Biomimetic Organocatalytic Asymmetric Synthesis of 2-Substituted Piperidine-Type Alkaloids and Their Analogues

Unveiling New Reactivities in Complex Mixtures: Synthesis of Tricyclic Pyridinium Derivatives

The discovery of new transformations drives the development of synthetic organic chemistry. While the main goal of synthetic chemists is to obtain the maximum yield of a desired product with minimal side product formation, meticulous characterization of the latter offers an opportunity for discovering new reaction pathways, alternative mechanisms, and new products. Herein, we present a case study on the discovery and development of a new chemical transformation using online mass spectrometry. This highly sensitive method enabled the discovery of a new reaction pathway in a catalyst-free cross-dehydrogenative coupling of 1,2,3,4-tetrahydroisoquinoline with acetone via peroxide intermediate, ultimately yielding a tricyclic pyridinium compound. Mass spectrometry was instrumental in detecting and identifying the structure of the pyridinium compound, initially formed as a trace byproduct, which allowed us to develop a general methodology for its exclusive formation.

The Chemistry of the Defensive Secretions of Three Species of Millipedes in the Genus Brachycybe

Millipedes have long been known to produce a diverse array of chemical defense agents that deter predation. These compounds, or their precursors, are stored in high concentration within glands (ozadenes) and are released upon disturbance. The subterclass Colobognatha contains four orders of millipedes, all of which are known to produce terpenoid alkaloids-spare the Siphonophorida that produce terpenes. Although these compounds represent some of the most structurally-intriguing millipede-derived natural products, they are the least studied class of millipede defensive secretions. Here, we describe the chemistry of millipede defensive secretions from three species of Brachycybe: Brachycybe producta, Brachycybe petasata, and Brachycybe rosea. Chemical investigations using mass spectrometry-based metabolomics, chemical synthesis, and 2D NMR led to the identification of five alkaloids, three of which are new to the literature. All identified compounds are monoterpene alkaloids with the new compounds representing indolizidine (i.e. hydrogosodesmine) and quinolizidine alkaloids (i.e. homogosodesmine and homo-hydrogosodesmine). The chemical diversity of these compounds tracks the known species phylogeny of this genus, rather than the geographical proximity of the species. The indolizidines and quinolizidines are produced by non-sympatric sister species, B. producta and B. petasata, while deoxybuzonamine is produced by another set of non-sympatric sister species, B. rosea and Brachycybe lecontii. The fidelity between the chemical diversity and phylogeny strongly suggests that millipedes generate these complex defensive agents de novo and begins to provide insights into the evolution of their biochemical pathways.

An acyl-CoA thioesterase is essential for the biosynthesis of a key dauer pheromone in C. elegans

Methyl ketone (MK)-ascarosides represent essential components of several pheromones in Caenorhabditis elegans, including the dauer pheromone, which triggers the stress-resistant dauer larval stage, and the male-attracting sex pheromone. Here, we identify an acyl-CoA thioesterase, ACOT-15, that is required for the biosynthesis of MK-ascarosides. We propose a model in which ACOT-15 hydrolyzes the β-keto acyl-CoA side chain of an ascaroside intermediate during β-oxidation, leading to decarboxylation and formation of the MK. Using comparative metabolomics, we identify additional ACOT-15-dependent metabolites, including an unusual piperidyl-modified ascaroside, reminiscent of the alkaloid pelletierine. The β-keto acid generated by ACOT-15 likely couples to 1-piperideine to produce the piperidyl ascaroside, which is much less dauer-inducing than the dauer pheromone, asc-C6-MK (ascr#2, 1). The bacterial food provided influences production of the piperidyl ascaroside by the worm. Our work shows how the biosynthesis of MK- and piperidyl ascarosides intersect and how bacterial food may impact chemical signaling in the worm.

The regioselective Wacker oxidation of internal allylamines: synthesis of functionalized and challenging β-amino ketones

A convenient and general protocol for the palladium-catalysed oxidation of internal allylamine derivatives to β-amino ketones is reported. The transformation occurs at room temperature and shows a wide substrate scope as well as high functional group and N-protecting group tolerance. We also describe potential applications of the method, e.g., the synthesis of bioactive molecules or simple transformations of selected β-amino ketones into other interesting building blocks.

Structure-Guided Design of Halofuginone Derivatives as ATP-Aided Inhibitors Against Bacterial Prolyl-tRNA Synthetase

Aminoacyl-tRNA synthetases (aaRSs) are promising antimicrobial targets due to their essential roles in protein translation, and expanding their inhibitory mechanisms will provide new opportunities for drug discovery. We report here that halofuginone (HF), an herb-derived medicine, moderately inhibits prolyl-tRNA synthetases (ProRSs) from various pathogenic bacteria. A cocrystal structure of Staphylococcus aureus ProRS (SaProRS) with HF and an ATP analog was determined, which guided the design of new HF analogs. Compound 3 potently inhibited SaProRS at IC₅₀ = 0.18 μM and K_d = 30.3 nM and showed antibacterial activities with an MIC of 1-4 μg/mL in vitro. The bacterial drug resistance to 3 only developed at a rate similar to or slower than those of clinically used antibiotics in vitro. Our study indicates that the scaffold and ATP-aided inhibitory mechanism of HF could apply to bacterial ProRS and also provides a chemical validation for using bacterial ProRS as an antibacterial target.

Stereoselective synthesis of analogues of deoxyfebrifugine

The preparation of six new optically active analogues of the natural product febrifugine (1) is reported. These analogues, lacking the hydroxy group from the natural product, were prepared from optically active N-protected S-pelletierine (7) and differ in terms of the specific quinazolinone portion included. The required S-7 (80% enantiomeric excess) was prepared from an asymmetric Mannich reaction between piperideine (8) and acetone in the presence of l-proline. The differently substituted quinazolinone used in this study (10a–10g) was either commercially available or was prepared from the corresponding substituted anthranilic acid and were installed via a bromination–alkylation sequence. N-Deprotection of the subsequent adducts (12a–12g) gave target compounds 13a–13f and completed the synthetic sequence.

Combining bio- and organocatalysis for the synthesis of piperidine alkaloids

There is continued interest in developing cascade processes for the synthesis of key chiral building blocks and bioactive natural products (or analogues). Here, we report a hybrid bio-organocatalytic cascade for the synthesis of a small panel of 2-substituted piperidines, relying on a transaminase to generate a key reactive intermediate for the complexity building Mannich reaction.

A metabolic regulon reveals early and late acting enzymes in neuroactive Lycopodium alkaloid biosynthesis

Plants synthesize many diverse small molecules that affect function of the mammalian central nervous system, making them crucial sources of therapeutics for neurological disorders. A notable portion of neuroactive phytochemicals are lysine-derived alkaloids, but the mechanisms by which plants produce these compounds have remained largely unexplored. To better understand how plants synthesize these metabolites, we focused on biosynthesis of the Lycopodium alkaloids that are produced by club mosses, a clade of plants used traditionally as herbal medicines. Hundreds of Lycopodium alkaloids have been described, including huperzine A (HupA), an acetylcholine esterase inhibitor that has generated interest as a treatment for the symptoms of Alzheimer's disease. Through combined metabolomic profiling and transcriptomics, we have identified a developmentally controlled set of biosynthetic genes, or potential regulon, for the Lycopodium alkaloids. The discovery of this putative regulon facilitated the biosynthetic reconstitution and functional characterization of six enzymes that act in the initiation and conclusion of HupA biosynthesis. This includes a type III polyketide synthase that catalyzes a crucial imine-polyketide condensation, as well as three Fe(II)/2-oxoglutarate-dependent dioxygenase (2OGD) enzymes that catalyze transformations (pyridone ring-forming desaturation, piperidine ring cleavage, and redox-neutral isomerization) within downstream HupA biosynthesis. Our results expand the diversity of known chemical transformations catalyzed by 2OGDs and provide mechanistic insight into the function of noncanonical type III PKS enzymes that generate plant alkaloid scaffolds. These data offer insight into the chemical logic of Lys-derived alkaloid biosynthesis and demonstrate the tightly coordinated coexpression of secondary metabolic genes for the biosynthesis of medicinal alkaloids.

Diversification of Unprotected Alicyclic Amines by C-H Bond Functionalization: Decarboxylative Alkylation of Transient Imines

Despite extensive efforts by many practitioners in the field, methods for the direct α-C-H bond functionalization of unprotected alicyclic amines remain rare. A new advance in this area utilizes N-lithiated alicyclic amines. These readily accessible intermediates are converted to transient imines through the action of a simple ketone oxidant, followed by alkylation with a β-ketoacid under mild conditions to provide valuable β-amino ketones with unprecedented ease. Regioselective α'-alkylation is achieved for substrates with existing α-substituents. The method is further applicable to the convenient one-pot synthesis of polycyclic dihydroquinolones through the incorporation of a SN Ar step.

Enantioselective Reductive Coupling of Imines Templated by Chiral Diboron

We herein report a general, practical, and highly efficient method for asymmetric synthesis of a wide range of chiral vicinal diamines via reductive coupling of imines templated by chiral diboron. The protocol features high enantioselectivity and stereospecificity, mild reaction conditions, simple operating procedures, use of readily available starting materials, and a broad substrate scope. The method signifies the generality of diboron-enabled [3,3]-sigmatropic rearrangement.

Switching Electrophile Intermediates to Nucleophiles: Michael and Oxa-Diels-Alder Reactions to Afford Polyoxy-Functionalized Piperidine Derivatives with Tetrasubstituted Carbon

Michael, Michael-annulation, and oxa-Diels-Alder reactions of carbohydrate derivatives that afford polyoxy-functionalized piperidine derivatives bearing tetrasubstituted carbon at the 3-position of the piperidine ring are reported. Iminium ions generated from carbohydrate derivatives with amines were converted to enamines in situ, which acted as nucleophiles. As a result, substituents were introduced at the 3-position or both 2- and 3-positions of the piperidines bearing polyoxy groups. This strategy will be useful in drug discovery efforts.

From synthetic control to natural products: a focus on N-heterocycles

Natural products containing a N-heterocycle motif are widespread in nature and medicinal plants, in particular, have proved to be a source of almost unlimited N-derived structures with high molecular diversity. Because of their intrinsic potential for use in both biomedical and agricultural applications, there is a general need for new compounds and for the synthesis of 'natural-inspired' analogues. Importantly, transition of a natural product from discovery to a 'market lead' is associated with an increasingly challenging demand for more of the compound, which cannot be met by isolation from natural plant sources, often due to low extraction yields and uneven availability of the plant source itself. Synthesis remains the most reliable approach to provide valuable products for the market. In this review, a comprehensive overview of our contribution to synthetic access to N-derived natural products is given. Major strengths of the proposed methodologies are discussed critically. © 2019 Society of Chemical Industry.

Biomimetic Organocatalytic Approach to 4-Arylquinolizidine Alkaloids and Application in the Synthesis of (-)-Lasubine II and (+)-Subcosine II

An enantioselective, biomimetic organocatalytic synthesis of 4-arylquinolizidin-2-ones, key intermediates in the synthesis of several Lythraceae alkaloids, was developed. The methodology features S-proline-mediated Mannich/aza-Michael reactions of readily available arylideneacetones and Δ¹-piperideine. The total syntheses of (-)-lasubine II and (+)-subcosine II as well as the formal syntheses of structurally related Lythraceae alkaloids were achieved. The use of Δ¹-pyrroline in the Mannich/aza-Michael reaction provides enantiomerically enriched 5-arylindolizidin-7-ones, which are precursors to nonopiate antinociceptive agents.

Mannich Reactions of Carbohydrate Derivatives with Ketones To Afford Polyoxy-Functionalized Piperidines

Mannich reactions of carbohydrate derivatives with ketones that afford polyoxy-functionalized piperidines are reported. Ketone nucleophiles (enamines/enolates) were generated in the presence of the amines used for the formation of the iminium ions of sugar derivatives with or without an additive. Conditions to preferentially generate piperidine derivatives rather than tetrahydrofurans were identified. Products from the reactions of allyl ketones were readily transformed to bicyclic piperidines.

Transition-Metal Acetate-Promoted Intramolecular Nitrene Insertion to Vinylogous Carbonates for Divergent Synthesis of Azirinobenzoxazoles and Benzoxazines

Synthesis and isolation of highly unstable azirinobenzoxazole and benzoxazines in a chemodivergent fashion from aryl azido vinylogous carbonates by simple change in transition metal acetate is described. Thermal or rhodium(II) acetate-mediated decomposition of these azides gave dihydroazirino benzoxazole. Their nickel(II) acetate-promoted reaction gave 4-dihydro-2H-benzoxazines, whereas copper(II) acetate led to the corresponding oxidized imine derivatives. Benzaoxazine derivative could be kinetically resolved using a proline-catalyzed Mannich reaction. The benzoxazines were rapidly elaborated to angularly fused tetracyclic systems and coumarin-fused derivatives in a "one pot" fashion.

Mannich-type Reactions of Cyclic Nitrones: Effective Methods for the Enantioselective Synthesis of Piperidine-containing Alkaloids

Even though there are dozens of biologically active 2-substituted and 2,6-disubstituted piperidines, only a limited number of approaches exist for their synthesis. Herein is described two Mannich-type additions to nitrones, one using β-ketoacids under catalyst-free conditions and another using methyl ketones in the presence of chiral thioureas, which can generate a broad array of such 2-substituted materials, as well as other ring variants, in the form of β-N-hydroxy-aminoketones. Both processes have broad scope, with the latter providing products with high enantioselectivity (up to 98 %). The combination of these methods, along with other critical steps, has enabled 8-step total syntheses of the 2,6-disubstituted piperidine alkaloids (-)-lobeline and (-)-sedinone.

Iridium-Catalyzed Highly Regioselective and Diastereoselective Allylic Etherification To Access cis-2,6-Disubstituted Dihydropyridinones

A highly regio- and diastereoselective method to access cis-2,6-disubstituted dihydropyridinones under mild conditions by an iridium-catalyzed allylic etherification is reported. cis-2,6-Disubstituted dihydropyridinones are important precursors for the de novo synthesis of the corresponding piperidine alkaloids and iminosugars. This strategy features a broad substrate scope, high yields, and excellent regio- and diastereoselectivities. A π-allyl-Ir intermediate is involved in the mechanism. The strong A^1,3-strain from the tosyl group may also favor the formation of cis-products in this transformation.

3-Hydroxy-1,5-dihydro-2H-pyrrol-2-ones as novel antibacterial scaffolds against methicillin-resistant Staphylococcus aureus

Herein, we report the synthesis and evaluation of 3-hydroxy-1,5-dihydro-2H-pyrrol-2-ones as antibacterial agents against methicillin-resistant S. aureus (MRSA) and methicillin-resistant S. epidermidis (MRSE). Lead compound 38 showed minimum inhibitory concentrations (MICs) of 8 and 4 μg/mL against MRSA and MRSE, respectively. Furthermore, compound 38 displayed a MIC of 8-16 μg/mL against linezolid-resistant MRSA. These molecules, previously underexplored as antibacterial agents, serve as a new scaffold for antimicrobial development.

Biomimetic synthesis of 2-substituted N-heterocycle alkaloids by one-pot hydrolysis, transamination and decarboxylative Mannich reaction

A novel multi-enzymatic approach enabled the facile synthesis of a broad range of biologically active 2-substituted piperidine and pyrrolidine alkaloids.

Direct Access to Highly Functionalized Heterocycles through the Condensation of Cyclic Imines and α-Oxoesters

A facile, gram-scale preparation of 2-hydroxy-5,6,7,7a-tetrahydro-3H-pyrrolizin-3-ones and 2-hydroxy-6,7,8,8a-tetrahydroindolizin-3(5H)-ones from a condensation cyclization of α-oxoesters with five- and six-membered cyclic imines, respectively, is reported. This transformation enables a concise, three-step synthesis of the natural products phenopyrrozin and p-hydroxyphenopyrrozin. Further, biologically relevant scaffolds, such as α-quaternary β-homo prolines and β-lactams, are also prepared in two- to three-steps from the versatile 2-hydroxy-5,6,7,7a-tetrahydro-3H-pyrrolizin-3-one core.

A telescopic one-pot synthesis of β-lactam rings using amines as a convenient source of imines

A facile synthetic approach to substituted β-lactams was designed, using secondary benzylic amines and acid chlorides as starting materials. The reactions proceeded smoothly and all the products were obtained in good yields.

Biomimetic Total Syntheses of (+)-Dihydrolyfoline and (-)-5-epi-Dihydrolyfoline

The first asymmetric total syntheses of (+)-dihydrolyfoline and (-)-5-epi-dihydrolyfoline have been achieved in five and six steps with 4.6% and 14% overall yields, respectively, in which the chiral biaryl axes were constructed in a highly regioselective and stereoselective manner via a biogenetic enzymatic oxidative couplings of phenols, and the requisite quinolizidinone cores were prepared by an enzymatic Mannich reaction.

Building polyfunctional piperidines: a stereoselective strategy of a three-component Mannich reaction inspired by biosynthesis and applications in the synthesis of natural alkaloids (+)-241D; (−)-241D; isosolenopsin A and (−)-epimyrtine

A stereoselective strategy for building polyfunctional piperidines by a three-component Mannich reaction and applications in the stereoselective synthesis of natural alkaloids.

A highly enantioselective Mannich reaction of aldehydes with cyclic N-acyliminium ions by synergistic catalysis

A novel enantioselective synthesis of carbamoyl isoquinoline and tetrahydropyridine derivatives is accomplished using matched combinations of Lewis or Brønsted acids and secondary amine organocatalysts.