Differentiation of solvatomorphs of active pharmaceutical ingredients (API) by solid-state vibrational circular dichroism (VCD)

Here, we present the new application of solid-state Vibrational Circular Dichroism (VCD) spectroscopy to differentiate several dutasteride (DS) solvatomorphs - the model active pharmaceutical ingredient (API). Several crystalline DS hydrochloride hydrates solvated with methanol, ethanol, acetonitrile, acetone, and acetic acid were prepared. In contrast to almost identical IR spectra, the VCD ones were very sensitive to changes in the sample composition. We marked significant differences in the shape of VCD spectra of studied DS solvatomorphs, DS hydrates, and DS polymorphic forms. Our findings, supported by DFT calculations, show that VCD spectroscopy has the pronounced ability to distinguish their crystal arrangements. We believe that this contribution will extend the use of VCD in the pharmaceutical industry for developing and designing new chiral drug products for the identification, description, and in-depth probing of several pharmaceutical solvatomorphs in the future.

VCD spectra of chiral naphthalene-1-carboxamides in the solid-state

The VCD spectra of chiral 2,3-dihydro-1H-benzo[de]isoquinolin-1-one (8-substituted naphthalene-1-carboxamide, BIQ) were studied in KBr pellets. The X-ray diffractometry revealed that the Me, Ph, and pClPh BIQs crystalize in the monoclinic P21, while nBu, pMePh, and oMeOPh BIQs in the orthorhombic P212121 space group. Only the Me-BIQ crystal exhibits the presence of cyclic amide dimers, while the others contain chains of the amid group hydrogen bonds. For all BIQs, except pMePh, the most intense IR band in the 1750-1550 cm-1 region is located at ca. 1680 cm-1 and is accompanied by two weak ones at ca. 1618 and 1590 cm-1. For the pMePh derivative, four almost equally intense IR bands at 1662, 1639, 1614, and 1588 cm-1 are observed. This region of the IR spectra of BIQs, but pMePh, is well reproduced by calculations based on BIQ monomers. On the other hand, the complex IR pattern of pMePh is computationally reproduced when larger crystal fragments, like octamers, are considered. Registration of the VCD spectra enabled recognizing the complexity of IR contours at ca. 1680 cm-1 by the corresponding VCD motives. For (i) Me, Ph and pClPh (R)-enantiomers, two (+)(-) bands were distinguished and for (ii) nBu and pMePh ones, one VCD band with right-side asymmetry was found. For (iii) oMeOPh the VCD pattern cannot be unambiguously assigned. Thus, the VCD spectra in the ν(C=O) range diverse the studied compounds. Among the set of molecules, pMePh has exceptional crystal geometry. Therefore, its most intense ν(C=O) band position and shape can be connected with the geometry of the hydrogen bonds, interactions, and crystal packing. Interpretation of the VCD spectra is based on linear and packed BIQ octamers. This cluster model can reproduce the main features of the solid-state VCD of BIQs.

Cycloartane-type triterpenoids and steroids from Trichilia connaroides and their multidrug resistance reversal activities

Four undescribed cycloartane-type triterpenoids (1-4) and seven undescribed steroids (6-12), along with five known analogues (5 and 13-16), were isolated from the leaves of Trichilia connaroides. Their structures were identified based on the NMR data and HRESIMS, and the absolute configurations were determined through single-crystal X-ray diffraction analysis, Mosher's method, and ECD calculations. The multidrug resistance (MDR) reversal activities of all the isolates were assessed, and compounds 10 and 11 showed significant activities to reverse the MDR of MCF-7/DOX cells with IC50 values of 2.90 and 3.76 μM, respectively. These bioactive compounds may bring fresh insights into the research and development of MDR reversal agents.

Guided Isolation of An Uncommon Cembranoid Orthoester, Sarcotortin A, and Three Skeletal Diverse Terpenoids from the Hainan Soft Coral Sarcophyton tortuosum Based on Molecular Networking Strategy

Applying the emerging molecular networking strategy, an uncommon cembranoid orthoester, sarcotortin A (1), featuring a 3/14/8/5-fused scaffold, an unusual eunicellane-type diterpenoid, sarcotorolide A (2), and two new biscembranoids, ximaolides M and N (7 and 8), along with nine known terpenoids 3-6 and 9-13 were isolated from the Hainan soft coral Sarcophyton tortuosum. The structure and absolute configuration of all new compounds were established by a combination of spectroscopic data, X-ray diffraction analysis, and/or quantum chemical computational approaches. The plausible biogenetic relationship among these skeletally different terpenoids was proposed and discussed. In in vitro bioassay, new compound 7 exhibited a remarkable inhibitory activity against protein tyrosine phosphatases 1B (PTP1B) with the IC₅₀ value of 8.06 μM. In addition, compounds 4 and 10 displayed significant inhibitory effects on lipopolysaccharide (LPS)-induced inflammatory responses in RAW264.7 macrophages cells with the IC₅₀ values of 19.13 and 16.45 μM, respectively. Compound 9 showed interesting cytotoxicity against H1975, MDA-MB231, A549, and H1299 cancer cell lines with IC₅₀ values of 31.59, 34.96, 43.87, and 27.93 μM, respectively.

Electronic Circular Dichroism Imaging (ECDi) Casts a New Light on the Origin of Solid-State Chiroptical Properties

Solid-state ECD (ss-ECD) spectra of a model microcrystalline solid, finasteride, dispersed into a KCl pellet were recorded by using the synchrotron radiation source at the Diamond B23 beamline. Scanning a surface of 36 mm² with a step of 0.5 mm, we measured a set of ECD imaging (ECDi) spectra very different from each other and from the ss-ECD recorded with a bench-top instrument (1 cm² area). This is due to the anisotropic part of the ECD (ACD), which averages to zero in solution or on a large number of randomly oriented crystallites, but can otherwise be extremely large. Two-way singular value decomposition (SVD) analysis, through experimental and simulated TDDFT spectra, disclosed that the measured and theoretical principal components are in line with each other. This finding demonstrates that the observed isotropic ss-ECD spectrum is governed by the anisotropy of locally oriented crystals. It also introduces a new quality for ss-ECD measurements and opens a new future for probing and mapping chiral materials in the solid state such as active pharmaceutical ingredients (APIs).

New lobane-type diterpenoids from the Xisha soft coral Sinularia polydactyla

Lobane-type diterpenoids are not frequently discovered from marine soft corals. In this paper, three new lobane type diterpenes, 13-methoxyloba-8,10,15(16),17(18)-tetraene (1), 8,10,13(15)Z,16E-lobatetraene (2) and 19-hydroxy-lobatetraene (3), and a new natural compound, 17,18-epoxyloba-16-acetoxy-8,10,13(15)-trien (4), co-occurring with a known related diterpenoid, 18-methoxyloba-8,10,13(15),16(17)-tetraene (5), were isolated from the South China Sea soft coral Sinularia polydactyla. The structures of new compounds were determined by extensive spectroscopic analysis and by comparison with those reported in the literature. In bioassay, all the isolates were inactive on antibacterial, PTP1B inhibitory, and immunological activities. This study increased the chemical diversity of marine diterpenoids.

New Biscembranoids Sardigitolides A-D and Known Cembranoid-Related Compounds from Sarcophyton digitatum: Isolation, Structure Elucidation, and Bioactivities

Chemical examination from the cultured soft coral Sarcophyton digitatum resulted in the isolation and structural identification of four new biscembranoidal metabolites, sardigitolides A-D (1-4), along with three previously isolated biscembranoids, sarcophytolide L (5), glaucumolide A (6), glaucumolide B (7), and two known cembranoids (8 and 9). The chemical structures of all isolates were elucidated on the basis of 1D and 2D NMR spectroscopic analyses. Additionally, in order to discover bioactivity of marine natural products, 1-8 were examined in terms of their inhibitory potential against the upregulation of inflammatory factor production in lipopolysaccharide (LPS)-stimulated murine macrophage J774A.1 cells and their cytotoxicities against a limited panel of cancer cells. The anti-inflammatory results showed that at a concentration of 10 µg/mL, 6 and 8 inhibited the production of IL-1β to 68 ± 1 and 56 ± 1%, respectively, in LPS-stimulated murine macrophages J774A.1. Furthermore, sardigitolide B (2) displayed cytotoxicities toward MCF-7 and MDA-MB-231 cancer cell lines with the IC50 values of 9.6 ± 3.0 and 14.8 ± 4.0 µg/mL, respectively.

Sarinfacetamides A and B, Nitrogenous Diterpenoids with Tricyclo[6.3.1.01,5]dodecane Scaffold from the South China Sea Soft Coral Sarcophyton infundibuliforme

Two novel nitrogenous diterpenoids, sarinfacetamides A (1) and B (2), featuring an uncommon tricyclo[6.3.1.0^1,5]dodecane scaffold, and a known related diterpene (3) were isolated from the South China Sea soft coral Sarcophyton infundibuliforme. Their structures, including the absolute configuration of 1, were established by extensive spectroscopic analysis and TDDFT-ECD calculation. Compounds 1 and 3 exhibited interesting promotion effects on the ConA-induced T lymphocyte proliferation. A plausible biosynthetic pathway for 1 and 2 was also proposed.

Daryamide Analogues from a Marine-Derived Streptomyces species

Three new cyclohexene amine derivatives, daryamides D-F (1-3), a new arylamine derivative, carpatamide D (4), and a new ornithine lactamization derivative, ornilactam A (5), were isolated from the marine-derived Streptomyces strain SNE-011. Their structures, including absolute configurations, were elucidated on the basis of spectroscopic analysis and chemical methods. The carpatamide skeleton could be considered as the biosynthetic precursor of the daryamides.

Biscembranoids and Cembranoids from the Soft Coral Sarcophyton elegans

Two novel biscembranoids, sarelengans A and B (1 and 2), five new cembranoids, sarelengans C–G (3–7), along with two known cembranoids (8 and 9) were isolated from the South China Sea soft coral Sarcophyton elegans. Their structures were determined by spectroscopic and chemical methods, and those of 1, 4, 5, and 6 were confirmed by single crystal X-ray diffraction. Compounds 1 and 2 represent the first example of biscembranoids featuring a trans-fused A/B-ring conjunction between the two cembranoid units. Their unique structures may shed light on an unusual biosynthetic pathway involving a cembranoid-∆⁸ rather than the normal cembranoid-∆¹ unit in the endo-Diels-Alder cycloaddition. Compounds 2 and 3 exhibited potential inhibitory effects on nitric oxide production in RAW 264.7 macrophages, with IC₅₀ values being at 18.2 and 32.5 μM, respectively.

Incensfuran: isolation, X-ray crystal structure and absolute configuration by means of chiroptical studies in solution and solid state

A new cembrane diterpene named incensfuran (1), biogenetically derived from incensole (2), was isolated from crude extracts of the Boswellia papyrifera Hochst.

Lignans from the Australian Endemic Plant Austrobaileya scandens

The sole species of the vascular plant family Austrobaileyaceae, Austrobaileya scandens, is endemic to the tropical rainforest of northeastern Queensland, Australia. A single lead-like enhanced fraction of A. scandens showed potent inhibition against human prostate cancer PC3 cells. Chemical investigation of this plant resulted in the isolation of two new aryltetralin lignans, austrobailignans 8 and 9 (1 and 2), and the synthetic compound nicotlactone B (3), newly identified as a natural product together with nine known lignans (4-12). Their structures were established on the basis of spectroscopic analyses. Absolute configurations of the new compounds were determined by quantum chemical electronic circular dichroism (ECD) calculations employing time-dependent density functional theory. The ECD calculations were also used to assign the absolute configuration of marphenol K (4) and revise the absolute configuration of kadsurindutin C (20). Ten out of the 12 isolated compounds inhibited the growth of PC3 cells with IC50 values ranging from micromolar to nanomolar. Marphenol A (5) was found for the first time to induce apoptosis and arrest the S cell cycle phase of PC3 cells.

Cytotoxic Biscembranoids from the Soft Coral Sarcophyton pauciplicatum

Ten biscembranoids (1-10), including the two new compounds sarcophytolides M and N (1 and 2), were isolated from the methanol extract of the Vietnamese soft coral Sarcophyton pauciplicatum. Their structures were elucidated by spectroscopic methods including one dimensional (1D)- and 2D-NMR, high-resolution electrospray ionization mass spectrometry (HR-ESI-MS), and circular dichroism (CD). The in vitro cytotoxic activity of all isolated compounds against a panel of eight human cancer cell lines including HepG2 (hepatoma cancer), HL-60 (acute leukemia), KB (epidermoid carcinoma), LNCaP (prostate cancer), LU-1 (lung cancer), MCF7 (breast cancer), SK-Mel2 (melanoma), and SW480 (colon adenocarcinoma) was evaluated using the sulforhodamine B colorimetric assay. Among the isolated biscembranoids, 1, 3, 4, 7, 9, and 10 exhibited significant cytotoxic effects and may be selected for further studies of their anticancer activity.

Stereochemistry of cleistanthane diterpenoid glucosides from Phyllanthus emblica

Theoretical calculated electronic circular dichroism and Mosher’s method were applied to elucidate the stereochemistry of cleistanthane diterpenoids.

Homoconjugation vs. exciton coupling in chiral α,β-unsaturated bicyclo[3.3.1]nonane dinitrile and carboxylic acids

The chiroptical properties of enantiomerically pure bicyclo[3.3.1]nona-2,6-diene-2,6-dicarbonitrile and related acids were studied by circular dichroism spectroscopy and theoretical computations. A consideration of the molecular structure of the synthesized difunctional compounds revealed that chromophores are predisposed to transannular through-space interaction due to a favourable conformation of the bicyclic skeleton and a rather small interchromophoric distance. Evidence for non-exciton-type coupling between the two acrylonitrile and acrylate moieties in 3 and 4, respectively, was obtained by chiroptical spectroscopy and DFT calculations.

Anisotropic dissymmetry factor, g: theoretical investigation on single molecule chiroptical spectroscopy

A formula for an anisotropic dissymmetry factor g evaluating the chiroptical response of orientationally fixed molecules is derived. Incorporating zeroth- and first-order multipole expansion terms, it is applied to bridged triarylamine helicene molecules to examine the experimental results of single-molecule chiroptical spectroscopy. The ground- and excited-state wave functions and a series of transition moments required for the evaluation of the anisotropic g value are calculated using time-dependent density functional theory (TDDFT). The probability histograms obtained for simulated g values, uniformly sampled in regard to the direction of light propagation toward the fixed molecule, show that even for a given diastereomer, the dissymmetry factors have positive and negative values and can deviate from their averages to a considerable extent when the angle between the electric dipole transition moment and the propagation vector of the incident light is near 0 or 180°.

Marine natural products

This review covers the literature published in 2012 for marine natural products, with 1035 citations (673 for the period January to December 2012) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1241 for 2012), together with the relevant biological activities, source organisms and country of origin. Biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.

Strong intermolecular exciton couplings in solid-state circular dichroism of aryl benzyl sulfoxides

A series of 13 enantiopure aryl benzyl sulfoxides () with different substituents on the two aromatic rings has been previously analyzed by means of electronic circular dichroism (CD) spectroscopy. Most of these compounds are crystalline and their X-ray structure is established. For almost one-half of the series, CD spectra measured in the solid state were quite different from those in acetonitrile solution. We demonstrate that the difference is due to strong exciton couplings between molecules packed closely together in the crystal. The computational approach consists of time-dependent density functional theory (TDDFT) calculations run on "dimers" composed of nearest neighbors found in the lattice. Solid-state CD spectra are well reproduced by the average of all possible pairwise terms. The relation between the crystal space group and conformation, and the appearance of solid-state CD spectra, is also discussed.

Structural revision of methyl tortuoate D, a bis-cembranoid from Hainan Sarcophyton tortuosum and its absolute stereochemistry

Methyl tortuoate D (1), together with five other known related bis-cembranoids, was isolated from Hainan soft coral Sarcophyton tortuosum. The structure of methyl tortuoate D (1a), firstly isolated and reported by Li et al. from the title organism, was corrected as 1 by an extensive analysis of its one-dimensional and two-dimensional nuclear magnetic resonance data and by comparison with those reported in the literature. In addition, lobophytone K (1b), recently isolated from Hainan soft coral Lobophytum pauciflorum by Lin et al., was proved to be the same compound as 1 and 1a. The absolute configuration of 1 was determined by comparing its electronic circular dichroism curve with that of co-occurring ximaolide A (2).

Circular dichroism calculation for natural products

Determination of the absolute configuration (AC) is often a challenging aspect in the structure elucidation of natural products. When chiral compounds possess appropriate chromophore(s), electronic circular dichroism (ECD) may provide a powerful approach to the determination of their absolute configuration. Recently, ECD calculations by time-dependent density functional theory (TDDFT) have come to be used more commonly. In the present review, we give several examples of recent studies using TDDFT-calculated ECD spectra for the AC determination of natural products.