Indolizidine and quinolizidine alkaloids

Marine Streptomyces-Derived Novel Alkaloids Discovered in the Past Decade

Natural alkaloids originating from actinomycetes and synthetic derivatives have always been among the important suppliers of small-molecule drugs. Among their biological sources, Streptomyces is the highest and most extensively researched genus. Marine-derived Streptomyces strains harbor unconventional metabolic pathways and have been demonstrated to be efficient producers of biologically active alkaloids; more than 60% of these compounds exhibit valuable activity such as antibacterial, antitumor, anti-inflammatory activities. This review comprehensively summarizes novel alkaloids produced by marine Streptomyces discovered in the past decade, focusing on their structural features, biological activity, and pharmacological mechanisms. Future perspectives on the discovery and development of novel alkaloids from marine Streptomyces are also provided.

The biological activities of quinolizidine alkaloids

Quinolizidine alkaloids isolated from various marine and terrestrial animals and plants are primarily composed of lupinine-, matrine-, and sparteine-type alkaloids. Matrine, phenanthroquinolizidines, bis-quinolizidines, and small molecules from amphibian skins are representative compounds of such alkaloids. Quinolizidine alkaloids harbor anticancer, antibacterial, antiinflammatory, antifibrosis, antiviral, and anti-arrhythmia. In this chapter, we comprehensively outline the biological activity and pharmacological action of quinolizidine alkaloids and discuss new avenues toward the discovery of novel and more efficient drugs based on these naturally occurring compounds. It is urgent for basic research and clinical practice to conduct more targeted comprehensive research based on the lead drugs of quinolizidine alkaloids with significant pharmacological activity.

Inverse hydride shuttle catalysis enables the stereoselective one-step synthesis of complex frameworks

The rapid assembly of complex scaffolds in a single step from simple precursors identifies as an ideal reaction in terms of efficiency and sustainability. Indeed, the direct single-step synthesis of complex alkaloid frameworks remains an unresolved problem at the heart of organic chemistry in spite of the tremendous progress of the discipline. Herein, we present a broad strategy in which dynamically assembled ternary complexes are converted into valuable azabicyclic scaffolds based on the concept of inverse hydride shuttle catalysis. The ternary complexes are readily constructed in situ from three simple precursors and enable a highly modular installation of various substitution patterns. Upon subjection to a unique dual-catalytic system, the transient intermediates undergo an unusual hydride shuttle process that is initiated by a hydride donation event. Furthermore, we show that, in combination with asymmetric organocatalysis, the product alkaloid frameworks are obtained in excellent optical purity.

Using interdisciplinary, phylogeny-guided approaches to understand the evolution of plant metabolism

To cope with relentless environmental pressures, plants produce an arsenal of structurally diverse chemicals, often called specialized metabolites. These lineage-specific compounds are derived from the simple building blocks made by ubiquitous core metabolic pathways. Although the structures of many specialized metabolites are known, the underlying metabolic pathways and the evolutionary events that have shaped the plant chemical diversity landscape are only beginning to be understood. However, with the advent of multi-omics data sets and the relative ease of studying pathways in previously intractable non-model species, plant specialized metabolic pathways are now being systematically identified. These large datasets also provide a foundation for comparative, phylogeny-guided studies of plant metabolism. Comparisons of metabolic traits and features like chemical abundances, enzyme activities, or gene sequences from phylogenetically diverse plants provide insights into how metabolic pathways evolved. This review highlights the power of studying evolution through the lens of comparative biochemistry, particularly how placing metabolism into a phylogenetic context can help a researcher identify the metabolic innovations enabling the evolution of structurally diverse plant metabolites.

SmI2‑Mediated Intermolecular Addition-Elimination of Piperidine and Pyrrolidine N-α-Radicals with Arylacetylene Sulfones

An efficient approach to access α-arylacetylene substituted pyrrolidine and piperidine derivatives has been developed through a samarium diiodide mediated addition-elimination process of pyrrolidine and piperidine N-α-radicals with arylacetylene sulfones.

Gold(I)-Catalyzed Tandem Synthesis of Polycyclic Dihydroquinazolinones

A gold-catalyzed cascade process for the synthesis of dihydroquinazolinone scaffolds was developed. A series of gold catalysts were screened for this tandem transformation, and the (PPh3)AuCl/AgOTf catalyst combination was found to be the best catalyst system. This method is characterized by good yields, high regioselectivity, and broad substrate scope. This method is also applicable to the synthesis of tetracyclic dihydroquinazolinones and seven-membered ring-fused dihydroquinazolinones.

Concise synthesis of bicyclic iminosugars via reductive functionalization of sugar-derived lactams and subsequent RCM reaction

An efficient method for the synthesis of bicyclic iminosugars has been developed. The strategy is based on the partial reduction of sugar-derived lactams by Schwartz's reagent and tandem stereoselective nucleophile addition dictated by Woerpel's model which affords polyhydroxylated cyclic amines as key intermediates. Introduction of a vinyl or allyl group to the iminosugar produces diene derivatives that can be subjected to the ring-closing metathesis reaction (RCM) to furnish polyhydroxylated pyrrolizidine, indolizidine and quinozilidine derivatives in good to excellent yields. This sequence of reactions has been applied to the formal synthesis of hyacinthacine A₂, a polyhydroxylated pyrrolizidine alkaloid.

Biologically active indolizidine alkaloids

Indolizidine alkaloids are chemical constituents isolated from various marine and terrestrial plants and animals, including but not limited to trees, fungi, ants, and frogs, with a myriad of important biological activities. In this review, we discuss the biological activity and pharmacological effects of indolizidine alkaloids and offer new avenues toward the discovery of new and better drugs based on these naturally occurring compounds.

Reductive cyclisations of amidines involving aminal radicals

The first general study of aminal radical cyclisations, triggered by reduction of amidines with SmI₂, delivers quinazolinones with complete diastereocontrol.

Pd-Catalyzed Hydroamination of Alkoxyallenes with Azole Heterocycles: Examples and Mechanistic Proposal

Palladium-catalyzed regio- and enantioselective addition of azole heterocycles to alkoxyallenes was developed (up to 92% yields and up to 94% ee). DFT calculations suggest a new Pd(0)-driven mechanistic pathway proceeding through protonation of the Pd-coordinated allene (4-PdL₂), which develops a strongly nucleophilic character at the central C atom.

Iodine-mediated synthesis of heterocycles via electrophilic cyclization of alkynes

Syntheses of heterocyclesvia5-exoor 6-endo-digiodocyclizations from alkynes.

Synthesis of tricyclic indolizidines from ethyl isocyanoacetate

Tricyclic indolizidines were synthesized in good yields from commercially available ethyl isocyanoacetate by a novel sequential alkylation, thiol-mediated radical cyclization, N-alkylation, and microwave-assisted Pauson-Khand reaction.

Simple Indolizidine and Quinolizidine Alkaloids

This review of simple indolizidine and quinolizidine alkaloids (i.e., those in which the parent bicyclic systems are in general not embedded in polycyclic arrays) is an update of the previous coverage in Volume 55 of this series (2001). The present survey covers the literature from mid-1999 to the end of 2013; and in addition to aspects of the isolation, characterization, and biological activity of the alkaloids, much emphasis is placed on their total synthesis. A brief introduction to the topic is followed by an overview of relevant alkaloids from fungal and microbial sources, among them slaframine, cyclizidine, Steptomyces metabolites, and the pantocins. The important iminosugar alkaloids lentiginosine, steviamine, swainsonine, castanospermine, and related hydroxyindolizidines are dealt with in the subsequent section. The fourth and fifth sections cover metabolites from terrestrial plants. Pertinent plant alkaloids bearing alkyl, functionalized alkyl or alkenyl substituents include dendroprimine, anibamine, simple alkaloids belonging to the genera Prosopis, Elaeocarpus, Lycopodium, and Poranthera, and bicyclic alkaloids of the lupin family. Plant alkaloids bearing aryl or heteroaryl substituents include ipalbidine and analogs, secophenanthroindolizidine and secophenanthroquinolizidine alkaloids (among them septicine, julandine, and analogs), ficuseptine, lasubines, and other simple quinolizidines of the Lythraceae, the simple furyl-substituted Nuphar alkaloids, and a mixed quinolizidine-quinazoline alkaloid. The penultimate section of the review deals with the sizable group of simple indolizidine and quinolizidine alkaloids isolated from, or detected in, ants, mites, and terrestrial amphibians, and includes an overview of the "dietary hypothesis" for the origin of the amphibian metabolites. The final section surveys relevant alkaloids from marine sources, and includes clathryimines and analogs, stellettamides, the clavepictines and pictamine, and bis(quinolizidine) alkaloids.

A direct method for the synthesis of indolizine derivatives from easily available aromatic ketones, pyridines, and acrylonitrile derivatives

A concise and efficient strategy has been proposed to synthesize indolizine derivatives from easily available aryl or heteroaryl methyl ketones, pyridines, and acrylonitriles. The mechanistic pathway involved the integration of iodination, pyridinium ylide synthesis, and 1,3-dipolar cycloaddition. The protocols were found to be highly efficient in terms of high yields, operational simplicity, mild reaction conditions, and easy workup. This method has provided an important supplement for the synthesis of indolizine derivatives via a novel tandem synthesis.

Synthesis of functionalized indolizidines through Pauson-Khand cycloaddition of 2-allylpyrrolidines

A concise entry to functionalized indolizidine scaffolds through a domino 2-aza-Cope-[3 + 2] dipolar cycloaddition and Pauson-Khand [2 + 2 + 1] cyclization has been accomplished. The process was conducted under mild conditions to afford diverse indolizidine systems as single diastereomers in good overall yields.

Copper-catalyzed multicomponent coupling/cycloisomerization reaction between substituted 1-formyl-9H-β-carbolines, secondary amines, and substituted alkynes for the synthesis of substituted 3-aminoindolizino[8,7-b]indoles

A copper-catalyzed efficient one step three component strategy for preparing a library of aminoindolizino[8,7-b]indoles from N-substituted 1-formyl-9H-β-carbolines, secondary amines, and substituted alkynes with high atom economy has been developed.

Palladium-catalyzed annulation of 2,2'-diiodobiphenyls with alkynes: synthesis and applications of phenanthrenes

A range of phenanthrene derivatives were efficiently synthesized by the palladium-catalyzed annulation of 2,2'-diiodobiphenyls with alkynes. The scope, limitations and regioselectivity of the reaction were investigated. The described method was adopted to synthesize 9,10-dialkylphenanthrenes, sterically overcrowded 4,5-disubstituted phenanthrenes and phenanthrene-based alkaloids. Reactions of highly methoxy-substituted biphenyls with 2-(2-propynyl)pyrrolidine and 2-(2-propynyl)piperidine gave 2-(9-phenanthylmethyl)pyrrolidines and 2-(9-phenanthylmethyl)piperidines, respectively. The products were transformed to phenanthroindolizidine and phenanthroquinolizidine alkaloids by the Pictet-Spengler reaction.

Enantiomeric natural products: occurrence and biogenesis

In nature, chiral natural products are usually produced in optically pure form-however, occasionally both enantiomers are formed. These enantiomeric natural products can arise from a single species or from different genera and/or species. Extensive research has been carried out over the years in an attempt to understand the biogenesis of naturally occurring enantiomers; however, many fascinating puzzles and stereochemical anomalies still remain.

The influence of lupin seed germination on the chemical composition and standardized ileal digestibility of protein and amino acids in pigs

The germination process can modify the chemical composition of nutrients in seeds, which can influence the digestibility and utilization of sprouts in animal diets compared to raw seeds. The aims of research were to provide controlled germination process of lupin seeds, monitor the changes in seed composition and determine the influence of the germination on the coefficients of standardized ileal digestibility (SID) of crude protein and amino acids in growing pigs, compared to raw lupin seeds. The seeds of two lupin species were used: yellow (RYL) (Lupinus luteus, cv. Lord) and blue (RBL) (Lupinus angustifolius, cv. Graf). Germination was provided in the dark at 24°C for 4 days. Nutritional and antinutritional compositions of raw and germinated seeds (GYL and GBL, respectively) were analysed. Digestibility study was performed on pigs with an average body weight of 25 kg, and the pigs were surgically fitted with a T-cannula in the distal ileum, with chromic oxide as an indicator. Seed germination increased the crude protein and fibre concentrations, but reduced the levels of the ether extract, nitrogen-free extracts and all amino acids in protein. The content of alkaloids and raffinose family oligosaccharides decreased in both lupin species. Germination had no positive impact (p>0.05) on the SID of crude protein and amino acids. Germination of lupin seeds negatively influenced the SID of lysine and methionine (p<0.05). The results of the research revealed a decrease in the concentrations of antinutritional factors in the sprouts of yellow and blue lupins compared to raw seeds; however, no positive effect was observed on the coefficients of the standardized ileal apparent digestibility of protein and amino acids.

Total synthesis of grandisine D

Total synthesis of grandisine D (5) was achieved by a Brønsted acid mediated Morita-Baylis-Hillman (MBH) ring-closure reaction and stereoselective aldol condensation with (S)-5-methylcyclohexenone (9) as key steps. The MBH approach was also applicable for the construction of the aza-fused bicyclic systems of pyrrolizidine and stemona alkaloids.