Chiral cruciferous phytoalexins: preparation, absolute configuration, and biological activity

Synthesized by an efficient one-pot spirocyclization method, two chiral cruciferous phytoalexins, 1-methoxyspirobrassinin (2) and 1-methoxyspirobrassinol methyl ether (4a), were prepared through optical resolution using the chiral HPLC method of corresponding racemates. The absolute configuration of natural (+)-2 was elucidated as R by using the direct comparison of ECD and VCD spectra with those of known (S)-(-)-spirobrassinin (1). Another chiral phytoalexin, (-)-4a, had its absolute configuration 2R,3R elucidated through the comparison of observed and calculated VCD. Interestingly, the absolute configurations of natural (S)-(-)-spirobrassinin (1) and (R)-(+)-1-methoxyspirobrassinin (2) were opposite of each other, even though their structures are almost similar, with the exception of an N-methoxy group. A significant difference in the antiproliferative activity between (2R,3R)-(-) and (2S,3S)-(+)-4a was observed.     

Five New Triene Derivatives from the Fungus Penicillium herquei JX4

Five undescribed triene derivatives, pinophols B–F ( 2 – 6 ), together with one known compound, pinophol A ( 1 ), were obtained from the mangrove endophytic fungus Penicillium herquei JX4. The structures of compounds 1 – 6 were elucidated using IR, HR-ESI-MS, and NMR methods. The absolute configurations of compounds 1 – 6 were confirmed by comparing their experimental or calculated ECD spectra. Pinophols C and D ( 3 and 4 ) showed inhibitory activities against LPS-induced NO production.     

Application of chiral technology in a pharmaceutical company. Enantiomeric separation and spectroscopic studies of key asymmetric intermediates using a combination of techniques. Phenylglycidols

Phenylglycidols substituted in the 2-, 3-, and 4- positions with fluorine, chlorine, and trifluoromethyl, and with methoxy in the 3- position, were synthesized from the corresponding E-cinnamic acids and separated into their (R,R)- and (S,S)- enantiomers using subcritical fluid chromatography with mixtures of MeOH in CO(2), on either a Chiralpak AD or AS chiral stationary phase. These compounds and commercially-available (R,R)- and (S,S)-phenylglycidol were analyzed for their vibrational circular dichroism (VCD), electronic circular dichroism (ECD), and optical rotation (OR) properties to exemplify a strategy whereby the absolute stereochemistry of common and key chiral intermediates is established early in the structure-activity and structure-property relationship phase of a drug discovery program in a pharmaceutical company. From this study, substituents in the phenyl group of the synthesized molecules were found not to grossly alter spectroscopic features, and therefore, diagnostic absorption bands in the respective VCD spectra, and the sign and shape of the measured ECD curves could be used to determine and track the absolute stereochemistry of analogs without necessarily requiring time-consuming ab initio calculations of all low energy conformers for all compounds. VCD, OR, and ECD calculations for the determination of absolute configuration carried out at the DFT level with the hybrid B3PW91 functional and the TZVP basis set were found to be especially useful in this study.     

Isoflavonoids from Crotalaria albida Inhibit Adipocyte Differentiation and Lipid Accumulation in 3T3-L1 Cells via Suppression of PPAR-γ Pathway

Two 2″-isopropenyl dihydrofuran isoflavonoids (1 and 3), one 2″-isopropenyl dihydrofuran chromone (2), as well as 13 known compounds were isolated from the herbs of Crotalaria albida. Their structures and relative configurations were elucidated via NMR and HRESIMS analyses. The 2″ S absolute configuration of 1 and 2 were deduced by comparing their NOESY spectra with that of 3, which was determined via single crystal X-ray diffraction (CuKα). The 3R absolute configuration of 1 was determined by CD. Compounds 1, 2, and 3 inhibit the adipocyte differentiation and lipid accumulation of 3T3-L1 through down-regulation of PPAR-γ activity.     

Phenolic derivatives with cytotoxic activities from the roots of Fallopia multiflora var. ciliinervis

Fourteen phenolic derivatives including two new compounds (1, 2), one new natural product (10) and eleven known ones (3 - 9 and 11–14) were obtained from the roots of Fallopia multiflora var. ciliinervis. The structures were determined using IR, MS, 1D and ²D NMR spectroscopy and the absolute configuration of the new structure (2) was deduced by ECD experiments. All of the isolated compounds were assayed for their cytotoxic activities against three tumor cell lines (A549, HCT116 and SW620) and the results showed that compounds 12－14 showed cytotoxicities against A549 cells while compounds 10 and 11 showed cytotoxicities against SW620 cells.     

A New Fusicoccane-Type Norditerpene and a New Indone from the Marine-Derived Fungus Aspergillus aculeatinus WHUF0198

A new norditerpene named aculeaterpene A ( 1 ) and a new indone named aculeaindone A ( 2 ), along with eight known compounds 3 – 10 were isolated from the culture extract of Aspergillus aculeatinus WHUF0198. The structural characterization of compounds 1 and 2 were performed by spectroscopic analysis, including 1D and 2D NMR and HR-ESI-MS experiments, whereas the absolute configurations were determined by comparing their experimental or calculated ECD spectra. Compound 1 was the first report of fusicoccane-based norditerpene, in which the C-20 was degraded and tured into a hydroxy group.     

Absolute stereochemistry of altersolanol A and alterporriols

The absolute stereochemistry of altersolanol A (1) was established by observing a positive exciton couplet in the circular dichroism (CD) spectrum of the C3,C4-O-bis(2-naphthoyl) derivative 10 and by chemical correlations with known compound 8. Before the discussion, the relative stereochemistry of 1 was confirmed by X-ray crystallographic analysis. The shielding effect at C7'-OMe group by C1-O-benzoylation established the relative stereochemical relationship between the C8-C8' axial bonding and the C1-C4/C1'-C4' polyol moieties of alterporriols E (3), an atropisomer of the C8-C8' dimer of 1. As 3 could be obtained by dimerization of 1 in vitro, the absolute configuration of its central chirality elements (C1-C4) must be identical to those of 1. Spectral comparison between the experimental and theoretical CD spectra supported the above conclusion. Axial stereochemistry of novel C4-O-deoxy dimeric derivatives, alterporriols F (4) and G (5), were also revealed by comparison of their CD spectra to those of 2 and 3.     

Absolute configuration of a polyol fragment of blasticidin A, a specific inhibitor of aflatoxin production

Blasticidin A (1) and aflastatin A (2), Streptomyces metabolites with similar structures, are specific inhibitors of aflatoxin production by Aspergillus parasiticus. The stereochemistry of the polyol fragment of 1 (3a) containing ten chiral centers was elucidated by applying acetonide and MTPA methods to a variety of acetonide derivatives of 3a, which determined the absolute configuration of 3a. By using the similar methods, the absolute configuration of the polyol fragment of 2 (4a) was determined, which was the same as that elucidated by J-based and other chemical methods previously.     

Xanthine oxidase inhibitors from an endophytic fungus Lasiodiplodia pseudotheobromae

Two new compounds, lasdiplactone (1) and lasdiploic acid (2) and one known compound 3 were isolated from the chloroform extract of cell free filtrate of the endophytic fungus Lasiosdiplodia pseudotheobromae. The structures of new compounds were determined by interplay of spectral techniques (IR, mass, ¹H NMR, ¹³C NMR, DEPT, and 2D NMR). The absolute configuration at C-4 position of 1 was established as S using a process similar to modified Mosher’s method. The absolute configuration of 2 was established by comparing its ECD spectrum with the calculated ECD spectra of all possible isomers. In the in vitro XO inhibition assay, the highest inhibition was exhibited by 3 with an IC₅₀ of 0.38 ± 0.13 μg/ml, followed by 2 with an IC₅₀ of 0.41 ± 0.1 μg/ml and the least in 1. The oxidized form of 1 also showed high XO inhibition with IC₅₀ of 0.35 ± 0.13 μg/ml.     

Central-to-axial chirality transfer revealed by liquid crystals: a combined experimental and computational approach for the determination of absolute configuration of carboxylic acids with an α chirality centre

The conversion into 6,7-dihydro-5H-dibenz[c,e]azepine (DAZ) N-protected amides is a viable route for the determination of the absolute configuration of chiral 2-substituted carboxylic acids. The biphenyl moiety of DAZ, besides being a probe of chirality for the electronic circular dichroism (ECD) spectroscopy, makes these systems suitable for configuration assignment by exploiting the chirality amplification which occurs in nematic liquid crystals. To assess the reliability of the liquid crystal method in detecting the absolute stereochemistry of chiral amides bound to a biphenyl group, we measured the helical twisting power of a series of DAZ-N-protected amides and compared these data with the results obtained from ECD measurements. We will show that the liquid crystal method, corroborated by HTP predictions, is trustworthy with our biphenyl derivatives, even when ECD spectra are ambiguous for the presence of aryl moieties displaying strong UV absorptions in the same range of the biphenyl chromophore.     

Ecdysteroids from the Aerial Parts of Paris verticillata

Two new ecdysteroids 14-epi-polypodine B ( 1 ) and 22-oxo-hydroxyecdysterone ( 2 ), along with nine known compounds, polypodine B ( 3 ), viticosterone E ( 4 ), 20-hdroxyecdysone-2-acetate ( 5 ), 22-oxo-20-hydroxyecdysone ( 6 ), 5-hydroxyecdysone ( 7 ), pinnatasterone ( 8 ), 3-epi-20-hydroxyecdysone ( 9 ), ecdysterone ( 10 ) and stachysterone B ( 11 ), were isolated from the aerial parts of Paris verticillata. The structures of all compounds were elucidated by extensive spectroscopic analysis, quantum chemical calculations and ANN-PRA/DP4⁺ probability analysis. Among them, the absolute configuration of compound 1 and 2 was unambiguous determined by ECD. Also, the isolated compounds were assessed for their cytotoxic activities. Compounds 2 , 3 and 7 exhibited significant cytotoxic activities against PC12, LN299 and SMCC7721 cells.     

HPLC‐ECD and TDDFT‐ECD study of hexahydropyrrolo[1,2‐a]quinoline derivatives

Synthesis of racemic hexahydropyrrolo[1,2‐a]quinoline derivatives ( 1 ‐ 8 ) was performed by utilizing the Knoevenagel‐[1,5]‐hydride shift‐cyclization domino reaction. Separation of the enantiomers of the chiral products ( 1 ‐ 8 ) was carried out by chiral high‐performance liquid chromatography, and online high‐performance liquid chromatography‐electronic circular dichroism (ECD) spectra were recorded to elucidate the absolute configuration by comparing the experimental and time‐dependent density functional theory‐ECD spectra obtained at various theoretical levels. For 1 of the products, the time‐dependent density functional theory‐ECD calculations allowed determining both the relative and the absolute configuration by distinguishing the 4 stereoisomers. One of the compounds with spiro 1,3‐cyclohexanedione moiety ( 7 ) possessed moderate acetylcholinesterase inhibitory activity, while 3 showed neuroprotective activity in oxygen‐glucose deprivation‐induced neurotoxicity in human neuroblastoma SH‐SY5Y cells.     

Configurational assignments of conformationally restricted bis-monoterpene hydroquinones: Utility in exploration of endangered plants

Background

Endangered plant species are an important resource for new chemistry. Lindera melissifolia is native to the Southeastern U.S. and scarcely populates the edges of lakes and ponds. Quantum mechanics (QM) used in combination with NMR/ECD is a powerful tool for the assignment of absolute configuration in lieu of X-ray crystallography.

Methods

The EtOAc extract of L. melissifolia was subject to chromatographic analysis by VLC and HPLC. Spin–spin coupling constant (SSCC) were calculated using DFT at the MPW1PW91/6-31G(d,p) level for all staggered rotamers. ECD calculations employed Amber* force fields followed by PM6 semi-empirical optimizations. Hetero- and homo-nuclear coupling constants were extracted from 1D ¹H, E.COSY and HETLOC experiments.

Results

Two meroterpenoids, melissifolianes A (1) and B (2) were purified and their 2-D structures elucidated using NMR and HRESIMS. The relative configuration of 1 was established using the combination of NOE-based distance restraints and the comparisons of experimental and calculated SSCCs. The comparison of calculated and experimental ECD assigned the absolute configuration of 1. The relative configuration of a racemic mixture, melissifoliane B (2) was established utilizing J-based analysis combined with QM and NMR techniques.Conclusion Our study of the Lindera melissifolia metabolome exemplifies how new chemistry remains undiscovered among the numerous endangered plant species and demonstrates how analysis by ECD and NMR combined with various QM calculations is a sensible approach to support the stereochemical assignment of molecules with conformationally restricted conformations.

General significance

QM–NMR/ECD combined approaches are of utility for unambiguous assignment of 3-D structures, especially with limited plant material and when a molecule is conformationally restricted. Conservation of an endangered plant species can be supported through identification of its new chemistry and utilization of that chemistry for commercial purposes.     

Hispanane‐Type Diterpenoid and Secoiridoid Glucosides from Viburnum cylindricum

Three hitherto unknown compounds, including one new hispanane‐type diterpenoid glucoside, namely viburnumoside ( 1 ), two new secoiridoid glucosides, 7α‐galloyloxysweroside ( 2 ), and 7β‐galloyloxysweroside ( 3 ), together with ten known compounds ( 4 – 13 ) were isolated from the ethanol extract of twigs and leaves of Viburnum cylindricum. Their structures were elucidated on the basis of extensive spectroscopic studies, and the absolute configuration of compound 1 was confirmed by the experimental and calculated electronic circular dichroism (ECD) data.     

Organocatalytic alkylation of carbohydrate‐containing aldehydes with dihydroquinoline N,O‐acetals: Absolute configuration of 1,2‐dihydroquinolines

The direct catalytic α‐amidoalkylation of dihydroquinolines with aldehydes bearing oxygen functionalities at different positions in a Mannich‐type reaction has been studied. β‐Alkoxy‐aldehyde 1d gave high enantioselectivity, albeit with an inherently poor diastereoselectivity, while the use of α‐alkoxy aldehydes 1c was detrimental also to enantioselectivity. Mannich‐type reactions have been studied for the first time using new chiral carbohydrate‐derived aldehydes 1a,b showing a reactivity markedly influenced by the presence of water. The chiral glycidic backbone showed a slight but significant influence on the overall stereochemical outcome only when present in α‐position of the aldehyde. The absolute stereochemistry of the products was studied by electronic circular dichroism (ECD) spectra and compared with theoretical calculations. ECD analysis easily provides the absolute configuration of 1,2‐dihydroquinoline derivatives such as quinoline‐1(2H)‐carboxylates.     

Absolute configuration determination of isoflavan-4-ol stereoisomers

Elucidation of the correct stereochemistry of the metabolite is essential for the mechanistic study of bioactive compounds. Isoflavan-4-ol has the same chiropical chromophore as THD, the biosynthetic precursor of the potent phytoestrogen S-equol. Interested in the correct absolute configuration of isoflavan-4-ol stereoisomers and to compare the available practical approaches for the absolute configuration determination, complete absolute configuration analysis of isoflavan-4-ol stereoisomers has been carried out with by means of ECD and VCD spectroscopy as well as modified Mosher method. Theoretical TD-DFT computations resulted in a poor simulation of the observed experimental ECD spectra, and thus inconclusive absolute configuration assignments of isoflavan-4-ol stereoisomers were obtained. However, DFT-assisted VCD spectroscopic analyses successfully determined correct absolute configurations, and further confirmed by modified Mosher method.     

Isolation,Stereochemical Study,and Antioxidant Activity of Benzofuranone Derivatives from a Mangrove‐derived Fungus Eurotium rubrum MA‐150

Enantiomers of a 2‐benzofuran‐1(3H)‐one derivative [(–)‐ 1 and (+)‐ 1 ] and four known analogs ( 2 , 3 , 4 , 5 ) were isolated and identified from the culture extract of Eurotium rubrum MA–150, a fungus obtained from the mangrove‐derived rizospheric soil. Their structures were established by detailed interpretation of nuclear magnetic resonance (NMR) data and the structure of (±)‐ 1 was confirmed by single‐crystal X‐ray diffraction analysis. The absolute configuration of the enantiomers (–)‐ 1 and (+)‐ 1 was determined by means of online high‐performance liquid chromatography – electronic circular dichroism (HPLC‐ECD) measurements and time‐dependent Density Functional Theory – electronic circular dichroism (TDDFT‐ECD) calculations. Compounds (±)‐ 1 as well as 2 and 3 exhibited potent DPPH radical scavenging activities with IC₅₀ values of 1.23, 2.26, and 3.99 μg/mL, respectively. Chirality 28:581–584, 2016. © 2016 Wiley Periodicals, Inc.     

Determination of 3(1)-stereochemistry in synthetic bacteriochlorophyll-d homologues and self-aggregation of their zinc complexes

Zinc complex of methyl 3(1)-octadecyl-bacteriopheophorbide-d was prepared from modification of naturally occurring chlorophyll-a. The 3(1)-epimerically pure samples were obtained by HPLC separation and their stereochemistry including the absolute configuration at the secondary alcoholic 3(1)-position was determined by combination of esterification to methoxy(trifluoromethyl)phenylacetate and NMR spectroscopy (Mosher's method). Both the epimers were monomeric in a polar organic solvent and self-aggregated in a non-polar solvent to give oligomers as well as dimers possessing red-shifted visible absorption bands. Visible spectra of the non-polar organic solutions were dependent upon the 3(1)-chirality and such a diastereoselective control on the self-aggregation led to the formation of self-aggregates with different supramolecular structures.     

Three new flavonoids from Millettia pachyloba

Two new prenylated isoflavones, pachyloisoflavone A (1), pachyloisoflavone B (2), and a new pterocarpan, pachylobin A (3), together with ten known flavonoids (4–13), were isolated from the vine stems and leaves of Millettia pachyloba. The structures of new compounds were elucidated on the basis of extensive spectroscopic data interpretation, including 1D, 2D NMR and HREIMS. The absolute configuration of 2 was established by the comparison of experimental and calculated electronic circular dichroism (ECD) spectra.     

Structure-chiroptical properties relationship in clavams: an experimental and theoretical study

It is well known that the biological activity of clavams depends strongly on the absolute configuration at the ring junction carbon atom. Therefore, development of the efficient stereo-controlled synthetic methods for the new oxygen analogs of penams, and the structure-activity relationship studies call for a reliable determination of the absolute stereochemistry of newly synthesized compounds. Recently, we proposed an empirical helicity rule relating the configuration of the bridgehead carbon atom to the sign of the 240 nm band observed in the electronic circular dichroism (ECD) spectrum of clavams. In the present work, we investigate the validity of this structure-property relationship for several enantiomeric pairs of model compounds possessing an additional, interfering chromophore in the molecule. For this purpose a combination of the ECD spectroscopy and the time-dependent density functional theory (TD-DFT) is used. A comparison of the ECD spectra with the theoretical ones obtained by the TD-DFT calculations gives a reasonable interpretation of the Cotton effects observed in the 250-220 nm spectral range. Moreover, the calculations confirm validity of the helicity rule for systems studied here and demonstrate that ECD spectroscopy may be used as a highly sensitive probe of the three-dimensional molecular structure of clavams.