On the Origin of the Haouamine Alkaloids

Aniline assisted dimerization of phenylalanines: convenient synthesis of 2-aroyl-3-arylquinoline in an I2-DMSO system

We herein report an efficient synthesis of 2-aroyl-3-arylquinolines from phenylalanines and anilines. The mechanism involves I₂-mediated Strecker degradation enabled catabolism and reconstruction of amino acids and a cascade aniline-assisted annulation. Both DMSO and water act as oxygen sources in this convenient protocol.

Discovery and Biosynthesis of Antimicrobial Phenethylamine Alkaloids from the Marine Flavobacterium Tenacibaculum discolor sv11

The bacterial genus Tenacibaculum has been associated with various ecological roles in marine environments. Members of this genus can act, for example, as pathogens, predators, or episymbionts. However, natural products produced by these bacteria are still unknown. In the present work, we investigated a Tenacibaculum strain for the production of antimicrobial metabolites. Six new phenethylamine (PEA)-containing alkaloids, discolins A and B (1 and 2), dispyridine (3), dispyrrolopyridine A and B (4 and 5), and dispyrrole (6), were isolated from media produced by the predatory bacterium Tenacibaculum discolor sv11. Chemical structures were elucidated by analysis of spectroscopic data. Alkaloids 4 and 5 exhibited strong activity against Gram-positive Bacillus subtilis DSM10, Mycobacterium smegmatis ATCC607, Listeria monocytogenes DSM20600, and Staphylococcus aureus ATCC25923, with minimum inhibitory concentration (MIC) values ranging from 0.5 to 4 μg/mL, and moderate activity against Candida albicans FH2173 and Aspergillus flavus ATCC9170. Compound 6 displayed moderate antibacterial activities against Gram-positive bacteria. Dispyrrolopyridine A (4) was active against efflux pump deficient Escherichia coli ATCC25922 ΔtolC, with an MIC value of 8 μg/mL, as well as against Caenorhabditis elegans N2 with an MIC value of 32 μg/mL. Other compounds were inactive against these microorganisms. The biosynthetic route toward discolins A and B (1 and 2) was investigated using in vivo and in vitro experiments. It comprises an enzymatic decarboxylation of phenylalanine to PEA catalyzed by DisA, followed by a nonenzymatic condensation to form the central imidazolium ring. This spontaneous formation of the imidazolium core was verified by means of a synthetic one-pot reaction using the respective building blocks. Six additional strains belonging to three Tenacibaculum species were able to produce discolins, and several DisA analogues were identified in various marine flavobacterial genera, suggesting the widespread presence of PEA-derived compounds in marine ecosystems.

Carbamoyl Functionalized Bent para-Phenylenes via an Unexpected Reaction of the Burgess Reagent with α-Ketols

The attempted dehydration of macrocyclic α-ketols with the Burgess reagent has resulted in the unexpected synthesis of carbamoylated, bent para-phenylene units. The same reaction with an acyclic analogue affords the intended dehydration product, indicating that the change in reactivity is conformationally controlled and a result of the bifunctional nature of the Burgess reagent.

Chichibabin pyridinium synthesis via oxidative decarboxylation of photoexcited α-enamine acids

A visible light-induced decarboxylative Chichibabin pyridinium synthesis between α-amino acids and aldehydes was developed. When the in situ generated α-enamine acids were photoexcited, they were oxidized by aerobic oxygen to give radical cation species. After decarboxylation and further oxidation, the generated iminium undergoes Chichibabin cyclization to afford pyridiniums. This photochemical protocol enables the synthesis of various tetra-substituted pyridiniums and related natural products in one-step.

Magnesiate-Utilized/Benzyne-Mediated Approach to Indenopyridones from 2-Pyridones: An Attempt To Synthesize the Indenopyridine Core of Haouamine

An efficient, short-staged synthesis of cis-fused indeno[2,1-b]- and indeno[1,2-c]pyridin-2-ones, starting from 2-pyridones, using magnesiates of type R₃MgLi as nucleophilic and deprotonation agents, mediated by benzyne generated in situ, under optimized conditions, is described. Following the developed protocol, rare C4a-arylsubstituted indeno[2,1-b]pyridones, resembling the core of haouamine, were obtained. The protocol offering the one-pot synthesis directly from 2-pyridone is also described.

Nickel-Catalyzed Dearomative trans-1,2-Carboamination

We describe the development of an arenophile-mediated, nickel-catalyzed dearomative trans-1,2-carboamination protocol. A range of readily available aromatic compounds was converted to the corresponding dienes using Grignard reagents as nucleophiles. This strategy provided products with exclusive trans-selectivity and high enantioselectivity was observed in case of benzene and naphthalene. The utility of this methodology was showcased by controlled and stereoselective preparation of small, functionalized molecules.

In situ generation of N-unsubstituted imines from alkyl azides and their applications for imine transfer via copper catalysis

Although azides have been widely used in nitrene transfer reactions, in situ generation of N-H imines from azides for downstream transformations has rarely been explored. We report copper-mediated formation of N-unsubstituted aliphatic imines from easily available aliphatic azides using a customized phenanthroline-based ligand (L₁*). Through trapping in situ-generated N-H imines, multisubstituted pyridines or indoles were readily synthesized. ¹³C-labeled azide was used as part of an isotope labeling study, which suggests that the construction of pyridine derivatives involves a three-component dehydrogenative condensation. The construction of 2,3,5-triaryl pyridines using this method provided evidence supporting a proposed pathway involving both imine formation and abnormal Chichibabin pyridine synthesis. The generation of N-unsubstituted imine intermediates was also confirmed by formation of indole derivatives from alkyl azides.

One-Pot Phosphate-Mediated Synthesis of Novel 1,3,5-Trisubstituted Pyridinium Salts: A New Family of S. aureus Inhibitors

Polysubstituted pyridinium salts are valuable pharmacophores found in many biologically active molecules. Their synthesis typically involves the use of multistep procedures or harsh reaction conditions. Here, we report water-based phosphate mediated reaction conditions that promote the condensation of arylacetaldehydes with amines to give 1,3,5-pyridinium salts. The reaction, carried out at pH 6, provides conditions suitable for the use of less stable aldehydes and amines in this Chichibabin pyridine condensation. The evaluation of selected 1,3,5-trisubstituted pyridinium salts highlighted that they can inhibit the growth of S. aureus in the low μg/mL range. The synthetic accessibility of these compounds and preliminary growth inhibition data may pave the way towards the discovery of new anti-bacterials based on the 1,3,5-trisubstituted pyridinium scaffold.

11-Step Total Synthesis of Araiosamines

A concise route to a small family of exotic marine alkaloids known as the araiosamines has been developed, and their absolute configuration has been assigned. The dense array of functionality, high polarity, and rich stereochemistry coupled with equilibrating topologies present an unusual challenge for chemical synthesis and an opportunity for innovation. Key steps involve the use of a new reagent for guanidine installation, a remarkably selective C-H functionalization, and a surprisingly simple final step that intersects a presumed biosynthetic intermediate. Synthetic araiosamines were shown to exhibit potency against Gram-positive and -negative bacteria despite a contrary report of no activity.

Iron-Mediated One-Pot Synthesis of 3,5-Diarylpyridines from β-Nitrostyrenes

An operationally simple and mild one-pot protocol for the synthesis of a variety of 3,5-diarylpyridines from β-nitrostyrenes was achieved by using elemental iron. This reaction proceeds via reduction of the nitro group, resulting in in situ imine formation followed by trimolecular condensation with concomitant debenzylative aromatization. By employing this method, a series of symmetrical and unsymmetrical 3,5-diarylpyridines were synthesized with good to excellent yields. In addition, this method was also utilized for the synthesis of Sch-21418, an anti-inflammatory agent on gram scale.

Mild and ambient annulations for pyrrole synthesis from amines and arylacetaldehydes

A mild and ambient pyrrole synthesis via copper-catalyzed aerobic cyclization is described.

Maleimide-assisted anti-Markovnikov Wacker-type oxidation of vinylarenes using molecular oxygen as a terminal oxidant

Arylacetaldehydes were successfully synthesized by anti-Markovnikov Wacker-type oxidation of vinylarenes using 1 atm O₂ as a terminal oxidant.

Academia-industry symbiosis in organic chemistry

Collaboration between academia and industry is a growing phenomenon within the chemistry community. These sectors have long held strong ties since academia traditionally trains the future scientists of the corporate world, but the recent drastic decrease of public funding is motivating the academic world to seek more private grants. This concept of industrial “sponsoring” is not new, and in the past, some companies granted substantial amounts of money per annum to various academic institutions in exchange for prime access to all their scientific discoveries and inventions. However, academic and industrial interests were not always aligned, and therefore the investment has become increasingly difficult to justify from industry’s point of view. With fluctuating macroeconomic factors, this type of unrestricted grant has become more rare and has been largely replaced by smaller and more focused partnerships. In our view, forging a partnership with industry can be a golden opportunity for both parties and can represent a true symbiosis. This type of project-specific collaboration is engendered by industry’s desire to access very specific academic expertise that is required for the development of new technologies at the forefront of science. Since financial pressures do not allow companies to spend the time to acquire this expertise and even less to explore fundamental research, partnering with an academic laboratory whose research is related to the problem gives them a viable alternative. From an academic standpoint, it represents the perfect occasion to apply “pure science” research concepts to solve problems that benefit humanity. Moreover, it offers a unique opportunity for students to face challenges from the “real world” at an early stage of their career. Although not every problem in industry can be solved by research developments in academia, we argue that there is significant scientific overlap between these two seemingly disparate groups, thereby presenting an opportunity for a symbiosis. This type of partnership is challenging but can be a win–win situation if both parties agree on some general guidelines, including clearly defined goals and deliverables, biweekly meetings to track research progress, and quarterly or annual meetings to recognize overarching, common objectives. This Account summarizes our personal experience concerning collaborations with various industrial groups and the way it impacted the research programs for both sides in a symbiotic fashion.

A twist of nature – the significance of atropisomers in biological systems

This review encompasses the synthesis and identification of recently detected natural atropisomers with potential therapeutic activity.

Metal free access to quinolines via C–C bond cleavage of styrenes

A new metal free self-sorting tandem reaction between styrenes and anilines to access 2,4-disubstituted quinolines, involving simultaneous C–C and C–N bond formation along with a C–C bond cleavage, has been developed.

Biomimetic Chichibabin pyridine synthesis of the COPD biomarkers and elastin cross-linkers isodesmosine and desmosine

The tetrasubstituted pyridinium amino acids isodesmosine and desmosine are cross-linkers of elastin and are attractive biomarkers for the diagnosis of chronic obstructive pulmonary disease (COPD). In this study, the biomimetic total synthesis of isodesmosine and desmosine via a lanthanide-promoted Chichibabin pyridine synthesis using the corresponding aldehyde and amine hydrochloride is reported.

Probing autoinducer-2 based quorum sensing: the biological consequences of molecules unable to traverse equilibrium states

Bacteria have developed a cell-to-cell communication system, termed quorum sensing (QS), which allows for the population-dependent coordination of their behavior via the exchange of chemical signals. Autoinducer-2 (AI-2), a class of QS signals derived from 4,5-dihydroxy-2,3-pentandione (DPD), has been revealed as a universal signaling molecule in a variety of bacterial species. In spite of considerable interest, the study of putative AI-2 based QS systems remains a challenging topic in part due to the rapid interconversion between the linear and cyclic forms of DPD. Herein, we report the design and development of efficient syntheses of carbocyclic analogues of DPD, which are locked in the cyclic form. The synthetic analogues were evaluated for the modulation of AI-2-based QS in Vibrio harveyi and Salmonella typhimurium. No agonists were uncovered in either V. harveyi or S. typhimurium assay, whereas weak to moderate antagonists were found against V. harveyi. On the basis of NMR analyses and DFT calculations, the heterocyclic oxygen atom within DPD appears necessary to promote hydration at the C3 position of cyclic DPD to afford the active tetrahydroxy species. These results also shed light on the interaction between the heterocyclic oxygen atom and receptor proteins as well as the importance of the linear form and dynamic equilibrium of DPD as crucial requirements for activation of AI-2 based QS circuits.

Use of a tandem Prins/Friedel-Crafts reaction in the construction of the indeno-tetrahydropyridine core of the haouamine alkaloids: formal synthesis of (-)-haouamine A

A tandem Prins/Friedel-Crafts reaction useful for the construction of the indeno-tetrahydropyridine core of the haouamine alkaloids and a formal synthesis of (-)-haouamine A are described.

Construction of the cyclophane core of the hirsutellones via a RCM strategy

Construction of the highly strained [10]-paracyclophane core of the hirsutellones has been completed via an effective RCM strategy.

Chemical and biological studies of nakiterpiosin and nakiterpiosinone

Nakiterpiosin and nakiterpiosinone are two related C-nor-D-homosteroids isolated from the sponge Terpios hoshinota that show promise as anticancer agents. We have previously described the asymmetric synthesis and revision of the relative configuration of nakiterpiosin. We now provide detailed information on the stereochemical analysis that supports our structure revision and the synthesis of the originally proposed and revised nakiterpiosin. In addition, we herein describe a refined approach for the synthesis of nakiterpiosin, the first synthesis of nakiterpiosinone, and preliminary mechanistic studies of nakiterpiosin's action in mammalian cells. Cells treated with nakiterpiosin exhibit compromised formation of the primary cilium, an organelle that functions as an assembly point for components of the Hedgehog signal transduction pathway. We provide evidence that the biological effects exhibited by nakiterpiosin are mechanistically distinct from those of well-established antimitotic agents such as taxol. Nakiterpiosin may be useful as an anticancer agent in those tumors resistant to existing antimitotic agents and those dependent on Hedgehog pathway responses for growth.

Scalable total synthesis and biological evaluation of haouamine A and its atropisomer

A total synthesis of the complex, bent aromatic ring-containing marine alkaloid haouamine A is achieved through a route in which every step (with the exception of the final deprotection) is performed on a gram-scale. This is accomplished through the development of a method for the dehydrogenation of cyclohexenones that allows for point-to-planar chirality transfer. This strategy makes it possible to program the desired atropisomeric outcome from a simple chiral cyclohexenone. By synthesizing atrop-haouamine A, this work has firmly established that natural haouamine exists as a single, nonequilibrating atropisomer. Finally, biological investigations demonstrate that the bent aromatic ring of this natural product is critical for anticancer activity against PC3 cells.

Total synthesis of haouamine A: the indeno-tetrahydropyridine core

A full account of synthetic efforts towards the indeno-tetrahydropyridine core of haouamine A is presented. Initial failed strategies led to the unexpected discovery of a mild abnormal Chichibabin pyridine synthesis and provided knowledge and inspiration for the development of a cascade annulation that has enabled rapid and scalable access to the core in either racemic or enantiopure form.

Chemoselectivity: the mother of invention in total synthesis

IUPAC defines chemoselectivity as "the preferential reaction of a chemical reagent with one of two or more different functional groups", a definition that describes in rather understated terms the single greatest obstacle to complex molecule synthesis. Indeed, efforts to synthesize natural products often become case studies in the art and science of chemoselective control, a skill that nature has practiced deftly for billions of years but man has yet to master. Confrontation of one or perhaps a collection of functional groups that are either promiscuously reactive or stubbornly inert has the potential to unravel an entire strategic design. One could argue that the degree to which chemists can control chemoselectivity pales in comparison to the state of the art in stereocontrol. In this Account, we hope to illustrate how the combination of necessity and tenacity leads to the invention of chemoselective chemistry for the construction of complex molecules. In our laboratory, a premium is placed upon selecting targets that would be difficult or impossible to synthesize using traditional techniques. The successful total synthesis of such molecules demands a high degree of innovation, which in turn enables the discovery of new reactivity and principles for controlling chemoselectivity. In devising an approach to a difficult target, we choose bond disconnections that primarily maximize skeletal simplification, especially when the proposed chemistry is poorly precedented or completely unknown. By choosing such a strategy--rather than adapting an approach to fit known reactions--innovation and invention become the primary goal of the total synthesis. Delivery of the target molecule in a concise and convergent manner is the natural consequence of such endeavors, and invention becomes a prerequisite for success.