Total synthesis of cyrneines A-B and glaucopine C

Neurotrophic Natural Products

Neurotrophins (NGF, BDNF, NT3, NT4) can decrease cell death, induce differentiation, as well as sustain the structure and function of neurons, which make them promising therapeutic agents for the treatment of neurodegenerative disorders. However, neurotrophins have not been very effective in clinical trials mostly because they cannot pass through the blood-brain barrier owing to being high-molecular-weight proteins. Thus, neurotrophin-mimic small molecules, which stimulate the synthesis of endogenous neurotrophins or enhance neurotrophic actions, may serve as promising alternatives to neurotrophins. Small-molecular-weight natural products, which have been used in dietary functional foods or in traditional medicines over the course of human history, have a great potential for the development of new therapeutic agents against neurodegenerative diseases such as Alzheimer's disease. In this contribution, a variety of natural products possessing neurotrophic properties such as neurogenesis, neurite outgrowth promotion (neuritogenesis), and neuroprotection are described, and a focus is made on the chemistry and biology of several neurotrophic natural products.

Directed evolution of an alcohol dehydrogenase for the desymmetric reduction of 2,2-disubstituted cyclopenta-1,3-diones by enzymatic hydrogen transfer

Directed evolution of carbonyl reductase TbADH created mutant Tb2 with balanced activity toward ethyl secodione and isopropanol, enabling the desymmetric reduction of ethyl secodione to give (13R,17S)-ethyl secol with 94% yield, >99% ee and >99% de.

Thirteen cyathane diterpenoids with acetylcholinesterase inhibitory effects from the fungus Cyathus africanus

Eight undescribed cyathane diterpenoids, representative specialised metabolites of the genus Cyathus, named cyathins Q-X, along with five known congeners, were isolated from the liquid fermentation of Cyathus africanus. Their structures and absolute configurations were elucidated by integrating NMR spectroscopic analyses, electronic circular dichroism (ECD) calculations, and X-ray diffraction. Reasonable correction to the C-12 configuration of cyathin I was corroborated by the crystal data. The structural identification in this research expanded the number of candidates to allow for more bioactivity-screening options. Among them, (12S)-11α,14α-epoxy-13α,14β,15-trihydroxycyath-3-ene displayed significant acetylcholinesterase (AChE) inhibitory effect with an IC₅₀ value of 4.60 ± 0.85 μM. Molecular docking studies were also performed to unravel the underlying modes of interactions with the active sites of AChE for active compounds.

The Phosphinamide-Based Catalysts: Discovery, Methodology Development, and Applications in Natural Product Synthesis

In the total synthesis of natural products, synthetic efficiency has been an important driver for designing and developing new synthetic strategies and methodologies. To this end, the step, atom, and time economy and the overall yield are major factors to be considered. On the other hand, developing unified routes that can be used for synthesizing multiple molecules, specifically skeletally different classes of molecules, are also important aspects with which to be concerned. In the efforts toward efficient and flexible synthesis of structurally unique terpenoid and indole alkaloid natural products, we have designed and developed several phosphinamide-based new catalysts and reaction methodologies that have been compellingly demonstrated to be widely useful as strategic protocols for the diverse synthesis of various complex terpenoids and indole alkaloids. The important progress of these results will be summarized in this Account.In the first part, we present the stories of successful design and establishment of a novel method for the synthesis of P-stereogenic phosphinamides (P-SPhos) via a Pd-catalyzed C-H desymmetric enantioselective arylation, as well as the flexible derivatization of the P-stereogenic phosphinamides into various types of skeletally unique tricyclic and N,P-bidentate P-stereogenic compounds. Subsequently, the discovery of P-stereogenic phosphinamides as chiral organocatalysts for the desymmetric enantioselective reduction of cyclic 1,3-diketones and of phosphinamide-based cyclopalladium complex (C-Pd) as precatalysts for highly efficient Suzuki-Miyaura cross-coupling reaction of sterically congested nonactivated enolates is introduced. The notable features of the P-stereogenic phosphinamide-catalyzed desymmetric enantioselective reduction are highlighted by the broad substrate compatibility and excellent stereoselectivity, as well as most significantly, the good recoverability and reusability of catalysts. With regard to the sterically congested nonactivated enolates, such substrates are challenging for Suzuki cross-coupling reactions. We demonstrate that the phosphinamide-based cyclopalladium is a type of highly active precatalyst that allows the reaction to proceed under mild conditions and to be easily scaled up. Following the methodology development, the practical applications of these methods serving as strategic transformations are highlighted by the unified synthesis of four cyathane-type and two hamigeran-type terpenoids.In the second part, we describe the development of a robust method for oxidative Heck cross-coupling of indolyl amides by using the phosphinamide-based cyclopalladium as catalyst or phosphinamide as coligand. The method provides a general and straightforward method for diverse synthesis of indolyl δ-lactam derivatives, which present as a common core in a variety of Aspidosperma-derived indole alkaloids. The successful demonstration of this protocol for a concise and divergent synthesis of leuconodine-type indole alkaloids is also presented. We believe the results presented in this Account would have significant implications beyond our results and would find further applications in the field of synthetic methodology and natural product synthesis.

Enantioselective Total Synthesis and Absolute Configuration Assignment of (+)-Toxicodenane A

We present the first enantioselective total synthesis and absolute configuration assignment of (+)-toxicodenane A via a nine-step sequence from the readily available material. The synthesis features a desymmetric enantioselective reduction of 2,2-disubstituted 1,3-cyclohexanedione for the synthesis of a chiral 2,2-disubstituted 3-hydroxy cyclohexanone building block, a highly diastereoselective Grignard reaction for the incorporation of an allyl group, and a Lewis acid-mediated intramolecular transacetalation and Prins cascade reaction for the construction of oxa-bridged bicyclic rings.

Reconstitution of biosynthetic pathway for mushroom-derived cyathane diterpenes in yeast and generation of new "non-natural" analogues

Mushroom-derived cyathane-type diterpenes possess unusual chemical skeleton and diverse bioactivities. To efficiently supply bioactive cyathanes for deep studies and explore their structural diversity, de novo synthesis of cyathane diterpenes in a geranylgeranyl pyrophosphate engineered Saccharomyces cerevisiae is investigated. Aided by homologous analyses, one new unclustered FAD-dependent oxidase EriM accounting for the formation of allyl aldehyde and three new NADP(H)-dependent reductases in the biosynthesis of cyathanes are identified and elucidated. By combinatorial biosynthetic strategy, S. cerevisiae strains generating twenty-two cyathane-type diterpenes, including seven "unnatural" cyathane xylosides (12, 13, 14a, 14b, 19, 20, and 22) are established. Compounds 12-14, 19, and 20 show significant neurotrophic effects on PC12 cells in the dose of 6.3-25.0 μmol/L. These studies provide new insights into the divergent biosynthesis of mushroom-originated cyathanes and a straightforward approach to produce bioactive cyathane-type diterpenes.

Desymmetric Enantioselective Reduction of Cyclic 1,3-Diketones Catalyzed by a Recyclable P-Chiral Phosphinamide Organocatalyst

The P-stereogenic phosphinamides are a structurally novel skeletal class which has not been investigated as chiral organocatalysts. However, chiral cyclic 3-hydroxy ketones are widely used as building blocks in the synthesis of natural products and bioactive compounds. However, general and practical methods for the synthesis of such chiral compounds remain underdeveloped. Herein, we demonstrate that the P-stereogenic phosphinamides are powerful organocatalysts for the desymmetric enantioselective reduction of cyclic 1,3-diketones, providing a useful method for the synthesis of chiral cyclic 3-hydroxy ketones. The protocol displays a broad substrate scope that is amenable to a series of cyclic 2,2-disubstituted five- and six-membered 1,3-diketones. The chiral cyclic 3-hydroxy ketone products bearing an all-carbon chiral quaternary center could be obtained with high enantioselectivities (up to 98% ee) and diastereoselectivities (up to 99:1 dr). Most importantly, the reactions could be practically performed on the gram scale and the catalysts could be reused without compromising the catalytic efficiency. Mechanistic studies revealed that an intermediate formed from P-stereogenic phosphinamide and catecholborane is the real catalytically active species. The results disclosed herein bode well for designing and developing other reactions using P-stereogenic phosphinamides as new organocatalysts.

Synthesis and Biological Evaluation of Diversified Hamigeran B Analogs as Neuroinflammatory Inhibitors and Neurite Outgrowth Stimulators

We describe the efficient synthesis of a series of new simplified hamigeran B and 1-hydroxy-9-epi-hamigeran B norditerpenoid analogs (23 new members in all), structurally related to cyathane diterpenoid scaffold, and their anti-neuroinflammatory and neurite outgrowth-stimulating (neurotrophic) activity. Compounds 9a, 9h, 9o, and 9q exhibited moderate nerve growth factor (NGF)-mediated neurite-outgrowth promoting effects in PC-12 cells at the concentration of 20 μm. Compounds 9b, 9c, 9o, 9q, and 9t showed significant nitric oxide (NO) production inhibition in lipopolysaccharide (LPS)-activated BV-2 microglial cells, of which 9c and 9q were the most potent inhibitors, with IC₅₀ values of 5.85 and 6.31 μm, respectively. Two derivatives 9q and 9o as bifunctional agents displayed good activities as NO production inhibitors and neurite outgrowth-inducers. Cytotoxicity experiments, H₂O₂-induced oxidative injury assay, and ELISA reaction speculated that compounds may inhibit the TNF-α pathway to achieve anti-inflammatory effects on nerve cells. Moreover, molecular docking studies provided a better understanding of the key structural features affecting the anti-neuroinflammatory activity and displayed significant binding interactions of some derivatives (like 9c, 9q) with the active site of iNOS protein. The structure-activity relationships (SARs) were also discussed. These results demonstrated that this structural class compounds offered an opportunity for the development of a new class of NO inhibitors and NGF-like promotors.

Erinacine A and related cyathane diterpenoids: Molecular diversity and mechanisms underlying their neuroprotection and anticancer activities

The presence of a fused 5/6/7 tricyclic core characterizes the group of cyathane diterpene natural products, that include more than 170 compounds, isolated from fungi such as Cyathus africanus and Hericium erinaceus. These compounds have a common biosynthetic precursor (cyatha-3,12-diene) and can be produced bio- or hemi-synthetically, or via total syntheses. Cyathane diterpenes display a range of pharmacological properties, including anti-inflammatory (possibly through binding to the iNOS protein) and neuroprotective effects. Many cyathanes like cyahookerin C, cyathin Q and cyafranines B and G can stimulate neurite outgrowth in cells, whereas conversely a few molecules (such as scabronine M) inhibit NGF-stimulated neurite outgrowth. The main anticancer cyathanes are erinacine A and cyathins Q and R, with a capacity to trigger cancer cell death dependent on the production of reactive oxygen species (ROS). These compounds, active both in vitro and in vivo, activate different signaling pathways in tumor cells to induce apoptosis (and autophagy) and to upregulate the expression of several proteins implicated in the organization and functioning of the actin cytoskeleton. An analysis of the functional analogy between erinacine A and other natural products known to interfere with the actin network in a ROS-dependent manner (notably cucurbitacin B) further supports the idea that erinacine A functions as a perturbator of the cytoskeleton organization. Collectively, we provide an overview of the molecular diversity of cyathane diterpenes and the main mechanisms of action of the lead compounds, with the objective to encourage further research with these fungal products. The anticancer potential of erinacine A deserves further attention but it will be necessary to better characterize the implicated targets and signaling pathways.

An improved synthesis of the [5.6.7]-tricyclic core of cyrneine B and glaucopine C

A strategically new route for an efficient synthesis of [5.6.7]-tricyclic 9, which could serve as an advanced intermediate, was presented.

Pd-Catalyzed Aerobic Oxidative Heck Cross-Coupling for the Straightforward Construction of Indole δ-Lactams

The [6.5.6]-tricyclic indole δ-lactam represents a common key intermediate for the synthesis of a broad variety of structurally intriguing indole alkaloids. The development of a method for the versatile and straightforward construction of such structural motif is of great importance for potential synthetic applications. Herein, we present a co-ligand-prompted Pd-catalyzed 6-exo-trig intramolecular cyclization of indolyl amides via the aerobic oxidative Heck cross-coupling. The method provided a general and efficient way for the construction of [6.5.6]-tricyclic indole δ-lactams. A mechanistic study suggests that a Pd^(I)/Pd^(III) catalytic cycle should be responsible for effective coupling, which represents a mechanistically alternative pathway when compared with the Pd⁽⁰⁾/Pd^(II) cycle proposed for other related coupling reactions.

A Total Synthesis of (-)-Hamigeran B and (-)-4-Bromohamigeran B

A concise synthesis of (-)-hamigeran B and (-)-4-bromohamigeran B is presented. The key reactions include a Suzuki coupling of enol triflate 15 with arylboronic ester for efficient synthesis of the densely 1,2,3-trisubstituted cyclopentene 23, a coordination-controlled intramolecular Friedel-Crafts cyclization of free phenol 13 for highly regioselective construction of tricyclic core 12, and a LiOH/O₂-promoted hydrolysis and concomitant aerobic oxidation of 31 for atom- and step-economic accessing of diketone 32. The application of these key transformations allowed for a rapid and efficient synthesis of (-)-hamigeran B and (-)-4-bromohamigeran B in 13 steps from the readily available chiral material 18.