Quantumchemical calculation of CD spectra: the absolute configuration of palmarumycins CP3 and C2

Catechol-Bearing Polyketide Derivatives from Sparticola junci

Sparticols A (1) and B (2), two catechol-bearing naphthalenedioxy derivatives, were isolated from the submerged culture of the Spanish broom inhabiting Dothideomycetes fungus, Sparticola junci. The structures of 1 and 2 were established by NMR spectroscopic analysis and high-resolution mass spectrometry. The 8S absolute configuration of their β-hydroxy functionalities was determined by ECD-TDDFT. Both compounds exhibited inhibitory activity against Staphylococcus aureus with an MIC value of 66.6 μg/mL. Polyketides 1 and/or 2 may be associated with pathways cascading to seco-spirodioxynapthalene derivatives.

In-peptide amino acid racemization via inter-residue oxazoline intermediates during acidic hydrolysis

Isopedopeptins are antibiotic cyclic lipodepsipeptides containing the subsequence L-Thr—L-2,3-diaminopropanoic acid—D-Phe—L-Val/L-3-hydroxyvaline. Acidic hydrolysis of isopedopeptins in D₂O showed the D-Phe residues to racemize extensively in peptides with L-3-hydroxyvaline but not in peptides with L-Val. Similarly, one Leu residue in pedopeptins, which are related peptides containing the subsequence Leu—2,3-diaminopropanoic acid—Leu—L-Val/L-3-hydroxyvaline, was found to racemize in peptides with L-3-hydroxyvaline. Model tetrapeptides, L-Ala—L-Phe—L-Val/3-hydroxyvaline—L-Ala, gave the corresponding results, i.e. racemization of L-Phe only when linked to a L-3-hydroxyvaline. We propose the racemization to proceed via an oxazoline intermediate involving Phe/Leu and the L-3-hydroxyvaline residues. The 3-hydroxyvaline residue may form a stable tertiary carbocation by loss of the sidechain hydroxyl group as water after protonation. Elimination of the Phe/Leu H-2 and ring-closure from the carbonyl oxygen onto the carbocation results in the suggested oxazoline intermediate. The reversed reaction leads to either retained or inversed configuration of Phe/Leu. Such racemization during acidic hydrolysis may occur whenever a 3-hydroxyvaline residue or any amino acid that can form a stable carbocation on the C-3, is present in a peptide. The proposed mechanism for racemization was supported by incorporation of ¹⁸O in the 3-hydroxyvaline sidechain when the acidic hydrolysis was performed in H₂O/H₂¹⁸O (1:1). The 2,3-diaminopropanoic residues of isopedopeptins and pedopeptins were also found to racemize during acidic hydrolysis, as previously described. Based on the results, the configuration of the Leu and 2,3-diaminopropanoic acid residues of the pedopeptins were reassigned to be L-Leu and D-Leu, and 2 × L-2,3-diaminopropanoic acid.

Total synthesis of Palmarumycin BGs, C1 and Guignardin E

The first total synthesis of Palmarumycin BG1–3, BG5–6, C₁ and Guignardin E (1–7) were achieved by the same intermediate Palmarumycin C₂ through a N-benzyl cinchoninium chloride-catalyzed epoxidation, an organoselenium-mediated reduction, and a cerium(iii) chloride hydrate-promoted regioselective ring-opening and elimination of cyclic α,β-epoxy ketone as the key steps via6–7 step routes using 1,8-dihydroxynaphthalene (DHN) and 5-methoxytetralone as the starting materials in overall yields of 1.0–17.4%, respectively. Their structures and absolute configurations were characterized and determined by ¹H, ¹³C NMR, IR, HR-ESI-MS and X-ray diffraction data. These compounds displayed significant inhibition activities against HCT116, U87-MG, HepG2, BGC823 and PC9 cell lines.

The first total syntheses of Palmarumycin BG1–3, BG5–6, C₁ and Guignardin E were achieved. These compounds displayed significant inhibition activities against HCT116, U87-MG, HepG2, BGC823 and PC9 cell lines.

Anteaglonialides A-F and Palmarumycins CE(1)-CE(3) from Anteaglonium sp. FL0768, a Fungal Endophyte of the Spikemoss Selaginella arenicola

Anteaglonialides A-F (1-6), bearing a spiro[6-(tetrahydro-7-furanyl)cyclohexane-1,2'-naphtho[1,8-de][1,3]-dioxin]-10-one skeleton, three new spirobisnaphthalenes, palmarumycins CE1-CE3 (7-9), nine known palmarumycin analogues, palmarumycins CP5 (10), CP4a (11), CP3 (12), CP17 (13), CP2 (14), and CP1 (15), CJ-12,371 (16), 4-O-methyl CJ-12,371 (17), and CP4 (18), together with a possible artifact, 4a(5)-anhydropalmarumycin CE2 (8a), and four known metabolites, O-methylherbarin (19), herbarin (20), herbaridine B (21), and hyalopyrone (22), were encountered in a cytotoxic extract of a potato dextrose agar culture of Anteaglonium sp. FL0768, an endophytic fungus of the sand spikemoss, Selaginella arenicola. The planar structures and relative configurations of the new metabolites 1-9 were elucidated by analysis of extensive spectroscopic data, and the absolute configuration of 1 was determined by the modified Mosher's ester method. Application of the modified Mosher's ester method combined with the NOESY data resulted in revision of the absolute configuration previously proposed for 10. Co-occurrence of 1-6 and 7-18 in this fungus led to the proposal that the anteagloniolides may be biogenetically derived from palmarumycins. Among the metabolites encountered, anteaglonialide F (6) and known palmarumycins CP3 (12) and CP1 (15) exhibited strong cytotoxic activity against the human Ewing's sarcoma cell line CHP-100, with IC50 values of 1.4, 0.5, and 1.6 μM, respectively.

Antimicrobial and antioxidant activities and effect of 1-hexadecene addition on palmarumycin C2 and C3 yields in liquid culture of endophytic fungus Berkleasmium sp. Dzf12

Two spirobisnaphthalenes, namely palmarumycins C₂ and C₃, were isolated from cultures of the endophytic fungus Berkleasmium sp. Dzf12 after treatment with 1-hexadecene. After addition of 1-hexadecene at 10% to the medium on day 6 of culture, the maximal yields of palmarumycins C₂ and C₃ were obtained as 0.40 g/L and 1.19 g/L, which were 40.00 fold and 59.50 fold higher, respectively, in comparison with those of the control (0.01 g/L and 0.02 g/L). The results indicated that addition of 1-hexadecene can be an effective strategy for enhancing the production of palmarumycins C₂ and C₃ in liquid culture of endophytic fungus Berkleasmium sp. Dzf12. Palmarumycin C₃ exhibited stronger antimicrobial and antioxidant activities than palmarumycin C₂.

Structure-activity relationship study of selective excitatory amino acid transporter subtype 1 (EAAT1) inhibitor 2-amino-4-(4-methoxyphenyl)-7-(naphthalen-1-yl)-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile (UCPH-101) and absolute configurational assignment using infrared and vibrational circular dichroism spectroscopy in combination with ab initio Hartree-Fock calculations

The excitatory amino acid transporters (EAATs) play essential roles in regulating the synaptic concentration of the neurotransmitter glutamate in the mammalian central nervous system. To date, five subtypes have been identified, named EAAT1-5 in humans, and GLAST, GLT-1, EAAC1, EAAT4, and EAAT5 in rodents, respectively. In this paper, we present the design, synthesis, and pharmacological evaluation of seven 7-N-substituted analogues of UCPH-101/102. Analogue 9 inhibited EAAT1 in the micromolar range (IC(50) value 20 μM), whereas analogues 8 and 10 were inactive (IC(50) values >100 μM). The diastereomeric pairs 11a/11b and 12a/12b were separated by HPLC and the absolute configuration assigned by VCD technique in combination with ab initio Hartree-Fock calculations. Analogues 11a (RS-isomer) and 12b (RR-isomer) inhibited EAAT1 (IC(50) values 5.5 and 3.8 μM, respectively), whereas analogues 11b (SS-isomer) and 12a (SR-isomer) failed to inhibit EAAT1 uptake (IC(50) values >300 μM).

Naphthoquinone spiroketal with allelochemical activity from the newly discovered endophytic fungus Edenia gomezpompae

Chemical investigation of the mycelium of Edenia gomezpompae, a newly discovered endophytic fungus isolated from the leaves of Callicarpa acuminata (Verbenaceae) collected from the ecological reserve El Eden, Quintana Roo, Mexico, resulted in the isolation of four naphthoquinone spiroketals, including three new compounds and palmarumycin CP2 (4). We elucidated the structures of the metabolites by extensive NMR spectroscopy studies, including DEPT, COSY, NOESY, HSQC, HMBC, and chiroptical methods. The trivial names proposed for these compounds are preussomerin EG1 (1), preussomerin EG2 (2) and preussomerin EG3 (3). In addition, the X-ray data for 4 were obtained. The bioactivity of the mycelial organic extracts and the pure compounds was tested against three endophytic fungi (Colletotrichum sp., Phomopsis sp., and Guignardia manguifera) isolated from the same plant species (C. acuminata, Verbenaceae) and against four economically important phytopathogenic microorganisms (two fungoid oomycetes, Phythophtora capsici and Phythophtora parasitica, and the fungi Fusarium oxysporum and Alternaria solani). Spiroketals 1-3 displayed significant growth inhibition against all the phytopathogens. IC50 values for the four phytopathogens were from 20 to 170 microg/ml. Palmarumycin CP2 (4) was not bioactive against any of the fungi tested. Compound 1 showed the strongest bioactivity. The acetylated derivatives of preussomerin EG1 (1), 1a and 1b, were obtained and their biological activity was tested on endophytes and phytopathogens. Preussomerin EG1 1, 1a and 1b exhibited significant bioactivity against all microorganisms tested with the exception of Alternaria solani. This is the first report of allelochemicals with antifungal activity from the newly discovered endophytic fungus E. gomezpompae.

Revision of the Absolute Configuration of Plumericin and Isoplumericin fromPlumeria rubra

The absolute configurations of plumericin (1) and isoplumericin (2), isolated from Plumeria rubra, were re-assigned based on a combination of X-ray crystal-structure determination and quantum-mechanical calculations of their circular dichroism (CD) spectra. The experimental CD spectra showed an excellent match with those calculated for the (1S,5R,8R,9R,10R) absolute configuration (corresponding to ent-1 and ent-2, resp.), opposite to that generally accepted and published in the literature. Since the (false) plumericin configuration has been often used to derive the absolute configuration of related iridoids by chemical correlation, their absolute configurations also have to be reconsidered.

Assignment of the molecular absolute configuration through the ab initio Hartree-Fock calculation of the optical rotation: can the circular dichroism data help in reducing basis set requirements?

In this paper the calculation of the optical rotation (OR) of some rigid organic molecules, using the Hartree-Fock method with small (6-31G, DZP) basis sets, has been studied thoroughly to carefully evaluate the scope and limitations of this method, previously introduced by other authors. Calculations on test molecules (compounds 1-13) together with a careful analysis of their CD spectra allow a simple criterion for the reliability of this approach to be formulated: for unsaturated and/or aromatic (i.e., absorbing in the near-UV region) molecules, if the [alpha](D) is quantitatively determined by the lowest energy Cotton effect (at wavelengths >220 nm), then the HF/6-31G result is reliable. The usefulness of this method for the experimental organic chemist has been further demonstrated because the OR (sign and order of magnitude) of compounds 14-19 (i.e., large molecules having considerable interest in organic chemistry), which fulfill the above criterion and for which an extended basis set treatment is not feasible owing to their size, is correctly predicted.

Full absolute stereostructures of natural products directly from crude extracts: The HPLC-MS/ MS-NMR-CD 'triad'

This chapter deals with an efficient methodology available in our Center of Excellence, BIOTECmarin: the novel analytical 'triad' HPLC-MS/MS-NMR-CD. By this method, which was, in this complemented form, first introduced into phytochemical research by our group, we can not only rapidly identify known structures, but can also investigate new metabolites and establish their full absolute stereostructures online, directly from crude extracts, without the necessity of first isolating the compounds. The LC-CD option, which we have been using for the first time in natural products analysis, becomes even more valuable by the possibility of interpreting the online CD spectra by their simulation or prediction through quantum chemical calculation, thus avoiding the usual, often risky, empirical comparison with the CD spectra of (sometimes not so related) compounds of known absolute stereostructure or the application of (sometimes not really applicable) likewise empirical CD rules. The hyphenated analytical methods are additionally complemented by our synthetic expertise, again involving new concepts and strategies developed in our group. The methods and their application will first be explained and exemplified for plant-derived ('terrestrial') natural products, for which they were initially developed, and will then be applied to the online structural elucidation of novel natural products from marine organisms.

Unified route to the palmarumycin and preussomerin natural products. Enantioselective synthesis of (-)-preussomerin G

The total syntheses of eight members of the palmarumycin family have been achieved, with identification of the absolute stereochemistry for three of these natural products. In addition, the ras-farnesyl transferase inhibitor (-)-preussomerin G has been synthesized, achieving the first enantioselective route for accessing this family of natural products. Highlights of the synthetic work include an asymmetric epoxidation of a cyclic enone in excellent yield and enantiomeric excess and a potentially biomimetic oxidative spirocyclization for the introduction of the bis-spiroketal array unique to the preussomerin natural products.

Absolute stereochemistry of the spiroxins

The absolute configuration of spiroxin A has been determined by exciton-coupled circular dichroism (CD). Namely, the difference CD between spiroxin A – bis-methoxycinnamate, as well as that between spiroxin A – bis-retinoate, both exhibit negative exciton couplings between the acylated phenolic hydroxyl group chromophores. This establishes the absolute configuration as 2S,3R,4S,2'S,3'R,4'S.Key words: circular dichroism, exciton coupling, spiroxins, antitumor, antifungal.

UV mutagenesis and enzyme inhibitors as tools to elucidate the late biosynthesis of the spirobisnaphthalenes

The metabolite pattern of UV mutants of the spirobisnaphthalene producing fungus F-24'707 by TLC and HPLC analysis has been investigated. Mutants with differences in colony morphology or colour compared to the parent strain were isolated. Cultivation in shaking flasks and P flasks showed differences in the metabolite pattern of some of the strains. Furthermore, enzyme inhibitors were used to block the spirobisnaphthalene biosynthesis of the parent strain at different steps. Feeding of precursors and intermediates of cladospirone bisepoxide (15) led to a two-fold increase of the production of 15. From these data and preceding biosynthetic studies we deduced a general pathway for the biosynthesis of all spirobisnaphthalenes of the fungus F-24'707. This enables us to present the hypothesis that all bisnaphthalenes described so far are produced using a common pathway with only a few intermediates as central branching points.