Discovery of neuritogenic compound classes inspired by natural products

Brønsted Acid Catalyzed Asymmetric Synthesis of Cyclopentenones with C4-Quaternary Centers Starting from Vinyl Sulfoxides and Allenyl Ketones or Allenoates

Starting with chiral vinyl sulfoxides and allenyl ketones or allenoates, a triflic acid-catalyzed asymmetric [3,3]-sigmatropic rearrangement of sulfoniums is reported to have a direct access to highly functionalized C4-chiral cyclopentenones (19 examples, up to 85% yield and >95% enantiomeric excesses). In addition to the use of these chiral compounds as key building blocks in organic synthesis, the antiproliferative activities of sulfoxide substrates and the corresponding cyclopentenones were evaluated, and promising cytotoxicity against the HL-60 human tumor cell line was found.

Phase-Transfer Catalyzed Asymmetric [4 + 1] Annulations for the Synthesis of Chiral 2,2-Disubstituted Tetrahydrothiophenes

An efficient catalytic asymmetric [4 + 1] reaction, which features the use of simple β-keto esters as one-carbon nucleophiles and 5-succinimidothio-pent-2-enoates as four-atom bielectrophiles, has been developed in the presence of a bifunctional chiral phase-transfer catalyst. The new annulation provides a distinct protocol to access the functionalized 2-acyl-2-carboxyl tetrahydrothiophenes bearing consecutive quaternary and tertiary carbon stereocenters in high diastereoselectivities and enantioselectivities. Moreover, the prepared products could be readily transformed into the chiral 2-alkyl-2-carboxyl tetrahydrothiophenes via two steps of debenzoylation and alkylation reactions.

Synthesis of Polycyclic Ether-Benzopyrans and In Vitro Inhibitory Activity against Leishmania tarentolae

Construction of a focused library of polycyclic ether-benzopyrans was undertaken in order to discover new therapeutic compounds that affect Leishmania growth and infectivity. This is especially of interest since there are few drug therapies for leishmaniasis that do not have serious drawbacks such high cost, side effects, and emerging drug resistance. The construction of these polycyclic ether-benzopyrans utilized an acetoxypyranone-alkene [5+2] cycloaddition and the Suzuki-Miyaura cross-coupling. The multi-gram quantity of the requisite aryl bromide was obtained followed by effective Pd-catalyzed coupling with boronic acid derivatives. Compounds were tested in vitro using the parasitic protozoan, Leishmania tarentolae. Effects of concentration, time, and exposure to light were evaluated. In addition, the effects on secreted acid phosphatase activity and nitric oxide production were investigated, since both have been implicated in parasite infectivity. The data presented herein are indicative of disruption of the Leishmania tarentolae and thus provide impetus for the development and testing of a more extensive library.

Phosphine-Catalyzed Cascade Michael Addition/[4+2] Cycloaddition Reaction of Allenoates and 2-Arylidene-1,3-indanediones

The phosphine-catalyzed cascade Michael addition/[4+2] cycloaddition reaction of tetrahydrobenzofuranone-derived allenoates and 2-arylidene-1,3-indanediones has been reported, affording spirocyclic 1,3-indanedione derivatives in moderate to high yields with moderate to good diastereoselectivities. A scaled-up reaction worked well under mild conditions, and a plausible mechanism is proposed.

Synthesis of C14-Desmethylene Corialactone D and Discovery of Inhibitors of Nerve Growth Factor Mediated Neurite Outgrowth

An asymmetric synthesis of C14-desmethylene corialactone D is described on the basis of strategic application of a metallacycle-mediated annulative cross-coupling reaction, a Still [2,3]-Wittig rearrangement, and Morken's hydroxyl-directed diboration reaction. While representing a convenient approach to access novel compositions of matter inspired by the sesquiterpenoid natural product class (including classic natural product synthesis targets including the picrotaxanes and dendrobine), these studies have led to the discovery of natural product-inspired agents that inhibit nerve growth factor (NGF)-mediated neurite outgrowth in PC-12 cells.

Creation of bispiro[pyrazolone-3,3′-oxindoles] via a phosphine-catalyzed enantioselective [3 + 2] annulation of the Morita–Baylis–Hillman carbonates with pyrazoloneyldiene oxindoles

A [3 + 2] annulation between the Morita–Baylis–Hillman (MBH) carbonates and pyrazoloneyldiene oxindoles catalyzed by (S)-SITCP has been developed.

Phosphine Organocatalysis

The hallmark of nucleophilic phosphine catalysis is the initial nucleophilic addition of a phosphine to an electrophilic starting material, producing a reactive zwitterionic intermediate, generally under mild conditions. In this Review, we classify nucleophilic phosphine catalysis reactions in terms of their electrophilic components. In the majority of cases, these electrophiles possess carbon-carbon multiple bonds: alkenes (section 2), allenes (section 3), alkynes (section 4), and Morita-Baylis-Hillman (MBH) alcohol derivatives (MBHADs; section 5). Within each of these sections, the reactions are compiled based on the nature of the second starting material-nucleophiles, dinucleophiles, electrophiles, and electrophile-nucleophiles. Nucleophilic phosphine catalysis reactions that occur via the initial addition to starting materials that do not possess carbon-carbon multiple bonds are collated in section 6. Although not catalytic in the phosphine, the formation of ylides through the nucleophilic addition of phosphines to carbon-carbon multiple bond-containing compounds is intimately related to the catalysis and is discussed in section 7. Finally, section 8 compiles miscellaneous topics, including annulations of the Hüisgen zwitterion, phosphine-mediated reductions, iminophosphorane organocatalysis, and catalytic variants of classical phosphine oxide-generating reactions.

Chemical probes and drug leads from advances in synthetic planning and methodology

Screening of small-molecule libraries is a productive method for identifying both chemical probes of disease-related targets and potential starting points for drug discovery. In this article, we focus on strategies such as diversity-oriented synthesis that aim to explore novel areas of chemical space efficiently by populating small-molecule libraries with compounds containing structural features that are typically under-represented in commercially available screening collections. Drawing from more than a decade's worth of examples, we highlight how the design and synthesis of such libraries have been enabled by modern synthetic chemistry, and we illustrate the impact of the resultant chemical probes and drug leads in a wide range of diseases.

Quantifying orientational regeneration of injured neurons by natural product concentration gradients in a 3D microfluidic device

Regeneration of injured neurons in complicated three-dimensional (3D) microenvironments is a key approach for treating neurodegenerative diseases. Microfluidics provides a versatile tool to recapitulate cellular microenvironments in vitro, but it still remains a big challenge to construct a microfluidic platform incorporating extracellular matrix (ECM) structures and highly controlled 3D gradients of soluble factors to study the regeneration of injured neurons. In this work, we developed a microfluidic device which can provide multiple adjustable gradients in a 3D ECM to investigate the regeneration of injured central nervous system (CNS) neurons in response to natural small molecules. With interconnecting but independently controlled central channels, asymmetrically designed side channels and a series of microgrooves connecting the central channels, spatially and temporally controlled 3D biochemical gradients can be generated inside collagen hydrogel in the central channels. This allows quantitative analysis of guided axon growth and the orientational regeneration of injured dopaminergic neurons by 3D chemical gradients of three natural molecules. This study demonstrates a promising microfluidic platform for the generation of highly controlled 3D biochemical gradients in an ECM to quantitatively study neuronal responses, thereby potentially facilitating drug screening and optimization of treatment protocols for neurodegenerative diseases.

Multifunctional chiral phosphine-catalyzed [3+2] annulation of Morita-Baylis-Hillman carbonates with cyclopentenones: asymmetric synthesis of 4-oxo-hexahydropentalenes

A multifunctional chiral phosphine-catalyzed enantioselective [3+2] annulation of Morita-Baylis-Hillman carbonates with cyclopentenones provides a direct approach to access functional 4-oxo-1,3a,4,5,6,6a-hexahydropentalene compounds. Moderate to good yields and excellent enantioselectivities (>90% ee) were obtained for a series of substrates under mild reaction conditions.

Catalyst-controlled regioselectivity in phosphine catalysis: the synthesis of spirocyclic benzofuranones via regiodivergent [3 + 2] annulations of aurones and an allenoate

Catalyst-controlled regiodivergent [3 + 2] annulations of aurones and allenoates have been developed. When a dipeptide phosphine catalyst with an l-d- configuration was employed, α-selective [3 + 2] annulation products could be obtained with good regioselectivities and enantioselectivities. With the employment of l-l- dipeptide phosphines, γ-selective annulation products could be selectively obtained with excellent enantioselectivities. By simply tuning the catalyst configurations, a wide range of α-selective or γ-selective spirocyclic benzofuranones with either aryl or alkyl substitutions could be readily prepared. DFT calculations suggest that the conformation of the dipeptide phosphines influences the hydrogen bonding interactions or the distortion energy, resulting in delicate energy differentiation in the transition states, and accounting for the observed regioselectivity.

Highly Stereoselective Synthesis of a Compound Collection Based on the Bicyclic Scaffolds of Natural Products

Despite the great contribution of natural products in the history of successful drug discovery, there are significant limitations that persuade the pharmaceutical industry to evade natural products in drug discovery research. The extreme scarcity as well as structural complexity of natural products renders their practical synthetic access and further modifications extremely challenging. Although other alternative technologies, particularly combinatorial chemistry, were embraced by the pharmaceutical industry to get quick access to a large number of small molecules with simple frameworks that often lack three-dimensional complexity, hardly any success was achieved in the discovery of lead molecules. To acquire chemotypes beholding structural features of natural products, for instance high sp³ character, the synthesis of compound collections based on core-scaffolds of natural products presents a promising strategy. Here, we report a natural product inspired synthesis of six different chemotypes and their derivatives for drug discovery research. These bicyclic hetero- and carbocyclic scaffolds are highly novel, rich in sp³ features and with ideal physicochemical properties to display drug likeness. The functional groups on the scaffolds were exploited further to generate corresponding compound collections. Synthesis of two of these collections exemplified with ca. 350 compounds are each also presented. The whole compound library is being exposed to various biological screenings within the European Lead Factory consortium.

Chiral phosphine-mediated intramolecular [3 + 2] annulation: enhanced enantioselectivity by achiral Brønsted acid

Enantioselective intramolecular [3 + 2] annulation of chalcones bearing an allene moiety has been successfully developed. The reaction was effectively promoted by amino acid-derived phosphines, in combination with achiral Brønsted acids. Dihydrocoumarin architectures were constructed in high yields and with excellent enantiomeric excesses. Theoretical studies via DFT calculations revealed that the hydrogen bonding network induced by achiral Brønsted acids/chiral phosphines could more efficiently distinguish between two enantioselective pathways, thus leading to enhanced enantioselectivity.

Enantiodivergent Combination of Natural Product Scaffolds Enabled by Catalytic Enantioselective Cycloaddition

An efficient strategy has been established for the enantiodivergent synthesis of natural product inspired compounds embodying both tropane and pyrrolidine natural product fragments. This strategy includes the enantioselective kinetic resolution of racemic tropanes by means of a copper(I)-catalyzed [3+2] cycloaddition and allows the preparation of two enantiopure products in a one-pot reaction in high yield and with high diastereo- and enantioselectivity by using one chiral catalyst.

Highly enantioselective synthesis of dihydrocoumarin-fused dihydropyrans via the phosphine-catalyzed [4 + 2] annulation of allenones with 3-aroylcoumarins

Phosphine-catalyzed [4 + 2] annulation between 3-aroylcoumarins and allenones has been developed. In the presence of a dipeptide phosphine catalyst 7, dihydrocoumarin-fused dihydropyrans were prepared in high yields and with excellent enantioselectivities.

Counting on natural products for drug design

Natural products and their molecular frameworks have a long tradition as valuable starting points for medicinal chemistry and drug discovery. Recently, there has been a revitalization of interest in the inclusion of these chemotypes in compound collections for screening and achieving selective target modulation. Here we discuss natural-product-inspired drug discovery with a focus on recent advances in the design of synthetically tractable small molecules that mimic nature's chemistry. We highlight the potential of innovative computational tools in processing structurally complex natural products to predict their macromolecular targets and attempt to forecast the role that natural-product-derived fragments and fragment-like natural products will play in next-generation drug discovery.

Enantioselective [3 + 2] annulation of α-substituted allenoates with β,γ-unsaturated N-sulfonylimines catalyzed by a bifunctional dipeptide phosphine

The first enantioselective [3 + 2] annulation of α-substituted allenoates with β,γ-unsaturated N-sulfonylimines is described. In the presence of a dipeptide phosphine catalyst, a wide range of highly functionalized cyclopentenes bearing an all-carbon quaternary center were obtained in moderate to good yields and with good to excellent enantioselectivities.

Synthesis of Neurotrophic Seco-prezizaane Sesquiterpenes (1R,10S)-2-Oxo-3,4-dehydroneomajucin, (2S)-Hydroxy-3,4-dehydroneomajucin, and (-)-Jiadifenin

An asymmetric approach to the synthesis of neurotrophic seco-prezizaane sesquiterpenes is described that is based on the strategic application of a hydroxyl-directed metallacycle-mediated [2 + 2 + 2] annulation and an intramolecular radical cyclization cascade. Targets prepared are among the most potent members of the natural product class and include (1R,10S)-2-oxo-3,4-dehydroneomajucin, (2S)-hydroxy-3,4-dehydroneomajucin, and (-)-jiadifenin. In addition to representing the first application of the alkoxide-directed metallacycle-mediated hydrindane-forming annulation reaction in natural product synthesis and the first total synthesis of (2S)-hydroxy-3,4-dehydroneomajucin, these pursuits have resulted in the elucidation of a complex radical cascade process for installation of the C5 quaternary center common to the natural product class.

l-Isoleucine derived bifunctional phosphine catalyses asymmetric [3 + 2]-annulation of allenyl-esters and -ketones with ketimines

l-Isoleucine derived bifunctional N-acylaminophosphine catalyzed a [3 + 2]-annulation reaction between allenyl carbonyl compounds and isatinimines to afford a facile and asymmetric access to 3,2′-dihydropyrrolyl spirooxindoles.

Hes1 inhibitor isolated by target protein oriented natural products isolation (TPO-NAPI) of differentiation activators of neural stem cells

Hes1 dimer inhibitor, agalloside (2), which can accelerate the differentiation of neural stem cells was isolated by using Hes1-immobilized beads.

The Hes1 dimer inhibitor, agalloside (2), which can accelerate the differentiation of neural stem cells is described. Six natural products, including one new natural product, which bind to Hes1 were rapidly isolated by a developed “target protein oriented natural products isolation” (TPO-NAPI) method using Hes1-immobilized beads. Of the six compounds, 2 inhibited Hes1 dimer formation at both the protein- and cellular level. Neural stem cells treated with 2 differentiated to neurons with longer neurites than cells treated with varproic acid or retinoic acid. Moreover, 2 exhibited specificity for neurons. This promotion of differentiation was supported by an increase in the mRNA expression of the proneural genes, Mash1 and Ngn2, which were inhibited by Hes1.

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.

A general catalytic reaction sequence to access alkaloid-inspired indole polycycles

A general two-step catalytic reaction sequence affording a range of indole alkaloid-inspired complex molecular frameworks is presented.

Phosphine-catalyzed dearomatizing [3+2] annulations of isoquinolinium methylides with allenes

A phosphine-catalyzed annulation provides an access to highly functionalized pyrroloisoquinolines with high regioselectivity.