Phenanthroquinolizidine Alkaloids Research Articles

Annulation of O-silyl N,O-ketene acetals with alkynes for the synthesis of dihydropyridinones and its application in concise total synthesis of phenanthroindolizidine alkaloids.

The formation of N-heterocycles with multiple substituents is important in organic synthesis. Herein, we report a novel method for the construction of functionalized dihydropyridinone rings through the annulation of an amide α-carbon with a tethered alkyne moiety. The reaction of the amide with the alkyne was achieved via O-silyl N,O-ketene acetal formation and silver-mediated addition. Furthermore, the developed method was applied for the total synthesis of phenanthroindolizidine and phenanthroquinolizidine alkaloids. By varying the coupling partners, a concise and collective total synthesis of these alkaloids was achieved.

Total Synthesis of Cryptopleurine and Its Analogues

AbstractTotal synthesis of phenanthroquinolizidine alkaloid cryptopleurine was achieved in 8 steps from commercially available 2-pyridinecarboxaldehyde and the epoxide derived from methyleugenol. The key intermediate vinyl triflate enables the divergent synthesis of crypto­pleurine derivatives by late-stage installation of various substituents on the C-ring.

Novel seco-phenanthroquinolizidine alkaloids from Indonesian Boehmeria virgata

• Two new seco -phenanthroquinolizidine alkaloids and eleven known compounds isolated from the Boehmeria virgata leaves. • The absolute configurations of new seco -phenanthroquinolizidines were established by CD spectra. • Compound 3 exhibited potent cytotoxic activity against A549, MDA-MB-231, MCF-7, KB and MDR (KB-VIN) cell lines. Two novel seco -phenanthroquinolizidine alkaloids, (−)-(14a R ,15 R )-hydroxyjulandine ( 1 ) and (+)-( S )-3- O -desmethyljulandine ( 2 ), together with eleven known compounds ( 3 – 13 ) were isolated from the leaves of Indonesian Boehmeria virgata . The structures of these compounds were assigned based on the interpretation of 1D/2D NMR and HRMS data. The absolute configurations of seco -phenanthroquinolizidines 1 and 2 were determined from their ECD spectra by comparison with the known phenanthroquinolizidine 3 . The isolated seco -phenathroquinolizidines 1 – 3 were evaluated for antiproliferative activity against five tumor cell lines. Compound 3 exhibited potent cytotoxic activity, while 1 and 2 were not active against all tested cell lines.

Design, Synthesis, Antivirus Activity, and SARs of Phenanthroquinolizidine Alkaloid Derivatives

Our previous investigation revealed that phenanthroindolizidine alkaloid derivatives have a good inhibitory effect against tabacco mosaic virus (TMV) and that the conformation of these alkaloid analogues has a big influence on the anti-TMV activity. Compared with the well-studied phenanthroindolizidine alkaloids, phenanthroquinolizidine alkaloids have gained less attention. To uncover their anti-TMV activity and structure and activity relationships, a series of D-ring opened phenanthroquinolizidine derivatives and 14a-substituted pentacyclic alkaloid analogues were designed, synthesized, and evaluated against TMV for the first time. Bioassay results revealed that most of these synthesized compounds exhibited a good to excellent inhibitory effect, and three of them displayed better activity than Ningnanmycin—one of the most successful plant virucides—thus providing a new anti-TMV lead for further development. Preliminary SARs showed that the pentacyclic structure, 14a-substituent, and hydrogen-bond donors in these alkaloids are crucial for keeping high potent activities.

Possible pharmaceutical applications can be developed from naturally occurring phenanthroindolizidine and phenanthroquinolizidine alkaloids.

Naturally occurring phenanthroindolizidine and phenanthroquinolizidine alkaloids (PIAs and PQAs) are two small groups of herbal metabolites sharing a similar pentacyclic structure with a highly oxygenated phenanthrene moiety fused with a saturated or an unsaturated N-heterocycle (indolizidine/quinolizidine moieties). Natural PIAs and PQAs only could be obtained from finite plant families (such as Asclepiadaceae, Lauraceae and Urticaceae families, etc.). Up to date, more than one hundred natural PIAs, while only nine natural PQAs had been described. PIA and PQA analogues have been applied to the development of potent anticancer agents all along because of their excellent cytotoxic activity. However, in the last two decades, other great biological properties, such as anti-inflammatory and antiviral activities were revealed successively by different pharmacological assays. Especially because of their potent antiviral activity against coronavirus (TGEV, SARS CoV and MHV) and tobacco mosaic virus, PIA and PQA analogues have attracted much pharmaceutical attention again, some of them have been used to present interesting targets for total or semi synthesis, and structure–activity relationship (SAR) study for the development of antiviral agents. In this review, natural PIA and PQA analogues obtained in the last two decades with their herbal origins, key spectroscopic characteristics for structural identification, biological activity with possible SARs and application prospects were systematically summarized. We hope this paper can stimulate further investigations on PIA and PQA analogues as an important source for potential drug discovery.

Antimalarial alkaloid from Hypoestes forskaolii

Following on from previous studies, we brought further our quest for anti-malarial agents isolated from plants grown in the Saudi Arabian Peninsula. Methanolic extracts were prepared from eighteen Saudi plants and then tested in vitro to assess their anti-malarial effects on Plasmodium falciparum K1, (a chloroquine-resistant strain) as well as their cytotoxicity on MRC5 (human diploid embryonic lung cell line) cells. Moderate anti-malarial activity was observed in extracts prepared from Hypoestes forskaolii (Vahl) R. Br. (IC50 value of 5.5 μg/ml) and Rhus retinorrhaea (IC50: 7.71 μg/ml). The remaining sixteen plant extracts appeared to be inactive (IC50 > 12.5 μg/ml). A novel phenanthro-quinolizidine alkaloid, 15β-hydroxycryptopleurine-N-oxide, was isolated from H. forskaolii using bio-guided fractionation procedures. Chloroquine-resistant (K1) and chloroquine-sensitive (FCR3) strains of P. falciparum appeared very sensitive to the anti-malarial activity of 15β-hydroxycryptopleurine-N-oxide, giving IC50 of 6.11 and 5.13 nM respectively. It showed cytotoxicity against MRC5 “IC50 of 24.45 nM” with selectivity indices of 4.0 and 4.76 against K1 and FCR3 strains, respectively. It is our understanding that this is the first account on phenanthro-quinolizidine alkaloids anti-malarial activity on a chloroquine-resistant P. falciparum strain.

Total Synthesis of Phenanthroquinolizidine Alkaloids Using a Building Block Strategy.

A concise and general strategy for the total synthesis of the phenanthroquinolizidine alkaloids has been developed. An iterative Suzuki-Miyaura coupling reaction between the requisite aryl boronic acid, 2-bromo-4,5-dimethoxyphenyl N-methyliminodiacetate (MIDA) boronate derived from boronic acid, and a suitable bromopyridine substrate bearing a homopropargyl alcohol at the 2-position generated the desired ortho-aza-terphenyl compounds. Hydrogenation of the triple bond followed by treatment with methanesulfonyl chloride afforded their corresponding tetrahydroquinolizinium ion intermediates, which were subsequently reacted with NaBH4 to provide the desired hexahydroquinolizine products. A final oxidative electrocyclization reaction gave the target phenanthroquinolizidine natural products. This synthetic approach only requires the use of three chromatographic separations throughout the entire synthesis.

Screening of Phenanthroquinolizidine Alkaloid Derivatives for Inducing Cell Death of L1210 Leukemia Cells with Negative and Positive P-glycoprotein Expression.

We describe the screening of a set of cryptopleurine derivatives, namely thienoquinolizidine derivatives and (epi-)benzo analogs with bioactive phenanthroquinolizidine alkaloids that induce cytotoxic effects in the mouse lymphocytic leukemia cell line L1210. We used three variants of L1210 cells: i) parental cells (S) negative for P-glycoprotein (P-gp) expression; ii) P-glycoprotein positive cells (R), obtained by selection with vincristine; iii) P-glycoprotein positive cells (T), obtained by stable transfection with a human gene encoding P-glycoprotein. We identified the most effective derivative 11 with a median lethal concentration of ≈13 μM in all three L1210 cell variants. The analysis of the apoptosis/necrosis induced by derivative 11 revealed that cell death was the result of apoptosis with late apoptosis characteristics. Derivative 11 did not induce a strong alteration in the proportion of cells in the G1, S or G2/M phase of the cell cycle, but a strong increase in the number of S, R and T cells in the subG1 phase was detected. These findings indicated that we identified the most effective inducer of cell death, derivative 11, and this derivative effectively induced cell death in S, R and T cells at similar inhibitory concentrations independent of P-gp expression.

Cytotoxic Alkaloids from Leaves of Pilea aff. martinii.

Two new phenanthroquinolizidine alkaloids (1: and 2: ) and a new piperidine derivative (3: ) were isolated from the leaves of Pilea aff. martinii together with 3 known alkaloids: julandine (4: ), cryptopleurine (5: ), and 1,3,6,6-tetramethyl-5,6,7,8-tetrahydro-isoquinolin-8-one (6: ). Their structures were determined by spectral data analyses including mass spectrometry and 2-dimensional nuclear magnetic resonance data. The absolute configurations of 1: -3: were established by comparison of their experimental circular dichroism data with the calculated electronic circular dichroism spectra. The isolated compounds were evaluated for their cytotoxicity against 4 cancer cell lines: KB (mouth epidermal carcinoma cells), HepG-2 (human liver hepatocellular carcinoma cells), LU-1 (human lung adenocarcinoma cells), and MCF-7 (human breast cancer cells). The new phenanthroquinolizidine pileamartine D (2: ) showed strong and selective proliferation inhibition toward KB and HepG-2 cells with IC50 values of 25 and 27 nM, respectively. Pileamartine C (1: ), julandine (4: ), and cryptopleurine (5: ) exhibited cytotoxicity against 4 tested cancer cell lines with IC50 values less than 1 µM.

Asymmetric synthesis and study of biological activity of (epi-)benzoanalogues of bioactive phenanthroquinolizidine alkaloids

The increasing microbial resistance to primary active structures remains alarming and the effort to look for new antibacterial active structures is still of scientific interest. One of the attractive ways to find new active structures is derivatization of well-known natural compounds. Alkaloids are a structurally diverse group of natural products with a wide range of biological effects. Historically, an attempt to increase the antimicrobial activity of alkaloids through chemical modifications has been successful. In this work, 12 new quinolizidine derivatives were synthesized and tested for their antimicrobial activity. The asymmetric synthesis of the benzoanalogue of the phenanthroquinolizidine bioactive alkaloid (−)-cryptopleurine and the epi-benzoanalogues of (−)-(15R)-hydroxycryptopleurine were achieved in six or seven steps starting from available enantiopure (S)-2-aminoadipic acid used as source of chirality as well as nitrogen. The highest antimicrobial activity was observed in the presence of the final saturated structure, the benzoanalogue of naturally occurring plant alkaloid cryptopleurine. It features selective toxicity, and significantly inhibits the growth of G+ bacteria, especially Staphylococcus sp. Tested derivatives have shown only a weak antifungal activity, but partial inhibition has been observed in the case of model yeasts.

Inner morphological and chemical differentiation of Boehmeria species.

The present study was designed to establish quality control parameters for pharmacognostic evaluation and differentiation of eight locally derived Boehmeria species, B. gracilis, B. nivea, B. pannosa, B. platanifolia, B. quelpaertensis, B. spicata, B. splitgerbera, B. tricuspis, and two varieties named B. japonica var. longispica, B. nivea var. concolor, which have been utilized as the folk medicine, 'Mo-Si-Pool' in Korea. Although the outer morphological study of these species had been reported, there is no pharmacognostical description yet. Therefore, inner morphological evaluation on leaf midrib, petiole and stem of eight Boehmeria species and two varieties was accomplished along with preliminary phytochemical analysis by HPLC-DAD profiling. The microscopic data showed discriminative inner morphological characteristics such as collenchyma cell layer, thickness of cortex and xylem, frequency of druse and hairs, and number of vascular bundles. The HPLC profiles exhibited more than four characteristic peaks. The molecular ions of the four peaks (1-4) were tentatively identified by ESI-MS, and their structures were identified by NMR spectroscopy to be the flavonoids, rutin (1), isoquercetin (2) and kaempferol-3-O-rutinoside (3), and a phenanthroquinolizidine alkaloid, (-)-cryptopleurine (4). While compounds 1 and 2 were detected in all samples, compound 4 was determined only in B. japonica var. longispica, B. pannosa and B. quelpaertensis and B. splitgerbera. These findings provide the initial scientific criteria for proper identification and establishment of standards for use of Boehmeria species in traditional medicine.

6-OH-Phenanthroquinolizidine Alkaloid and Its Derivatives Exert Potent Anticancer Activity by Delaying S Phase Progression.

To discover new phenanthroindolizidine and phenanthroquinolizidine alkaloids as potential anticancer drug candidates, non-natural 6-O-desmethylcryptopleurine (2) and its derivatives were prepared. Most of the new compounds exhibited potent antiproliferative activity against A549 and BEL-7402 cells, with the lowest IC50 being 3 nM. Optically pure 2-R was further evaluated against a panel of 30 cancer cell lines and found to inhibit the proliferation of all tested cell lines, including three multidrug-resistant cell lines, with an average IC50 value of 2.1 nM, which is much lower than that of previously reported phenanthroindolizidine DCB-3503 (1, IC50: 166.7 nM). A mechanistic evaluation showed that 2-R potently inhibited cell growth and colony formation, which are associated with a delay in S phase progression through the inhibition of DNA synthesis. These results along with further study on the safety profile of these compounds will facilitate the discovery of new phenanthroindolizidine and phenanthroquinolizidine alkaloids for use as anticancer drug candidates.

Use of chiral-pool approach into epi-thieno analogues of the scarce bioactive phenanthroquinolizidine alkaloids

The stereoselective synthesis of epi-thieno analogues of the phenanthroquinolizidine bioactive alkaloids (−)-Cryptopleurine and (−)-(15R)-Hydroxycryptopleurine was achieved in five steps starting from easily available enantiopure (S)-2-aminoadipic acid used as chiral pool and nitrogen atom source. During these investigations, both π-cationic cyclization of chiral N-thienylmethyl-6-oxopipecolinic acids into pure (S)-keto-lactams and theirs regioselective and diastereoselective reduction, considered as key steps of this sequence, were studied. Of particular interest, the Friedel–Crafts cyclization using (CF3CO)2O/BF3·Et2O show that near the expected keto-lactams, enamides and enamidones containing trifluoromethyl residue were isolated. A mechanism leading to the latter products with high synthetic potential was discussed.

Design, Synthesis, and Biological Activity of Sulfonamide Analogues of Antofine and Cryptopleurine as Potent and Orally Active Antitumor Agents.

Due to their profound antiproliferative activity and unique mode of action, phenanthroindolizidine and phenanthroquinolizidine alkaloids, represented by antofine and cryptopleurine, have attracted attention recently as potential therapeutic agents. We have designed, synthesized, and evaluated the methanesulfonamide analogues of these natural alkaloids with the hope of improving their druglikeness. The analogues showed enhanced growth inhibition of human cancer cells compared with the parent natural products. In particular, a methanesulfonamide analogue of cryptopleurine (5b) exhibited improved bioavailability and significant antitumor activity, which suggests that 5b is a promising new anticancer agent. Our studies suggest that the inhibition of cancer cell growth by 5b is associated with the induction of G0/G1 cell cycle arrest via nicotinamide N-methyltransferase-dependent JNK activation in Caki-1 renal cancer cells. In addition, compound 5b significantly inhibited the migration and invasion of Caki-1 cancer cells by modulating the p38 MAPK signaling pathway.

Design, synthesis, anti-tobacco mosaic virus (TMV) activity, and SARs of 7-methoxycryptopleurine derivatives.

A series of 7-methoxycryptopleurine derivatives 2-23 were prepared and evaluated for their antiviral activity against tobacco mosaic virus (TMV) for the first time. The bioassay results showed that most of these compounds exhibited excellent in vivo anti-TMV activity, of which 7-methoxycryptopleurine salt derivatives 16, 19, and 23 displayed significantly higher activity than 7-methoxycryptopleurine (1) and commercial ribavirin and ningnanmycin. Salification, the most commonly employed method for modifying physical-chemical properties, did significantly increase antiviral activity, and different salt forms displayed different antiviral effect. This study provides fundamental support for development and optimization of phenanthroquinolizidine alkaloids as potential inhibitors of plant virus.

Collective Asymmetric Synthesis of (−)-Antofine, (−)-Cryptopleurine, (−)-Tylophorine, and (−)-Tylocrebrine with tert-Butanesulfinamide as a Chiral Auxiliary

A collective asymmetric synthesis of phenanthroindolizidine and phenanthroquinolizidine alkaloids (-)-antofine, (-)-cryptopleurine, (-)-tylophorine, and (-)-tylocrebrine was achieved by means of a reaction sequence involving efficient generation of chiral homoallylic amine intermediates by asymmetric allylation of the corresponding tert-butanesulfinyl imine. From these intermediates, the pyrrolidine and piperidine rings were constructed by means of an intramolecular SN2 substitution reaction and a ring-closing metathesis reaction, respectively. The unusual C5-methoxy-substituted phenanthrene moiety of (-)-tylocrebrine was generated by means of an InCl3-catalyzed cycloisomerization reaction of an o-propargylbiaryl compound.

ChemInform Abstract: Palladium‐Catalyzed Annulation of 2,2′‐Diiodobiphenyls with Alkynes: Synthesis and Applications of Phenanthrenes.

AbstractThe present new method is also applicable to chiral alkynes like (XI) giving optically active products which can be smoothly converted into phenanthroindolizidine and phenanthroquinolizidine alkaloids by Pictet—Spengler reaction as shown for (XIV).

First Discovery and Stucture-Activity Relationship Study of Phenanthroquinolizidines as Novel Antiviral Agents against Tobacco Mosaic Virus (TMV)

A series of phenanthroquinolizidine alkaloids 1–24 were prepared and first evaluated for their antiviral activity against tobacco mosaic virus (TMV). The bioassay results showed that most of these compounds exhibited good to excellent in vivo anti-TMV activity, of which compounds 1, 2, 15 and 16 displayed significantly higher activity than (R)-antofine and commercial Ningnanmycin at the same test condition. The substituents on the phenanthrene moiety play an important role for maintaining high in vivo antiviral activity. The introduction of 6-hydroxyl, which is proposed to interact with TMV RNA, did increased anti-TMV activity. The 14aR-configuration was confirmed to be the preferred antiviral configuration for phenanthroquinolizidine alkaloids. Introduction of hydroxy group at 15-position of phenanthroquinolizidine alkaloids increased activity for S-configuration but decreased activity for R-configuration. Present study provides fundamental support for development and optimization of phenanthroquinolizidine alkaloids as potential inhibitors of plant virus.

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.

ChemInform Abstract: Synthesis and SAR Studies of Phenanthroindolizidine and Phenanthroquinolizidine Alkaloids as Potent Antitumor Agents.

AbstractThe phenanthroindolizidine alkaloids (X) are prepared by using a similar route that is used for the synthesis of racemic tylophorine.