Exploration of cascade cyclizations terminated by tandem aromatic substitution: total synthesis of (+)-schweinfurthin A

Substitution of a triazole for the central olefin in biologically active stilbenes

The stilbene moiety is commonly found in natural products and these compounds display an extraordinary range of biological activity. Efforts to derive useful drugs from stilbenes must address the potential liabilities of this structure, including a propensity for cis/trans isomerization. To identify olefin replacements that address this limitation while preserving biological activity we have prepared analogues of two bioactive stilbenes, a pawhuskin and a schweinfurthin, where a 1,2,3-triazole ring formally replaces the stilbene double bond. The new schweinfurthin analogue (23) has been tested for anti-proliferative activity against 60 cell lines, and shows a strong correlation of bioactivity when compared to the compound that inspired its synthesis (22).

RNAseq reveals extensive metabolic disruptions in the sensitive SF-295 cell line treated with schweinfurthins

The schweinfurthin family of natural compounds exhibit a unique and potent differential cytotoxicity against a number of cancer cell lines and may reduce tumor growth in vivo. In some cell lines, such as SF-295 glioma cells, schweinfurthins elicit cytotoxicity at nanomolar concentrations. However, other cell lines, like A549 lung cancer cells, are resistant to schweinfurthin treatment up to micromolar concentrations. At this time, the precise mechanism of action and target for these compounds is unknown. Here, we employ RNA sequencing of cells treated with 50 nM schweinfurthin analog TTI-3066 for 6 and 24 h to elucidate potential mechanisms and pathways which may contribute to schweinfurthin sensitivity and resistance. The data was analyzed via an interaction model to observe differential behaviors between sensitive SF-295 and resistant A549 cell lines. We show that metabolic and stress-response pathways were differentially regulated in the sensitive SF-295 cell line as compared with the resistant A549 cell line. In contrast, A549 cell had significant alterations in response genes involved in translation and protein metabolism. Overall, there was a significant interaction effect for translational proteins, RNA metabolism, protein metabolism, and metabolic genes. Members of the Hedgehog pathway were differentially regulated in the resistant A549 cell line at both early and late time points, suggesting a potential mechanism of resistance. Indeed, when cotreated with the Smoothened inhibitor cyclopamine, A549 cells became more sensitive to schweinfurthin treatment. This study therefore identifies a key interplay with the Hedgehog pathway that modulates sensitivity to the schweinfurthin class of compounds.

Synthesis of a Coumarin-Based Analogue of Schweinfurthin F

The natural schweinfurthins are stilbenes with significant antiproliferative activity and an uncertain mechanism of action. To obtain a fluorescent analogue with minimal deviation from the natural structure, a coumarin ring system was annulated to the D-ring, creating a new analogue of schweinfurthin F. This stilbene was prepared through a convergent synthesis, with a Horner-Wadsworth-Emmons condensation employed to form the central stilbene olefin. After preparation of a tricyclic phosphonate via a recent and more efficient modification of the classic Arbuzov reaction, condensation was attempted with an appropriately substituted bicyclic aldehyde but the coumarin system did not survive the reaction conditions. When olefin formation preceded generation of the coumarin, the stilbene formation proceeded smoothly and ultimately allowed access to the targeted coumarin-based schweinfurthin analogue. This analogue displayed the desired fluorescence properties along with significant biological activity in the National Cancer Institute's 60-cell line bioassay, and the pattern of this biological activity mirrored that of the natural product schweinfurthin F. This approach gives facile access to new fluorescent analogues of the natural schweinfurthins and should be applicable to other natural stilbenes as well.

Pivotal Reactions in the Creation of the Polycyclic Skeleton of Cryptotrione

Three pivotal reactions, namely, enyne cycloisomerization, polyene cyclization, and quinone methide formation, are applied to synthesize the complex polycyclic skeleton of cryptotrione. This review summarizes the most prominent applications of these three reactions to the total syntheses of natural products, covering results published in the literature between 2011 and 2020.

1 Introduction

2 Three Pivotal Reactions Applied to Create the Polycyclic Framework of Cryptotrione

2.1 Enyne Cycloisomerization

2.2 Polyene Cyclization

2.3 Quinone Methide Formation

3 Conclusion

A semi-rigid isoindoline-derived nitroxide spin label for RNA

A new isoindoline-derived benzimidazole nitroxide spin label, ImUm, was synthesized and incorporated into RNA oligoribonucleotides. ImUm is the first example of a conformationally unambiguous spin label for RNA, in which the nitroxide N-O bond lies on the same axis as the single bond used to attach the rigid isoindoline-based spin label to a uridine base. This results in minimal displacement of the nitroxide upon rotation of this single bond, which is a useful property for a label to be used for distance measurements. Continuous-wave (CW) EPR measurements of RNA duplexes containing ImUm indicate a restricted rotation around this single bond, presumably due to an intramolecular hydrogen bond between the benzimidazole N-H and O4 of the uracil. Orientation-selective pulsed electron-electron double resonance (PELDOR, also called double electron-electron resonance, or DEER) distance measurements between two spin labels in two RNA duplexes showed in one case a strong orientation dependence, further confirming the restricted motion of the spin labels in RNA duplexes.

Total Synthesis of Antiproliferative Parvifloron F

The first total synthesis of parvifloron F, a bioactive highly oxidized abietane diterpene, was achieved. The abietane skeleton was constructed by Lewis acid promoted cyclization. Preliminary structure-activity relationship correlations were established for the synthetic intermediates against human tumor cell lines. Certain compounds showed unique selective antiproliferative activity against triple-negative breast cancer. The oxidation level of the abietane ring affected the selectivity.

Schweinfurthins A–Q: isolation, synthesis, and biochemical properties

Stilbene analogues have shown remarkable structural diversity constituting simple or tangled structures, which have attracted the synthetic as well as the medicinal chemistry communities. Schweinfurthins are a family of prenylated/geranylated/farnesylated stilbenes that are isolated from an African plant belonging to the Macaranga species. These compounds have displayed potency towards central nervous system, renal and breast cancer cell lines. Specifically, these compounds have been found to be potent and selective inhibitors of cell growth in the National Cancer Institute's 60 cell-line screen. In this review article, we described the isolation, synthesis, and biochemical properties of schweinfurthins.

An overview of the isolation, synthesis, and biochemical properties of the stilbene-based natural products schweinfurthins A–Q (1999–2017).

Synthesis of amide isosteres of schweinfurthin-based stilbenes

The schweinfurthins are plant-derived stilbenes with an intriguing profile of anti-cancer activity. To obtain analogues of the schweinfurthins that might preserve the biological activity but have greater water solubility, a formal replacement of the central olefin with an amide has been explored. Two pairs of amides have been prepared, each containing the same hexahydroxanthene "left half" joined through an amide linkage to two different "right halves." In each series, the amide has been inserted in both possible orientations, placing the carbonyl group on the tricyclic ABC ring system and the amine on the D-ring, or placing the amine on the hexahydroxanthene and the carbonyl group on the D-ring. The four new schweinfurthin analogues have been tested in the NCI 60 cell line screen, and in both cases the more active isomer carried the carbonyl group on the C-ring.

Chemoselective and Sequential Palladium-Catalyzed Couplings for the Generation of Stilbene Libraries via Immobilized Substrates

A versatile palladium-catalyzed tandem synthetic sequence to afford E-stilbenes libraries has been developed. Excellent regio- and stereocontrol have been achieved by means of the sequence of Hiyama and Heck cross-couplings. Undesirable homocoupling byproducts were avoided employing immobilized substrates.

A selective delta opioid receptor antagonist based on a stilbene core

Studies of directed ortho metalation reactions on an aromatic substrate with multiple potential directing groups have identified conditions that favor either of two regioisomers. One of these regioisomers has been converted to an analogue of the stilbene pawhuskin A, and been shown to have high selectivity as an antagonist of the delta opioid receptor. Docking studies have suggested that this compound can adopt a conformation similar to naltrindole, a known delta antagonist.

3-Deoxyschweinfurthin B Lowers Cholesterol Levels by Decreasing Synthesis and Increasing Export in Cultured Cancer Cell Lines

The schweinfurthins have potent antiproliferative activity in multiple glioblastoma multiforme (GBM) cell lines; however, the mechanism by which growth is impeded is not fully understood. Previously, we demonstrated that the schweinfurthins reduce the level of key isoprenoid intermediates in the cholesterol biosynthetic pathway. Herein, we describe the effects of the schweinfurthins on cholesterol homeostasis. Intracellular cholesterol levels are greatly reduced in cells incubated with 3-deoxyschweinfurthin B (3dSB), an analog of the natural product schweinfurthin B. Decreased cholesterol levels are due to decreased cholesterol synthesis and increased cholesterol efflux; both of these cellular actions can be influenced by liver X-receptor (LXR) activation. The effects of 3dSB on ATP-binding cassette transporter 1 levels and other LXR targets are similar to that of 25-hydroxycholesterol, an LXR agonist. Unlike 25-hydroxycholesterol, 3dSB does not act as a direct agonist for LXR α or β. These data suggest that cholesterol homeostasis plays a significant role in the growth inhibitory activity of the schweinfurthins and may elucidate a mechanism that can be targeted in human cancers such as GBM.

Synthesis and structure activity relationships of schweinfurthin indoles

As part of a program to explore the biological activity of analogues of the natural schweinfurthins, a set of compounds has been prepared where an indole system can be viewed as a substitution for the resorcinol substructure of the schweinfurthin's D-ring. Twelve of these schweinfurthin indoles have been prepared and evaluated in the 60 cell line screen of the National Cancer Institute. While a range of activity has been observed, it is now clear that schweinfurthin indoles can demonstrate the intriguing pattern of activity associated with the natural stilbenes. In the best cases, these indole analogues display both potency and differential activity across the various cell lines comparable to the best resorcinol analogues.

Stilbenes as κ-selective, non-nitrogenous opioid receptor antagonists

The natural stilbene pawhuskin A has been shown to function as an opioid receptor antagonist, with preferential binding to the κ receptor. This finding encouraged assembly of a set of analogues to probe the importance of key structural features. Assays on these compounds determined that one (compound 29) shows potent opioid receptor binding activity and significantly improved selectivity for the κ receptor. These studies begin to illuminate the structural features of these non-nitrogenous opioid receptor antagonists that are required for activity.

Electrophilic aromatic prenylation via cascade cyclization

To gain access to prenylated hexahydroxanthenes, tandem cascade cyclization-electrophilic aromatic substitution reactions have been studied on substrates bearing allylic and propargylic substituents. Both BF₃·OEt₂ and TMSOTf can be used to initiate this reaction sequence, resulting in different ratios of the C-2 and C-6 substitution products. Even though allylic transposition is observed in some cases, the results of a crossover experiment are consistent with an intramolecular reaction sequence. Taken together, these studies now allow preparation of either the C-2 or C-6 prenylated hexahydroxanthene products.

Synthesis of indole analogues of the natural schweinfurthins

An interest in the schweinfurthins, natural stilbenes with significant antiproliferative activity, has prompted efforts to prepare a set of indole analogues. To approach the desired compounds through a Horner-Wadsworth-Emmons condensation, new indole derivatives bearing a phosphonomethyl substituent in the B-ring were required. The parent indole system with the necessary substitution pattern was obtained through Stobbe condensation and cyclization. A prenyl substituent was incorporated at the C3 position of a 4,6-disubstituted indole through a highly regioselective electrophilic aromatic substitution reaction, while metalation and alkylation provided the C2-prenylated indole. After introduction of the phosphonate group through classical reactions, the new indole phosphonates were found to undergo the desired condensation with nonracemic aldehydes representing the schweinfurthin left half. This approach provides facile access to new heteroaromatic analogues of the natural schweinfurthins and should be applicable to many other natural stilbenes as well.

Relevance of the C-5 position to schweinfurthin induced cytotoxicity

The schweinfurthins are an intriguing group of anti-proliferative agents that display low nanomolar activities against several cell types, including the human-derived glioblastoma cell line SF-295, but have little impact on other cell lines even at micromolar concentrations. This activity has inspired the synthesis of seven of the natural schweinfurthins, all with the correct absolute stereochemistry, and a variety of analogues designed to probe different facets of the pharmacophore. Reported herein is the synthesis of several new schweinfurthin analogues varied at the C-5 position along with data on their biological activity in the NCI 60 cell-line assay.

First total synthesis of (+)-Vedelianin, a potent antiproliferative agent

The total synthesis of (+)-vedelianin has been accomplished in 18 steps from vanillin. Preparation of a key intermediate in nonracemic form through a Shi epoxidation has allowed determination of the absolute stereochemistry of the natural product as the (2S, 3R, 4aR, 9aR)-isomer.

Direct conversion of benzylic and allylic alcohols to phosphonates

Benzyl phosphonate esters often serve as reagents in Horner-Wadsworth-Emmons reactions. In most cases, they can be prepared from benzylic alcohols via formation of the corresponding halide followed by an Arbuzov reaction. To identify a more direct synthesis of phosphonate esters, we have developed a one-flask procedure for conversion of benzylic and allylic alcohols to the corresponding phosphonates through treatment with triethyl phosphite and ZnI(2).