closing alkyne metathesis followed by an intermolecular enyne metathesis of the resulting cycloalkyne with ethene was used to forge the macrocyclic skeleton and to set the vicinal exo-methylene branches characteristic for the cytotoxic marine natural product amphidinolide V (1). Comparison of the synthetic material with an authentic sample of this extremely scarce metabolite isolated from a dinoflagellate of the Amphidinium sp. eliminated any doubts about its structure and allowed the absolute configuration of amphidinolide V to be determined as 8R,9S,10S,13R. Moreover, the flexibility inherent to the underlying synthesis blueprint also opened access to a comprehensive set of diastereomers of 1 as well as to synthetic analogues differing from the natural lead in the lipophilic chains appended to the macrocyclic core. This set of designed analogues gave first insights into structure-activity relationships, which revealed that the stereostructure of the macrolactone is a highly critical parameter, whereas the examined alterations of the side chain did not diminish the cytotoxicity of the compounds to any notable extent.