Chiral-phase HPLC Research Articles

The Kavaratamides: Discovery of Linear Lipodepsipeptides from the Marine Cyanobacterium Moorena bouillonii Using a Comparative Chemogeographic Analysis.

Kavaratamide A (1), a new linear lipodepsipeptide possessing an unusual isopropyl-O-methylpyrrolinone moiety, was discovered from the tropical marine filamentous cyanobacterium Moorena bouillonii collected from Kavaratti, India. A comparative chemogeographic analysis of M. bouillonii collected from six different geographical regions led to the prioritized isolation of this metabolite from India as distinctive among our data sets. AI-based structure annotation tools, including SMART 2.1 and DeepSAT, accelerated the structure elucidation by providing useful structural clues, and the full planar structure was elucidated based on comprehensive HRMS, MS/MS fragmentation, and NMR data interpretation. Subsequently, the absolute configuration of 1 was determined using advanced Marfey's analysis, modified Mosher's ester derivatization, and chiral-phase HPLC. The structures of kavaratamides B (2) and C (3) are proposed based on a detailed analysis of their MS/MS fragmentations. The biological activity of kavaratamide A was also investigated and found to show moderate cytotoxicity to the D283-medullablastoma cell line.

Bioassay-Guided Isolation and Identification of Cytotoxic Compounds from Melaleuca quinquenervia Fruits.

The fruit extract of Melaleuca quinquenervia yielded a total of 19 compounds, including two novel spiro-biflavonoid enantiomers (1a and 1b) and a chalcone derivative (3). Their structures were determined through spectroscopic analysis. The enantiomers of the racemic mixture of compound 1 were successfully resolved into (+)-1 and (-)-1 using chiral-phase HPLC. Single-crystal X-ray diffraction analysis was also used to confirm the structure of 1. The enantiomeric configurations of 1 and 2 were determined through a comparison of the calculated and experimental electronic circular dichroism spectra. Compounds 2 (melanervin), 14 (methyl betulinate), 15 (3-O-acetylbetulinic acid), and 16 (pyracrenic acid) were found to be highly cytotoxic, with compound 16 showing superior growth inhibition of nonsmall cell lung cancer cells (A549 cells) (IC50 2.8 ± 0.1 μM) compared to cisplatin (IC50 3.3 ± 0.0 μM), a positive control chemotherapeutic drug. Both compound 16 and cisplatin were significantly more cytotoxic toward A549 lung cancer cells compared to nontumorigenic Vero E6 cells.

Neuroprotective azaphilones from a deep-sea derived fungus Penicillium sp. SCSIO41030.

Ten azaphilones including one pair of new epimers and three new ones, penineulones A-E (1-5) with the same structural core of angular deflectin, were obtained from a deep-sea derived Penicillium sp. SCSIO41030 fermented on a liquid medium. Their structures including absolute configurations were elucidated using chiral-phase HPLC analysis, extensive NMR spectroscopic and HRESIMS data, ECD and NMR calculations, and by comparing NMR data with literature data. Biological assays showed that the azaphilones possessed no antitumor and anti-viral (HSV-1/2) activities at concentrations of 5.0 μM and 20 μM, respectively. In addition, azaphilones 8 and 9 showed neuroprotective effects against Aβ25-35-induced neurotoxicity in primary cultured cortical neurons at a concentration of 10 μM. Azaphilones 8 and 9 dramatically promoted axonal regrowth against Aβ25-35-induced axonal atrophy. Our study indicated that azaphilones could be promising lead compounds for neuroprotection.

Diterpenolignans and cephalotane diterpenoids from Cephalotaxus oliveri mast. with antitumor activity

Four undescribed naturally diterpenolignans, and two cephalotane diterpenoids, along with seven known compounds, including two pairs of enantiomers, were isolated from the twigs and leaves of Cephalotaxus oliveri Mast. Their structures were elucidated via spectroscopic data interpretation, chiral-phase HPLC analysis, NMR calculations, and electronic circular dichroism analysis. All the isolated compounds were evaluated for their cytotoxic activities against three kinds of human tumor cell lines. Among them, compound 8 exhibited the most potent activities against MCF-7, HepG2 and A549 cell lines with IC50 values of 2.83, 4.75 and 2.77 μM, respectively.

Stereochemical insights into structurally diverse lignanamides from the herbs of Solanum lyratum Thunb.

A chemical investigation of Solanum lyratum Thunb. (Solanaceae) afforded six pairs of enantiomeric lignanamides consisting of twelve undescribed compounds, along with two undescribed racemic mixtures, and the separations of the enantiomers were accomplished by chiral-phase HPLC. The structures of these undescribed compounds were elucidated by the analysis of spectroscopic data, NMR and electronic circular dichroism calculations. All isolated compounds were assessed for neuroprotective activities in H2O2-induced human neuroblastoma SH-SY5Y cells, and acetylcholinesterase (AChE) inhibitory activities. Among tested isolates, some enantiomeric lignanamides exhibited conspicuous neuroprotective effects and AChE inhibitory effect.

Functionalization of Pentacene: A Facile and Versatile Approach to Contorted Polycyclic Aromatic Hydrocarbons.

In this work, a facile and versatile strategy for the synthesis of contorted polycyclic aromatic hydrocarbons (PAHs) starting from the functionalized pentacene was established. A series of novel PAHs 1-4 and their derivatives were synthesized through a simple two-step synthesis procedure involving an intramolecular reductive Friedel-Crafts cyclization of four newly synthesized pentacene aldehydes 5-8 as a key step. All the molecules were confirmed by single-crystal X-ray diffraction and their photophysical and electrochemical properties were studied in detail. Interestingly, the most striking feature of 1-4 is their highly contorted carbon structures and the accompanying helical chirality. In particular, the optical resolution of 2 was successfully achieved by chiral-phase HPLC, and the enantiomers were characterized by circular dichroism and circularly polarized luminescence spectroscopy. Despite the highly nonplanar conformations, these contorted PAHs exhibited emissive properties with moderate-to-good fluorescence quantum yields, implying the potential utility of this series PAHs as high-quality organic laser dyes. By using a self-assembly method with the help of epoxy resin, a bottle microlaser based on 3 a was successfully illustrated with a lasing wavelength of 567.8 nm at a threshold of 0.3 mJ/cm2 . We believe that this work will shed light on the chemical versatility of pentacene and its derivatives in the construction of novel functionalized PAHs.

Four Pairs of Neuroprotective Aryldihydronapthalene-type Lignanamide Enantiomers from the Herbs ofSolanum lyratum.

Four pairs of aryldihydronapthalene-type lignanamide enantiomers were isolated from Solanum lyratum (Solanaceae). The enantiomeric separation was accomplished by chiral-phase HPLC, and five undescribed compounds were elucidated. Analysis by various spectroscopy and ECD calculations, the structures of undescribed compounds were illuminated. The neuroprotective effects of all compounds were evaluated using H2O2-induced human neuroblastoma SH-SY5Y cells and AchE inhibition activity. Among them, compound 4a exhibited remarkable neuroprotective effects at high concentrations of 25 and 50 μmol/L comparable to Trolox. Compound 1a showed the highest AchE inhibition with the IC50 value of 3.06 ± 2.40 μmol/L. Molecular docking of the three active compounds was performed and the linkage between the compounds and the active site of AchE was elucidated.

New Meroterpenoids and Anti-Osteoclastogenic Polyketides from the Mangrove-Derived fungus Arthrinium sp. SCSIO 41306.

Two new meroterpenoids, arthrinones A and B (1 and 2), along with six known compounds (3-8), were obtained from the fungus Arthrinium sp. SCSIO 41306. Comprehensive methods such as chiral-phase HPLC analysis and ECD calculations were applied to determine the absolute configurations. Griseofulvin (5), kojic acid (6), and 1H-indole-3-carboxaldehyde (8) showed inhibition of NF-κB in RAW 264.7 macrophages induced by lipopolysaccharide (LPS) with IC50 values of 22.21, 13.87 and 19.31 μM, respectively. In addition, griseofulvin (5) inhibited receptor activator of NF-κB ligand (RANKL)-induced osteoclast differentiation in a dose-dependent manner without visible evidence of cytotoxicity in bone marrow macrophages (BMMs). This is the first report on the activity of griseofulvin (5) to inhibit osteoclast formation (IC50 10.09±0.21 μM).

Platelet-inhibitory phenolic constituents from the fruits of Daemonorops draco

Eight previously undescribed phenolic compounds, dracoropins A – H (1–8), along with two known analogues (9 and 10) were isolated from the fruits of Daemonorops draco. Four pairs of isomers (1a/1b, 2a/2b, 3a/3b, and 4a/4b) were resolved by using chiral-phase HPLC separation. Their structures, including the absolute configurations of the resolved isomers, were elucidated by analysis of spectroscopic data (1D and 2D NMR, IR, and HRESIMS), single-crystal X-ray diffraction, and electronic circular dichroism (ECD) calculations. Compounds 1, 2, and 3 bear a rare 2-phenylbenzo[d]-1,3-dioxepine skeleton. All the isolates were evaluated for their inhibitory activity against ATP release in thrombin-activated platelets. Compounds 2b, 3a, and 6 could significantly inhibit ATP release in thrombin-activated platelets.

Chiral-phase HPLC separation of (divinyl-)protochlorophyllide-a enantiomers as key precursors in chlorophyll biosynthesis from their 132-stereoisomeric prime forms

Protochlorophyllide(PChlide)-a and its 8-vinylated analog, divinyl(DV)-PChlide-a, are common and essential intermediates in the biosynthesis of all naturally occurring chlorophyll (Chl) pigments. These porphyrinoid-type pigments have a single optically active (asymmetric) carbon atom at the 132-position, so their stereoisomers are (132R)- and (132S)-enantiomers. The former and latter are called (DV-)PChlide-a and (DV-)PChlide-a′, respectively. In this study, chiral-phase HPLC separation of enantiomeric (DV-)PChlides-a/a′ was demonstrated. The (132R)-enantiomeric PChlide-a was eluted more slowly than the corresponding (132S)-enantiomeric PChlide-a′ under the present HPLC conditions. On the other hand, the elution order of (132R)-DV-PChlide-a and (132S)-DV-PChlide-a′ was reverse to that of PChlides-a/a′. After the separation of each enantiomer by the chiral-phase HPLC, the stereoisomeric configuration at the 132-position was characterized by means of circular dichroism spectroscopy. The present chiral-phase HPLC method enables us to evaluate optical purities of (DV-)PChlide-a species. For example, PChlide-a and/or DV-PChlide-a extracted from the spent medium and harvested cells of cultured purple photosynthetic bacterial mutants, the former of which has been often used as the source of (DV-)PChlide-a substrates for enzymatic reactions, were revealed to be mostly racemized, giving enantiomeric mixtures of (DV-)PChlides-a/a′.

Ochrathinols A and B, two pairs of sulfur-containing racemates from an Antarctic fungus Aspergillus ochraceopetaliformis SCSIO 05702 inhibit LPS-induced pro-inflammatory cytokines and NO production

Ochrathinols A and B ((±)-1 and (±)-2), two undescribed sulfur-containing racemates, and ochracids A and B (3 and 4), two unprecedented pyrrolizidine alkaloids, were isolated from an Antarctic soil-derived fungus Aspergillus ochraceopetaliformis SCSIO 05702. Their structures including absolute configurations were determined through extensive spectroscopic analysis, chiral-phase HPLC analysis, quantum ECD calculations, and X-ray single-crystal diffraction. Ochrathinols A and B are unprecedented sulfur natural products featuring a novel 3-methylhexahydro-2H-cyclopenta [b]thiophene core. Interestingly, ochrathinol A ((±)-1) outstandingly suppressed the release of LPS-induced IL-1β, IL-6, and TNF-α inflammatory cytokines with concentration of 10 μM and alleviated the unbalanced NAD+/NADH ratio caused by LPS in RAW264.7 macrophages.

Structurally various p-terphenyls with neuraminidase inhibitory from a sponge derived fungus Aspergillus sp. SCSIO41315

Guided by Global Natural Products Social molecular networking, 14 new p-terphenyl derivatives, asperterphenyls A–N (1–14), together with 20 known p-terphenyl derivatives (15–34), were obtained from a sponge derived fungus Aspergillus sp. SCSIO41315. Among them, new compounds 2–8 and 15–17 were ten pairs of enantiomers. Comprehensive methods such as chiral-phase HPLC analysis, ECD calculations and X-ray diffraction analysis were applied to determine the absolute configurations. Asperterphenyls B (2) and C (3) represented the first reported natural p-terphenyl derivatives possessing a dicarboxylic acid system. Asperterphenyl A (1) displayed neuraminidase inhibitory activity with an IC50 value of 1.77 ± 0.53 µM and could efficiently inhibit infection of multiple strains of H1N1 with IC50 values from 0.67 ± 0.28 to 1.48 ± 0.60 µM through decreasing viral plaque formation in a dose-dependent manner, which suggested that asperterphenyl A (1) might be exploited as a potential antiviral compound in the pharmaceutical fields.

Bioactive constituents from the rhizomes of Atractylodes macrocephala

Twelve undescribed compounds including five sesquiterpenes (1–5), one monoterpene (6), and four lignans (7a/7b and 8a/8b), along with two other types (9 and 10) were isolated from the rhizomes of Atractylodes macrocephala. Among them, two pairs of enantiomers (7a/7b and 8a/8b) were successfully separated by chiral-phase HPLC, while racemate 9 could not be resolved. Their structures and absolute configurations were unambiguously elucidated by spectroscopic data analysis and electronic circular dichroism (ECD) calculations. Notably, compounds 1 and 2 are rare sesquiterpene hybrids featuring an eudesmanolactam linked to a resorcinol or methyl 2-methylpentanoat through a CN bond. Compound 3 represents the first example of eudesmanolide sesquiterpene with an oxygen-bridge between C-8 and C-14. Compounds 7a and 7b are a pair of rare enantiomeric benzodioxane norneolignans. Additionally, compound 2 exhibited weak cytotoxicity against SGC-7901 cells. Compound 4 significantly promoted the proliferation of LPS-induced IEC-6 cells with the rate of 117.2%.

Dimeric aporphine alkaloids from the twigs of Trivalvaria costata (Hook.f. & Thomson) I.M.Turner

A phytochemical investigation of the twig extract of Trivalvaria costata (Hook.f. & Thomson) I.M.Turner has identified ten undescribed dimeric aporphine alkaloids, trivalcostatines A−J, one undescribed isoquinoline alkaloid, trivalcostaisoquinoline, and four known aporphine alkaloids. Their structures were elucidated by detailed analysis of NMR and HRESITOFMS data. Three of the dimeric aporphine structures were confirmed by single crystal X-ray diffraction analysis. All of the dimeric aporphine alkaloids were isolated as mixtures of atropisomers. Several of them were resolved by chiral-phase HPLC and the absolute configurations of the pure atropisomers were assigned by calculated and experimental ECD analysis. Bidebilines A and B, heteropsine, and urabaine showed α-glucosidase inhibitory activities with IC50 values in the range of 4.1–11 μM.

Acylphloroglucinol-monoterpene meroterpenoids from Eucalyptus tereticornis and their inhibitory activity against ATP citrate lyase

Five undescribed enantiomeric pairs of acylphloroglucinol-monoterpene meroterpenoids ((+)-/(−)-eucateretins A−E) resolved by chiral-phase HPLC were obtained from the leaves of Eucalyptus tereticornis Smith, along with nine known analogues. Their structures were elucidated by spectroscopic methods and ECD calculations. This is the first report of meroterpenoid enantiomers from this plant. Some of the isolates, (−)-eucateretin A, (+)-/(−)-eucateretins E, 7′α-eucalrobusone X, eucalrobusone X, and robustadial B, exhibited inhibitory effects on ATP citrate lyase, and 7′α-eucalrobusone X significantly suppressed the hepatocyte lipogenesis.

Photochemical Dimerization of Plakinidine B Leads to Potent Inhibition of the E3 Ubiquitin-Protein Ligase CBL-B.

A new dimeric alkaloid plakoramine A [(±)-1] was identified from a marine sponge Plakortis sp. Chiral-phase HPLC separation of (±)-1 led to the purified enantiomers (+)-1 and (-)-1 which both potently inhibited CBL-B E3 ubiquitin ligase activities. The absolute configurations of the enantiomers were determined by quantum chemical calculations. Scrutinization of the purification conditions revealed a previously undescribed, nonenzymatic route to form (±)-1 via photochemical conversion of its naturally occurring monomeric counterpart, plakinidine B (2).

Eudesmane−type sesquiterpenes from the rhizomes of Atractylodes macrocephala and their bioactivities

Fifteen undescribed eudesmane−type sesquiterpenes, named atramacronoids D−R, along with fourteen known analogues were isolated from the rhizomes of Atractylodes macrocephala. The structures of atramacronoids D−R were elucidated based on extensive spectroscopic data analysis, Snatzke's rule, electronic circular dichroism (ECD) calculations, and X-ray crystallographic analysis. Notably, of the undescribed isolates, atramacronoids D and E are the first example of eudesmanolactam−phenol and eudesmanolactam−ethyl hybrids obtained from plants, respectively. A pair of enantiomers, (+)- and (−)-atramacronoids F, were successfully resolved by chiral-phase HPLC. Atramacronoid D exhibited weak cytotoxicity against SGC-7901 cells. Atramacronoid E significantly promoted the proliferation of LPS-induced IEC-6 cells.

Isolation, Synthesis and Absolute Configuration of the Pericharaxins A and B, Epimeric Hydroxy-Polyene Glycerol Ethers from the Calcarean Sponge Pericharax heteroraphis.

Naturally occurring epimeric hydroxy-polyene glycerol ether pericharaxins A (1a) and B (1b) were isolated from the calcarean sponge Pericharax heteroraphis. The structural and stereochemical characterization of both diastereoisomers were established on the basis of spectroscopic data analysis and total synthesis in seven steps. The mixture of pericharaxins A (1a) and B (1b) was proven to be epimeric by chiral-phase HPLC analysis of both synthetic and natural samples. Further separation of the epimers and application of Mosher’s method to the synthetic compounds allowed unequivocal absolute configuration assignment. While natural products and the synthetic intermediates were shown to be non-cytotoxic on the HCT116 cell line, the endochondral differentiation activity using human type X collagen transcription activity in ATDC5 cells is interesting.

Spiroisoxazoline Inhibitors of Acetylcholinesterase from Pseudoceratina verrucosa. Quantitative Chiroptical Analysis of Configurational Heterogeneity, and Total Synthesis of (±)-Methyl Purpuroceratate C.

Examination of the MeOH extract of the sponge, Pseudoceratina cf. verrucosa, Berquist 1995 collected near Ningaloo Reef, Western Australia for selective acetylcholinesterase (AChE) inhibitors, yielded five new bromotyrosine alkaloids, methyl purpuroceratates A and B (1b and 2b), purpuroceratic acid C (3a), and ningalamides A and B (4 and 5). The structures of 1-4 share the dibromo-spirocyclohexadienyl-isoxazoline (SIO) ring system found in purealidin-R, while ketoxime 5 is analogous to ianthelline and purpurealidin I. The planar structures of all five compounds were obtained from analysis of MS, 1D and 2D NMR data, and the absolute configuration of the spiroisoxazoline (SIO) unit was assigned by electronic circular dichroism (ECD) and comparison with standards prepared by total synthesis of methyl purpuroceratate C, (±)-3b. Compound 4 is the most complex SIO described, to date. The configuration of the homoserine module (C) in 4 was ascertained, after acid hydrolysis, by derivatization of an l-tryptophanamide derivative based on Marfey's reagent. Chiral-phase HPLC, with comparison to synthetic standards, revealed that most SIOs isolated from P. cf. verrucosa were configurationally heterogeneous; some, essentially racemic. Chiral-phase HPLC, with UV-ECD detection, is demonstrated as a superlative method for configurational assignment and quantitation of the enantiomeric composition of SIOs. Two SIOs─aerophobin-1 and aplysinamisine II─emerged as selective inhibitors of AChE over butyrylcholinesterase (BuChE, IC50 ratio >10), while aplysamine-2 moderately inhibited both cholinesterases (ChEs, IC50, (AChE) 0.46 μM; IC50, (BuChE) 1.03 μM). SIO alkaloids represent a potential new structural manifold for lead-discovery of new therapeutics for treatment of Alzheimer's disease.

Nostosin G and Spiroidesin B from the Cyanobacterium Dolichospermum sp. NIES-1697.

Chemical investigation of the cyanobacterium Dolichospermum sp. NIES-1697 afforded nostosin G (1), a linear tripeptide, spiroidesin B (2), and two known compounds, anabaenopeptins I (3) and J (4). Planar structures and absolute configurations for 1 and 2 were determined by 2D NMR, HRMS, Marfey's methodology, chiral-phase HPLC, and enzymatic degradation. Nostosin G (1) is a unique example of a linear peptide containing three subunits, 4-hydroxyphenyllactic acid (Hpla), homotyrosine (Hty), and argininal, with potent trypsin inhibitory properties. The biosynthetic gene clusters for nostosin G (1) and spiroidesin B (2) were investigated based on the genome sequence of Dolichospermum sp. NIES-1697.