Discovery and development of novel 10,12-disubstituted aloperine derivatives against HCoV-OC43 by targeting allosteric site of host TMPRSS2.

By inducing steric activation of the 10CH bond with a 12-acyl group to form a key imine oxime intermediate, 20 novel (10S)-10,12-disubstituted aloperine derivatives were successfully synthesized and assessed for their antiviral efficacy against HCoV-OC43. Of them, compound 3i exhibited the moderate activities against HCoV-OC43, as well as against the SARS-CoV-2 variant EG.5.1 with the comparable EC50 values of 4.7 and 4.1 µM. A mechanism study revealed that it inhibited the protease activity of host TMPRSS2 by binding to an allosteric site, rather than the known catalytic center, different from that of camostat. Also, the combination of compound 3i and molnupiravir, as an RdRp inhibitor, showed an additive antiviral effect against HCoV-OC43. The results provide a new binding mode and lead compound for targeting TMPRSS2, with an advantage in combating broad-spectrum coronavirus.

Structure-activity relationship and biological evaluation of 12 N-substituted aloperine derivatives as PD-L1 down-regulatory agents through proteasome pathway.

Twenty-nine 12 N-substituted aloperine derivatives were synthesized and screened for suppression on PD-L1 expression in H460 cells, as a continuation of our work. Systematic structural modifications led to the identification of compound 6b as the most active PD-L1 modulator. Compound 6b could significantly down-regulate both constitutive and inductive PD-L1 expression in NSCLC cells, and successively enhance the cytotoxicity of co-cultured T cells against tumor cells at the concentration of 20 µM. Also, it exhibited a moderate in vivo anticancer efficacy against Lewis tumor xenograft with a stable PK and safety profile. The mechanism study indicated that 6b mediated the degradation of PD-L1 through a proteasome pathway, rather than a lysosome route. These results provided the powerful information for cancer immunotherapy of aloperine derivatives with unique endocyclic skeleton by targeting PD-L1 to activate immune cells to kill cancer cells.

Discovery and evolution of 12N-substituted aloperine derivatives as anti-SARS-CoV-2 agents through targeting late entry stage.

So far, there is still no specific drug against COVID-19. Taking compound 1 with anti-EBOV activity as the lead, fifty-four 12N-substituted aloperine derivatives were synthesized and evaluated for the anti-SARS-CoV-2 activities using pseudotyped virus model. Among them, 8a exhibited the most potential effects against both pseudotyped and authentic SARS-CoV-2, as well as SARS-CoV and MERS-CoV, indicating a broad-spectrum anti-coronavirus profile. The mechanism study disclosed that 8a might block a late stage of viral entry, mainly via inhibiting host cathepsin B activity rather than directly targeting cathepsin B protein. Also, 8a could significantly reduce the release of multiple inflammatory cytokines in a time- and dose-dependent manner, such as IL-6, IL-1ß, IL-8 and MCP-1, the major contributors to cytokine storm. Therefore, 8a is a promising agent with the advantages of broad-spectrum anti-coronavirus and anti-cytokine effects, thus worthy of further investigation.

Recent progress on the total syntheses of macrocyclic diamine alkaloids.

Covering: 2006 to 2019Macrocyclic diamine alkaloids derived from 3-alkyldihydropyridine dimers comprise a diverse and highly complex family of natural products. The macrocyclic and caged structural features of these alkaloids have inspired many creative solutions from the synthetic organic community over the past 30 years. This review will cover the successful synthetic campaigns over the past decade, with a focus on (1) key bond disconnections and advances and (2) remaining challenges and opportunities for innovation within this natural product class.

Stereodirected synthesis of alkaloid-like quinolizidine systems.

New stereoselective methods for the chemical modification of cytisine based on T-reactions are reported. A reaction of cytisine with 2-chloro-5-nitrobenzaldehyde and followed condensation with 1,3-dimethylbarbituric acid affords N-(5-nitro-2-{1,3-dimethylperhydropyrimidine-2,4,6-trione-5-methynyl})cytisine, which undergoes a cyclization with the tetrahydropyridine ring closure. The cyclization proceeds via two competing routes yielding 5,5-spirobarbituric acid derivatives with 11,19-diaza-pentacyclo[11.7.1.02,11.05,10.014,19]henicosane and 11,15-diazapentacyclo-[11.7.1.02,11.05,10.015,20]henicosane skeletons. The cyclization reaction in solutions afford either 24.25-trans and 15,16-trans isomers or trans and cis isomer mixtures, depending on the specific solvent. Meanwhile, 24,25-cis and 15,16-cis isomers are formed stereoselectively under heterogeneous conditions in water suspensions. Trans-5,5-spirobarbiturates under similar conditions undergo isomerization into more stable cis-analogs by the configuration inversion at the C7 atom. The synthesized 5,5-spirobarbituric acid derivatives were successfully converted into alkaloid-like quinolizidine systems (1R,2R,3R,13S)-7-nitro-18-oxo-11,19-diazapentacyclo[11.7.1.02,11.05,10.014,19]henicosa-5(10),6,8,14,16-pentaene-3-carboxylic acid and (1R,2S,3S,13S)-nitro-16-oxo-11,15-diazapentacyclo[11,7,1.02,11,05,10,015,20]henicosa-5,7,9,17,19-pentaene-3-carboxylic acid and their derivatives via the spiropyrimidine moiety removal by the stereoselective hydrolysis. The molecular and crystal structures of the target substances were elucidated by X-ray crystallography and NMR.

Advances on the Bioactivities, Total Synthesis, Structural Modification, and Structure-Activity Relationships of Cytisine Derivatives.

Cytisine is a quinolizidine alkaloid isolated from various Leguminosae plants. Cytisine and its derivatives exhibit a broad range of biological properties, such as smoking cessation aid, antidepressant, neuroprotective, nootropic, anticancer, antiviral, antiparasitic, antidiabetic, insecticidal, and nematicidal activities. In this review, the progress of cytisine and its derivatives in regard to bioactivities, total synthesis, structural modifications focusing on their N-12 position and lactam ring is reported. Additionally, the structure-activity relationships of cytisine and its derivatives are also discussed.

Synthesis of new derivatives of boehmeriasin A and their biological evaluation in liver cancer.

Two series of boehmeriasin A analogs have been synthesized in short and high yielding processes providing derivatives differing either in the alkaloid's pentacyclic scaffold or its peripheral substitution pattern. These series have enabled, for the first time, comparative studies into key biological properties revealing a new lead compound with exceptionally high activity against liver cancer cell lines in the picomolar range for both well (Huh7, Hep3B and HepG2) and poorly (Mahlavu, FOCUS and SNU475) differentiated cells. The cell death was characterized as apoptosis by cytochrome-C release, PARP protein cleavage and SubG1 cell cycle arrest. Subsequent testing associated apoptosis via oxidative stress with in situ formation of reactive oxygen species (ROS) and altered phospho-protein levels. Compound 19 decreased Akt protein phosphorylation which is crucially involved in liver cancer tumorigenesis. Given its simple synthetic accessibility and intriguing biological properties this new lead compound could address unmet challenges within liver cancer therapy.

Oxidative Asymmetric Formal Aza-Diels⁻Alder Reactions of Tetrahydro-ß-carboline with Enones in the Synthesis of Indoloquinolizidine-2-ones.

Ru-catalyzed tandem amine oxidative dehydrogenation/formal aza-Diels⁻Alder reaction for enantio- and diastereoselective synthesis of indoloquinolizidine-2-ones from tetrahydro-ß-carbolines and α,ß-unsaturated ketones is described. The reaction proceeds via tandem ruthenium-catalyzed amine dehydrogenation using tert-butyl hydroperoxide (TBHP) as the oxidant and a chiral thiourea-catalyzed formal aza-[4 + 2] cycloaddition, providing a step-economical strategy for the synthesis of these valuable heterocyclic products.

Enantioselective Total Syntheses of Methanoquinolizidine-Containing Akuammiline Alkaloids and Related Studies.

The akuammiline alkaloids are a structurally diverse class of bioactive natural products isolated from plants found in various parts of the world. A particularly challenging subset of akuammiline alkaloids are those that contain a methanoquinolizidine core. We describe a synthetic approach to these compounds that has enabled the first total syntheses of (+)-strictamine, (-)-2( S)-cathafoline, (+)-akuammiline, and (-)-Ψ-akuammigine. Our strategy relies on the development of the reductive interrupted Fischer indolization reaction to construct a common pentacyclic intermediate bearing five contiguous stereocenters, in addition to late-stage formation of the methanoquinolizidine framework using a deprotection-cyclization cascade. The total syntheses of (-)-Ψ-akuammigine and (+)-akuammiline mark the first preparations of akuammiline alkaloids containing both a methanoquinolizidine core and vicinal quaternary centers. Lastly, we describe the bioinspired reductive rearrangements of (+)-strictamine and (+)-akuammiline to ultimately provide (-)-10-demethoxyvincorine and a new analogue thereof.

Enantiopure Indolo[2,3-a]quinolizidines: Synthesis and Evaluation as NMDA Receptor Antagonists.

Enantiopure tryptophanol is easily obtained from the reduction of its parent natural amino acid trypthophan (available from the chiral pool), and can be used as chiral auxiliary/inductor to control the stereochemical course of a diastereoselective reaction. Furthermore, enantiopure tryptophanol is useful for the syntheses of natural products or biological active molecules containing the aminoalcohol functionality. In this communication, we report the development of a small library of indolo[2,3-a]quinolizidines and evaluation of their activity as N-Methyl d-Aspartate (NMDA) receptor antagonists. The indolo[2,3-a]quinolizidine scaffold was obtained using the following key steps: (i) a stereoselective cyclocondensation of (S)- or (R)-tryptophanol with appropriate racemic δ-oxoesters; (ii) a stereocontrolled cyclization on the indole nucleus. The synthesized enantiopure indolo[2,3-a]quinolizidines were evaluated as NMDA receptor antagonists and one compound was identified to be 2.9-fold more potent as NMDA receptor blocker than amantadine (used in the clinic for Parkinson's disease). This compound represents a hit compound for the development of novel NMDA receptor antagonists with potential applications in neurodegenerative disorders associated with overactivation of NMDA receptors.

Novel Benzo[a]quinolizidine Analogs Induce Cancer Cell Death through Paraptosis and Apoptosis.

Paraptosis is nonapoptotic cell death characterized by massive endoplasmic reticulum (ER)- or mitochondria-derived vacuoles. Induction of paraptosis offers significant advantages for the treatment of chemotherapy-resistant tumors compared with anticancer drugs that rely on apoptosis. Because some natural alkaloids induce paraptotic cell death, a novel series of benzo[a]quinolizidine derivatives were synthesized, and their antiproliferative activity and ability to induce cytoplasmic vacuolation were analyzed. Structural optimization led to the identification of the potent compound 22b, which inhibited cancer cell proliferation in vitro and in vivo and profoundly facilitated paraptosis-like cell death and induced caspase-dependent apoptosis. Further investigation revealed that 22b-mediated vacuolation originated from persistent ER stress and upregulation of LC3B. Paraptosis induced by benzo[a]quinolizidine derivatives thus represents an alternative strategy for cancer chemotherapy.

Formal Synthesis of (+)-Lasubine II and (-)-Subcosine II via Organocatalytic Michael Addition of a Ketone to an α-Nitrostyrene.

The first examples of an organocatalytic Michael addition of a ketone to in situ generated α-nitrostyrenes are reported. A suitably functionalized γ-nitroketone obtained from the organocatalyzed Michael addition was converted into (+)-2-epi-lasubine II, the immediate synthetic precursor of (+)-lasubine II and (-)-subcosine II (enantiomers of the natural quinolizidine alkaloids). Two of the three stereocenters in (+)-2-epi-lasubine II are set by the Michael reaction.

Synthesis of Azabicycles via Cascade Aza-Prins Reactions: Accessing the Indolizidine and Quinolizidine Cores.

The first detailed studies of intramolecular aza-Prins and aza-silyl-Prins reactions, starting from acyclic materials, are reported. The methods allow rapid and flexible access toward an array of [6,5] and [6,6] aza-bicycles, which form the core skeletons of various alkaloids. On the basis of our findings on the aza-Prins and aza-silyl-Prins cyclizations, herein we present simple protocols for the intramolecular preparation of the azabicyclic cores of the indolizidines and quinolizidines using a one-pot cascade process of N-acyliminium ion formation followed by aza-Prins cyclization and either elimination or carbocation trapping. It is possible to introduce a range of different substituents into the heterocycles through a judicial choice of Lewis acid and solvent(s), with halo-, phenyl-, and amido-substituted azabicyclic products all being accessed through these highly diastereoselective processes.

Total Synthesis of the Galbulimima Alkaloids Himandravine and GB17 Using Biomimetic Diels-Alder Reactions of Double Diene Precursors.

The enantioselective total syntheses of himandravine and GB17 were completed through a common biomimetic strategy involving Diels-Alder reactions of unusual double diene containing linear precursors. The double diene precursors, containing or lacking a C12 substituent as required to produce GB17 or himandravine, respectively, were found to undergo Diels-Alder reactions to afford mixtures of regioisomeric cycloadducts that map onto the alternative carbocyclic frameworks of both himandravine and GB17. Computational investigations revealed that these Diels-Alder reactions proceed via transition state structures of similar energy that have a high degree of bispericyclic character and that the low levels of regioselectivity observed in the reactions are a consequence of competing orbital interaction and distortion energies. The combined experimental and computational results provide valuable insights into the biosynthesis of the Galbulimima alkaloids.

A solid-phase combinatorial approach for indoloquinolizidine-peptides with high affinity at D(1) and D(2) dopamine receptors.

Ligands acting at multiple dopamine receptors hold potential as therapeutic agents for a number of neurodegenerative disorders. Specifically, compounds able to bind at D1R and D2R with high affinity could restore the effects of dopamine depletion and enhance motor activation on degenerated nigrostriatal dopaminergic systems. We have directed our research towards the synthesis and characterisation of heterocycle-peptide hybrids based on the indolo[2,3-a]quinolizidine core. This privileged structure is a water-soluble and synthetically accessible scaffold with affinity for diverse GPCRs. Herein we have prepared a solid-phase combinatorial library of 80 indoloquinolizidine-peptides to identify compounds with enhanced binding affinity at D2R, a receptor that is crucial to re-establish activity on dopamine-depleted degenerated GABAergic neurons. We applied computational tools and high-throughput screening assays to identify 9a{1,3,3} as a ligand for dopamine receptors with nanomolar affinity and agonist activity at D2R. Our results validate the application of indoloquinolizidine-peptide combinatorial libraries to fine-tune the pharmacological profiles of multiple ligands at D1 and D2 dopamine receptors.

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.

Boehmeriasin A as new lead compound for the inhibition of topoisomerases and SIRT2.

Two synthetic approaches to boehmeriasin A are described. A gram scale racemic preparation is accompanied by an efficient preparation of both the pure enantiomers using the conformationally stable 2-piperidin-2-yl acetaldehyde as starting material. The anti-proliferative activity in three cancer cell lines (CEM, HeLa and L1210) and two endothelial cell lines (HMEC-1, BAEC) indicates promising activity at the nanomolar range. Topoisomerases and SIRT2 are identified as biological targets and the experimental data has been supported by docking studies.

Aza-Michael reaction of 12-N-carboxamide of (-)-cytisine under high pressure conditions.

The first example of aza-Michael reaction of 12-N-carboxamide of quinolizidine alkaloid (-)-cytisine with α,ß-unsaturated ketones, dimethyl acetylenedicarboxylate and ß-nitrostyrene under high pressure condition has been described. It has been shown that the [4+2]-cycloaddition takes place in the case with N-phenylmaleimide.

Synthesis of polyhydroxylated quinolizidine and indolizidine scaffolds from sugar-derived lactams via a one-pot reduction/Mannich/Michael sequence.

A direct approach to the synthesis of indolizidine and quinolizidine scaffolds of iminosugars is described. The presented strategy is based on a one-pot sugar lactam reduction with Schwartz's reagent followed by a diastereoselective Mannich/Michael tandem reaction of the resulting sugar imine with Danishefsky's diene. The stereochemical course of the investigated reaction has been explained in detail. The obtained bicyclic products are attractive building blocks for the synthesis of various naturally occurring polyhydroxylated alkaloids and their derivatives.

Colorimetric and fluorescent determination of sulfide and sulfite with kinetic discrimination.

Two fluorescent probes, m-PSP and p-PSP , for sulfite and/or sulfide were constructed by connecting a pyridinium ion to a coumarin fluorophore through an α,ß-unsaturated ketone. The presence of the pyridinium salt promoted the nucleophilic addition of sulfite and sulfide to the α,ß-unsaturated ketone, which could be visualized by dramatic changes in the solution's color and fluorescence intensity. Both probes exhibit good selectivity (the selectivity coefficients toward major interferences are less than 0.07) and high sensitivity for sulfite and sulfide over biothiols and other potential analytes. The detection limits of m-PSP for the analysis of sulfite and sulfide are calculated to 8.5 × 10(-7) M and 2.7 × 10(-7) M, respectively. Living cell imaging results indicate that both probes can be applied in biological systems.