Fluevirosines A–C: A Biogenesis Inspired Example in the Discovery of New Bioactive Scaffolds from Flueggea virosa

Interaction of Norsecurinine-Type Oligomeric Alkaloids with α-Tubulin: A Molecular Docking Study

The medicinal plant Securinega virosa (Roxb ex. Willd) Baill., also known as Flueggea virosa (Roxb. ex Willd.) Royle, is commonly used in traditional medicine in Africa and Asia for the management of diverse pathologies, such as parasite infections, diabetes, and gastrointestinal diseases. Numerous alkaloids have been isolated from the twigs and leaves of the plant, notably a variety of oligomeric indolizidine alkaloids derived from the monomers securinine and norsecurinine which both display anticancer properties. The recent discovery that securinine can bind to tubulin and inhibit microtubule assembly prompted us to investigate the potential binding of two series of alkaloids, fluevirosines A-H and fluevirosinine A-J, with the tubulin dimer by means of molecular modeling. These natural products are rare high-order alkaloids with tri-, tetra-, and pentameric norsecurinine motifs. Despite their large size (up to 2500 Å3), these alkaloids can bind easily to the large drug-binding cavity (about 4800 Å3) on α-tubulin facing the β-tubulin unit. The molecular docking analysis suggests that these hydrophobic macro-alkaloids can form stable complexes with α/β-tubulin. The tubulin-binding capacity varies depending on the alkaloid size and structure. Structure-binding relationships are discussed. The docking analysis identifies the trimer fluevirosine D, tetramer fluevirosinine D, and pentamer fluevirosinine H as the most interesting tubulin ligands in the series. This study is the first to propose a molecular target for these atypical oligomeric Securinega alkaloids.

Synthesis of the Indolizidine Core of Virosinine A via a Microwave-Promoted Cascade Cyclization Involving Iminyl Radicals

The indolizidine core of virosinine A was synthesized by means of a microwave-promoted cascade reaction featuring 5-exo-trig iminyl radical cyclization, thiyl radical elimination, and intramolecular imine alkylation. The resulting bicyclic iminium ion underwent stereoselective reduction by Red-Al to deliver the target compound. DFT calculations suggested that both the radical cyclization and thiyl radical elimination steps are reversible at high reaction temperatures.

Synthesis of High-Order and High-Oxidation State Securinega Alkaloids

Securinega alkaloids, composed of more than 100 members characterized by the compact tetracyclic scaffold, have fascinated the synthetic community with their structural diversity and notable bioactivities. On the basis of the structural phenotype, oligomerizations and oxidations are major biosynthetic diversification modes of the basic Securinega framework. Despite the rich history of synthesis of basic monomeric Securinega alkaloids, the synthesis of oligomeric Securinega alkaloids, as well as oxidized derivatives, has remained relatively unexplored because of their extra structural complexity. In the first half of this Account, our synthetic studies toward high-order Securinega alkaloids are described. We aimed to establish a reliable synthetic method to form C14-C15' and C12-C15' bonds, which are prevalent connection modes between monomers. During our total synthesis of flueggenine C (9), we have invented an accelerated Rauhut-Currier reaction capable of forming the C14-C15' bond stereoselectively. Installation of the nucleophilic functionality to the Michael acceptor, which ushers the C-C bond forming conjugate addition to follow the intramolecular pathway, was the key to success. The C12-C15' linkage, which was inaccessible via an accelerated Rauhut-Currier reaction, was established by devising a complementary cross-coupling/conjugate reduction-based dimerization strategy that enabled the total synthesis of flueggenines D (11) and I (14). In this approach, the C12-C15' linkage was established via a Stille cross-coupling, and the stereochemistry of the C15' position was controlled during the following conjugate reduction step. In the later half of this Account, our achievements in the field of high-oxidation state Securinega alkaloids synthesis are depicted. We have developed oxidative transformations at the N1 and C2-C4 positions, where the biosynthetic oxidation event occurs most frequently. The discovery of a VO(acac)₂-mediated regioselective Polonovski reaction allowed us to access the key 2,3-dehydroallosecurinine (112). Divergent synthesis of secu'amamine A (62) and fluvirosaones A (60) and B (61) was accomplished by exploiting the versatile reactivities of the C2/C3 enamine moiety in 112. We have also employed a fragment-coupling strategy between menisdaurilide and piperidine units, which allowed the installation of various oxygen-containing functionality on the piperidine ring. Combined with the late-stage, light-mediated epimerization and well-orchestrated oxidative modifications, collective total synthesis of seven C4-oxygenated securinine-type natural products was achieved. Lastly, the synthesis of flueggeacosine B (70) via two synthetic routes from allosecurinine (103) was illustrated. The first-generation synthesis (seven overall steps) employing Pd-catalyzed cross-coupling between stannane and thioester to form the key C3-C15' bond enabled the structural revision of the natural product. In the second-generation synthesis, we have invented visible-light-mediated, Cu-catalyzed cross-dehydrogenative coupling (CDC) between an aldehyde and electron-deficient olefin, which streamlined the synthetic pathway into four overall steps. Organisms frequently utilize dimerization (oligomerization) and oxidations during the biosynthesis as a means to expand the chemical space of their secondary metabolites. Therefore, methods and strategies for dimerizations and oxidations that we have developed using the Securinega alkaloids as a platform would be broadly applicable to other alkaloids. It is our sincere hope that lessons we have learned during our synthetic journey would benefit other chemists working on organic synthesis.

Nonenzymatic Reactions in Natural Product Formation

Biosynthetic mechanisms of natural products primarily depend on systems of protein catalysts. However, within the field of biosynthesis, there are cases in which the inherent chemical reactivity of metabolic intermediates and substrates evades the involvement of enzymes. These reactions are difficult to characterize based on their reactivity and occlusion within the milieu of the cellular environment. As we continue to build a strong foundation for how microbes and higher organisms produce natural products, therein lies a need for understanding how protein independent or nonenzymatic biosynthetic steps can occur. We have classified such reactions into four categories: intramolecular, multicomponent, tailoring, and light-induced reactions. Intramolecular reactions is one of the most well studied in the context of biomimetic synthesis, consisting of cyclizations and cycloadditions due to the innate reactivity of the intermediates. There are two subclasses that make up multicomponent reactions, one being homologous multicomponent reactions which results in dimeric and pseudodimeric natural products, and the other being heterologous multicomponent reactions, where two or more precursors from independent biosynthetic pathways undergo a variety of reactions to produce the mature natural product. The third type of reaction discussed are tailoring reactions, where postmodifications occur on the natural products after the biosynthetic machinery is completed. The last category consists of light-induced reactions involving ecologically relevant UV light rather than high intensity UV irradiation that is traditionally used in synthetic chemistry. This review will cover recent nonenzymatic biosynthetic mechanisms and include sources for those reviewed previously.

Indigenous Uses, Phytochemical Analysis, and Anti-Inflammatory Properties of Australian Tropical Medicinal Plants

Australian tropical plants have been a rich source of food (bush food) and medicine to the first Australians (Aboriginal people), who are believed to have lived for more than 50,000 years. Plants such as spreading sneezeweed (Centipeda minima), goat’s foot (Ipomoea pes-caprae), and hop bush (Dodonaea viscosa and D. polyandra) are a few popular Aboriginal medicinal plants. Thus far, more than 900 medicinal plants have been recorded in the tropical region alone, and many of them are associated with diverse ethnomedicinal uses that belong to the traditional owners of Aboriginal people. In our effort to find anti-inflammatory lead compounds in collaboration with Aboriginal communities from their medicinal plants, we reviewed 78 medicinal plants used against various inflammation and inflammatory-related conditions by Aboriginal people. Out of those 78 species, we have included only 45 species whose crude extracts or isolated pure compounds showed anti-inflammatory properties. Upon investigating compounds isolated from 40 species (for five species, only crude extracts were studied), 83 compounds were associated with various anti-inflammatory properties. Alphitolic acid, Betulinic acid, Malabaric acid, and Hispidulin reduced proinflammatory cytokines and cyclooxygenase enzymes (COX-1 and 2) with IC₅₀ values ranging from 11.5 to 46.9 uM. Other promising anti-inflammatory compounds are Brevilin A (from Centipeda minima), Eupalestin, and 5′-methoxy nobiletin (from Ageratum conyzoides), Calophyllolide (from Calophyllum inophyllum), and Brusatol (from Brucea javanica). D. polyandra is one example of an Aboriginal medicinal plant from which a novel anti-inflammatory benzoyl ester clerodane diterpenoid compound was obtained (compound name not disclosed), and it is in the development of topical medicines for inflammatory skin diseases. Medicinal plants in the tropics and those associated with indigenous knowledge of Aboriginal people could be a potential alternative source of novel anti-inflammatory therapeutics.

Total synthesis of dimeric Securinega alkaloids (-)-flueggenines D and I

We describe the total synthesis of (-)-flueggenines D and I. This features the first total synthesis of dimeric Securinega alkaloids with a C(α)-C(δ') connectivity between two monomeric units. The key dimerization was enabled by a sequence that involves Stille reaction and conjugate reduction. The high chemofidelity of the Stille reaction enabled us to assemble two structurally complex fragments that could not be connected by other methods. Stereochemical flexibility and controllability at the δ'-junction of the dimeric intermediate render our synthetic strategy broadly applicable to the synthesis of other high-order Securinega alkaloids.

Synthesis and biological evaluation of suffrutines A, B and their N-fused analogues

The synthesis, structure confirmation, stability and isomerization features of suffrutines A, B and their N-fused analogues were reported. Biological tests showed that the introduction of nitrogen atom might be beneficial to the anticancer activity.

Flueggeacosines A-C, Dimeric Securinine-Type Alkaloid Analogues with Neuronal Differentiation Activity from Flueggea suffruticosa

Flueggeacosines A-C (1-3), three dimeric securinine-type alkaloid analogues with unprecedented skeletons, were isolated from Flueggea suffruticosa. Compounds 1 and 2 are the first examples of C-3-C-15' connected dimeric securinine-type alkaloids. Compound 3 is an unprecedented heterodimer of securinine-type and benzoquinolizidine alkaloids. Biosynthetic pathways for 1-3 were proposed on the basis of the coexisting alkaloid monomers as the precursors. Compound 2 exhibited significant activity in promoting neuronal differentiation of Neuro-2a cells.

Design and Synthesis of Dimeric Securinine Analogues with Neuritogenic Activities

Neurite outgrowth is crucial during neuronal development and regeneration, and strategies that aim at promoting neuritogenesis are beneficial for reconstructing synaptic connections after neuronal degeneration and injury. Using a bivalent analogue strategy as a successful approach, the current study identifies a series of novel dimeric securinine analogues as potent neurite outgrowth enhancers. Compounds 13, 14, 17-19, and 21-23, with different lengths of carbon chain of N,N-dialkyl substituting diacid amide linker between two securinine molecules at C-15 position, exhibited notable positive effects on both neuronal differentiation and neurite extension of neuronal cells. Compound 14, one of the most active compounds, was used as a representative compound for mechanistic studies. Its action on neurite outgrowth was through phosphorylation/activation of multiple signaling molecules including Ca²⁺/calmodulin-dependent protein kinase II (CaMKII), extracellular signal-regulated kinase (ERK) and Akt. These findings collectively identify a new group of beneficial compounds for neuritogenesis, and may provide insights on drug discovery of neural repair and regeneration.

Flueggether A and Virosinine A, Anti-HIV Alkaloids from Flueggea virosa

Two new alkaloids, flueggether A (1) and virosinine A (2), were isolated from a Chinese medicinal plant, Flueggea virosa. Their structures were assigned via spectroscopic methods with the absolute configurations of 1 and 2 being established by X-ray diffraction analysis and calculated electronic circular dichroism data, respectively. Compound 1 represents the first example with an ether bridge of Securinega alkaloid oligomers, and 2 bears a new heterocyclic backbone. Both alkaloids showed mild in vitro anti-HIV activity.

Terpenoids and Steroids from Euphorbia hypericifolia

Two new triterpenoids and two new sterols, named euphyperins A-D (1–4), including an oleanane-type triterpenoid (1), a lupane-type nortriterpenoid (2), and two cholestane-type steroids (3 and 4), along with five known compounds (5–9) were isolated from the twigs and leaves of Euphorbia hypericifolia. Euphyperin B (2) represents a rare lupane-type nortriterpenoid, and euphyperin C (3) is a novel 8,14-secocholestane-type steroid. Euphyperin A (1) exhibited moderate PTP1B inhibitory activity with an IC50 = 17.05 ± 1.12 μg/mL.

New Securinega alkaloids with anti-HIV activity from Flueggea virosa

A series of Securinega alkaloid hybrids and oligomers with anti-HIV activity were isolated and identified from Flueggea virosa.

Novel monoamine oxidase inhibitors: a patent review (2012 - 2014)

INTRODUCTION

Monoamine oxidase (MAO) inhibitors, despite the initial pharmacological interest, are used in clinic for their antidepressant effect and in the management of Parkinson symptoms, due to the established neuroprotective action. Efficacy and tolerability emerged from large-scale and randomized clinical trials.

AREAS COVERED

Thirty-six patents range from April 2012 to September 2014. The number of chemotypes with inhibitory effects on MAO is truly high (40 synthetic compounds, 22 natural products and 6 plant extracts reported and licensed), and the present review is comprehensive of all compounds, which have been patented for their relevance to clinical medicine in this period range (27 patents). Moreover, some of the collected patents deal with new formulations of compounds endowed with MAO inhibitory properties (two patents) and new therapeutic options/drug associations for already known MAO inhibitors (seven patents).

EXPERT OPINION

The patents reported in this review showed that the interest in this field is constant and mainly devoted to the study of selective MAO-B inhibitors, used as drugs for the treatment of neurological disorders. The development of novel human MAO inhibitors took advantage of the discovery of new therapeutic targets (cancer, hair loss, muscle dystrophies, cocaine addiction and inflammation), the recognized role of MAOs as molecular biomarkers and their activity in other tissues.

Eurifoloids A-R, structurally diverse diterpenoids from Euphorbia neriifolia

Eighteen new diterpenoids, named eurifoloids A-R (1-18), including ingenane (1 and 2), abietane (3-7), isopimarane (8-12), and ent-atisane (13-18) types, along with four known analogues were isolated from Euphorbia neriifolia. Eurifoloid M (13) represents a rare class of ent-atisane-type norditerpenoid. Eurifoloids E (5) and F (6) exhibited significant anti-HIV activities, with EC50 values of 3.58 ± 0.31 (SI = 8.6) and 7.40 ± 0.94 μM (SI = 10.3), respectively.

Suffrutines A and B: a pair of Z/E isomeric indolizidine alkaloids from the roots of Flueggea suffruticosa

Suffrutines A (1) and B (2), a pair of novel photochemical Z/E isomeric indolizidine alkaloids, with a unique and highly conjugated C20 skeleton, were isolated from the roots of Flueggea suffruticosa. The structures were elucidated by extensive analysis of NMR spectra and single-crystal X-ray diffraction. The light-induced isomerization and hypothetical biogenetic pathway to 1 and 2, as well as their activity for regulating the morphology of Neuro-2a cells are also discussed.

Anti-hepatitis C virus dinorditerpenes from the roots of Flueggea virosa

Along with four known terpenoids (1-4), eight new dinorditerpenes (5-12) were isolated and identified from the roots of Flueggea virosa. The absolute configurations of 4-6 were determined by the Mosher's method, and that of 5 was confirmed by single-crystal X-ray diffraction analysis. Using the hepatitis C virus cell culture infection system, compounds 1, 3, 11, and 12 exhibited significant anti-HCV activity with EC50 values of 5.6, 5.0, 7.5, and 6.6 μM, respectively. Compounds 11 and 12 were nontoxic toward the tested Huh7.5 cell lines.

Trinorditerpenes from the roots of Flueggea virosa

Two trinorditerpenes, flueggrenes A and B (1 and 2), have been isolated from the roots of Flueggea virosa. Their structures were established by extensive analyses of spectroscopic data. The isolates were evaluated for anti-HCV activity, as well as the inhibition of superoxide anion generation and elastase release in response to FMLP/cytochalasin B.