A Unified Approach for the Stereoselective Total Synthesis of Pyridone Alkaloids and Their Neuritogenic Activity

Enantioselective Synthesis of the 1,3-Dienyl-5-Alkyl-6-Oxy Motif: Method Development and Total Synthesis

The 1,3-dienyl-5-alkyl-6-oxy motif is widely found in various types of bioactive natural products. However, present synthesis is mainly non-asymmetric which relied upon different olefination or transition metal-catalyzed cross-coupling reactions using enantioenriched precursors. Herein, based upon a newly developed enantioselective α-alkylation of conjugated polyenoic acids, a variety of 1,3-dienyl-5-alkyl-6-oxy motif (with E-configured internal olefin) was generated as the corresponding α-adducts in a highly enantioselective and diastereoselective manner. Utilizing 1,3-dienyl-5-alkyl-6-oxy motif as key intermediates, we further demonstrated their synthetic potential by expedient total syntheses of three types of natural products (glutarimide antibiotics, α-pyrone polyketides and Lupin alkaloids) within 4-7 steps.

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.

Direct C-H Cyanation by ICN Formed In Situ: Nannozinone B

A novel method for C-H cyanation of different pyrans, pyrroles, indoles, and acyclic nucleophilic double bonds using TMSCN, NIS, and Zn(OTf)₂ as a catalyst is described. The transformation is conducted under mild conditions tolerating a variety of functional groups. Zn(OTf)₂ is likely to serve a dual catalytic role as an activator for TMSCN and for the cyanogen iodide generated in situ. Optimization, the substrate scope, and mechanistic observations are reported. Furthermore, this method is applied in the first total synthesis of the natural product nannozinone B.

Stereostructure Clarifying Total Synthesis of the (Polyenoyl)tetramic Acid Militarinone B. A Highly Acid-Labile N-Protecting Group for Amides†

The 5S, 8'R, and 10'R configurations of militarinone B (3), which is a natural product from Paecilomyces militaris, should equal those in its biosynthetic precursor, militarinone C. The configuration at C-1' emerged from syntheses of the militarinone B candidates 1''S- and 1''R-(5S,8'R,10'R)-3 from the building blocks 9, 11, 14, and 15a while introducing TMB as a more acid-labile N-protecting group for β-ketoamides than DMB. Comparisons of 1''S- and 1''R-(5S,8'R,10'R)-3 with natural militarinone B (3; reisolated from Nature) revealed identity versus distinctness.

Diversity-oriented functionalization of 2-pyridones and uracils

Heterocycles 2-pyridone and uracil are privileged pharmacophores. Diversity-oriented synthesis of their derivatives is in urgent need in medicinal chemistry. Herein, we report a palladium/norbornene cooperative catalysis enabled dual-functionalization of iodinated 2-pyridones and uracils. The success of this research depends on the use of two unique norbornene derivatives as the mediator. Readily available alkyl halides/tosylates and aryl bromides are utilized as ortho-alkylating and -arylating reagents, respectively. Widely accessible ipso-terminating reagents, including H/DCO2Na, boronic acid/ester, terminal alkene and alkyne are compatible with this protocol. Thus, a large number of valuable 2-pyridone derivatives, including deuterium/CD3-labeled 2-pyridones, bicyclic 2-pyridones, 2-pyridone-fenofibrate conjugate, axially chiral 2-pyridone (97% ee), as well as uracil and thymine derivatives, can be quickly prepared in a predictable manner (79 examples reported), which will be very useful in new drug discovery.

Synthesis of Colibactin Pyrrolidono[3,4-d]pyridones via Regioselective C(sp3)-H Activation

The synthesis of pyrrolidono[3,4-d]pyridones of relevance to putative genotoxic colibactin structures featuring a doubly conjugated 1,6-Michael acceptor system is reported. We investigated and implemented a highly selective Pd-catalyzed C(sp³)-H activation reaction as a key step and further functionalized the pyridone core. Evaluating the role of this structural unit of relevance to colibactin, we found that this structure displayed a high degree of stability toward both acidic conditions and nucleophiles.

Structure-Elucidating Total Synthesis of the (Polyenoyl)tetramic Acid Militarinone C§

The (polyenoyl)tetramic acid militarinone C (1) heads a family of seven members. Before our work, the configuration of C-5 was unknown whereas the configurations of C-8' and C-10' were either (R,R) or (S,S). We synthesized the four stereoisomers of constitution 1, which conform with these insights. This included cross-coupling both enantiomers of the western building block (8) with both enantiomers of the eastern building block (9). The specific rotations of the resulting 1 isomers suggested that natural 1 is configured like the coupling partners (S)-8 and (R,R)-9. This conclusion was corroborated by degrading natural 1 to alcohol 35 and by proving its configurational identity with synthetic (R,R)-35.

Structural Simplification of Natural Products

Natural products (NPs) are important sources of clinical drugs due to their structural diversity and biological prevalidation. However, the structural complexity of NPs leads to synthetic difficulties, unfavorable pharmacokinetic profiles, and poor drug-likeness. Structural simplification by truncating unnecessary substructures is a powerful strategy for overcoming these limitations and improving the efficiency and success rate of NP-based drug development. Herein, we will provide a comprehensive review of the structural simplification of NPs with a focus on design strategies, case studies, and new technologies. In particular, a number of successful examples leading to marketed drugs or drug candidates will be discussed in detail to illustrate how structural simplification is applied in lead optimization of NPs.

Scaffold Diversity Synthesis Delivers Complex, Structurally, and Functionally Distinct Tetracyclic Benzopyrones

Complexity-generating chemical transformations that afford novel molecular scaffolds enriched in sp3 character are highly desired. Here, we present a highly stereoselective scaffold diversity synthesis approach that utilizes cascade double-annulation reactions of diverse pairs of zwitterionic and non-zwitterionic partners with 3-formylchromones to generate highly complex tetracyclic benzopyrones. Each pair of annulation partners adds to the common chroman-4-one scaffold to build two new rings, supporting up to four contiguous chiral centers that include an all-carbon quaternary center. Differently ring-fused benzopyrones display different biological activities, thus demonstrating their immense potential in medicinal chemistry and chemical biology research.

Total synthesis of natural productsviairidium catalysis

An overview of the highlights in total synthesis of natural products using iridium as a catalyst is given.

Enantio- and Regioselective Ir-Catalyzed Hydrogenation of Di- and Trisubstituted Cycloalkenes

A number of cyclic olefins were prepared and evaluated for the asymmetric hydrogenation reaction using novel N,P-ligated iridium imidazole-based catalysts (Crabtree type). The diversity of these cyclic olefins spanned those having little functionality to others bearing strongly coordinating substituents and heterocycles. Excellent enantioselectivities were observed both for substrates having little functionality (up to >99% ee) and for substrates possessing functional groups several carbons away from the olefin. Substrates having functionalities such as carboxyl groups, alcohols, or heterocycles in the vicinity of the C═C bond were hydrogenated in high enantiomeric excess (up to >99% ee). The hydrogenation was also found to be regioselective, and by controlling the reaction conditions, selective hydrogenation of one of two trisubstituted olefins can be achieved. Furthermore, trisubstituted olefins can be selectively hydrogenated in the presence of tetrasubstituted olefins.

Capturing Biological Activity in Natural Product Fragments by Chemical Synthesis

Natural products have had an immense influence on science and have directly led to the introduction of many drugs. Organic chemistry, and its unique ability to tailor natural products through synthesis, provides an extraordinary approach to unlock the full potential of natural products. In this Review, an approach based on natural product derived fragments is presented that can successfully address some of the current challenges in drug discovery. These fragments often display significantly reduced molecular weights, reduced structural complexity, a reduced number of synthetic steps, while retaining or even improving key biological parameters such as potency or selectivity. Examples from various stages of the drug development process up to the clinic are presented. In addition, this process can be leveraged by recent developments such as genome mining, antibody–drug conjugates, and computational approaches. All these concepts have the potential to identify the next generation of drug candidates inspired by natural products.

Neuritogenic militarinone-inspired 4-hydroxypyridones target the stress pathway kinase MAP4K4

Progressive loss and impaired restoration of neuronal activity are hallmarks of neurological diseases, and new small molecules with neurotrophic activity are in high demand. The militarinone alkaloids and structurally simplified analogues with 4-hydroxy-2-pyridone core structure induce pronounced neurite outgrowth, but their protein target has not been identified. Reported herein is the synthesis of a militarinone-inspired 4-hydroxy-2-pyridone collection, its investigation for enhancement of neurite outgrowth, and the discovery of the stress pathway kinase MAP4K4 as a target of the discovered neuritogenic pyridones. The most potent 4-hydroxy-2-pyridone is a selective ATP-competitive inhibitor of MAP4K4 but not of the other stress pathway related kinases, as proven by biochemical analysis and by a crystal structure of the inhibitor in complex with MAP4K4. The findings support the notion that MAP4K4 may be a new target for the treatment of neurodegenerative diseases.

Copy, edit, and paste: natural product approaches to biomaterials and neuroengineering

Progress in the chemical sciences has formed the world we live in, both on a macroscopic and on a nanoscopic scale. The last century witnessed the development of high performance materials that interact with humans on many layers, from clothing to construction, from media to medical devices. On a molecular level, natural products and their derivatives influence many biological processes, and these compounds have enormously contributed to the health and quality of living of humans. Although coatings of stone materials with oils or resins (containing natural products) have led to improved tools already millennia ago, in contrast today, natural product approaches to designer materials, that is, combining the best of both worlds, remain scarce. In this Account, we will summarize our recent research efforts directed to the generation of natural product functionalized materials, exploiting the strategy of "copy, edit, and paste with natural products". Natural products embody the wisdom of evolution, and only total synthesis is able to unlock the secrets enshrined in their molecular structure. We employ total synthesis ("copy") as a scientific approach to address problems related to molecular structure, the biosynthesis of natural products, and their bioactivity. Additionally, the fundamental desire to investigate the mechanism of action of natural products constitutes a key driver for scientific inquiry. In an emerging area of relevance to society, we have prepared natural products such as militarinone D that can stimulate neurite outgrowth and facilitate nerve regeneration. This knowledge obtained by synthetic organic chemistry on complex natural products can then be used to design structurally simplified compounds that retain the biological power of the parent natural product ("edit"). This process, sometimes referred to as function-oriented synthesis, allows obtaining derivatives with better properties, improving their chemical tractability and reducing the step count of the synthesis. Along these lines, we have demonstrated that militarinone D can be truncated to yield structurally simplified analogs with improved activity. Finally, with the goal of designing bioactive materials, we have immobilized functionally optimized, neuritogenic natural products ("paste"). These materials could facilitate nerve regeneration, act as nerve guidance conduits, or lead to new approaches in neuroengineering. Based on the surface-adhesive properties of electron-deficient catecholates and the knowledge gathered on neuritogenic natural product derivatives, two mechanistically different design principles have been applied to generate neuritogenic materials. In conclusion, natural products, and their functionally optimized analogs, present a large, mostly untapped reservoir of powerful modulators of biological systems, and their hybridization with materials can lead to new approaches in various fields, from biofilm prevention to neuroengineering.

Direct arginine modification in native peptides and application to chemical probe development

An efficient method for the direct labeling of the Arg guanidinium group in native peptides is reported. This straightforward procedure allows modifying the arginine moiety in peptides with various reporter groups, such as fluorophores, biotin, etc., under mild conditions in an operationally simple procedure. The scope of this method tolerates various functionalized amino acids such as His, Ser, Trp, Tyr, Glu, etc., while the only limitations uncovered so far are restricted to cysteine and free amine residues. The utility of this late-stage diversification method was demonstrated in direct labeling of leuprolide, a clinically used drug, for distribution monitoring in Daphnia, and in labeling of microcystin, a cyanobacterial toxin.

Indole alkaloids with new skeleton activating neural stem cells

Alstoscholarisines A-E (1-5), five unprecedented monoterpenoid indole alkaloids with 6/5/6/6/6 fused-bridge rings, were isolated from Alstonia scholaris. They promoted adult neuronal stem cells (NSCs) proliferation significantly, in which the most active one (1) functioned from a concentration of 0.1 μg/mL in a dosage-dependent manner. Furthermore, 1 enhanced NSC sphere formation and neurogenic fate commitment through activation of a Wnt signaling pathway and promoted NSC differentiation but did not affect proliferation of neuroblastoma cells.

Neuritogenic surfaces using natural product analogs

Neuritogenic surfaces are generated by a simple dip-coating procedure, as glass slides are coated with a neurotrophin-like small organic molecule in the presence of a collagen matrix. The surfaces retain their biological activity for multiple cycles and the protocol is suitable for various substrates and coating conditions.

Caged retinoids as photoinducible activators: implications for cell differentiation and neurite outgrowth

Aiming to control neurite formation and navigate the axonal growth by an extrinsic guidance, we report on the design, synthesis and biological evaluation of caged retinoids. Agonists of RARβ have been temporarily blocked either by the [(α-methyl-2-nitropiperonyl)oxy]carbonyl (MeNPOC) group or by immobilization using nitrocatechol linkers on TiO2 particles. Release on demand has been achieved by release under UV irradiation, leading to the biologically active compounds, which have been shown to induce neuronal differentiation and neurite outgrowth.

Asymmetric total synthesis of a putative sex pheromone component from the parasitoid wasp Trichogramma turkestanica

Virgin females of the parasitoid wasp Trichogramma turkestanica produce minute amounts of a sex pheromone, the identity of which has not been fully established. The enantioselective synthesis of a putative component of this pheromone, (6S,8S,10S)-4,6,8,10-tetramethyltrideca-2E,4E-dien-1-ol (2), is reported as a contribution to this identification. Catalytic asymmetric conjugate addition of methylmagnesium bromide and stereoselective Horner–Wadsworth–Emmons olefinations are used as the key steps, and 2 was obtained in 16 steps with an overall yield of 4.4%.

Functionally Optimized Neuritogenic Farinosone C Analogs: SAR-Study and Investigations on Their Mode of Action

Several natural products derived from entomopathogenic fungi have been shown to initiate neuronal differentiation in the rat pheochromocytoma PC12 cell line. After the successful completion of the total synthesis program, the reduction of structural complexity while retaining biological activity was targeted. In this study, farinosone C served as a lead structure and inspired the preparation of small molecules with reduced complexity, of which several were able to induce neurite outgrowth. This allowed for the elaboration of a detailed structure-activity relationship. Investigations on the mode of action utilizing a computational similarity ensemble approach suggested the involvement of the endocannabinoid system as potential target for our analogs and also led to the discovery of four potent new endocannabinoid transport inhibitors.

Neurotrophic natural products: chemistry and biology

Neurodegenerative diseases and spinal cord injury affect approximately 50 million people worldwide, bringing the total healthcare cost to over 600 billion dollars per year. Nervous system growth factors, that is, neurotrophins, are a potential solution to these disorders, since they could promote nerve regeneration. An average of 500 publications per year attests to the significance of neurotrophins in biomedical sciences and underlines their potential for therapeutic applications. Nonetheless, the poor pharmacokinetic profile of neurotrophins severely restricts their clinical use. On the other hand, small molecules that modulate neurotrophic activity offer a promising therapeutic approach against neurological disorders. Nature has provided an impressive array of natural products that have potent neurotrophic activities. This Review highlights the current synthetic strategies toward these compounds and summarizes their ability to induce neuronal growth and rehabilitation. It is anticipated that neurotrophic natural products could be used not only as starting points in drug design but also as tools to study the next frontier in biomedical sciences: the brain activity map project.