DFT investigation of coupling constant anomalies in substituted β-lactams

β-lactams are a chemically diverse group of molecules with a wide range of biological activities. Having recently observed curious trends in 2JHH coupling values in studies on this structural class, we sought to obtain a more comprehensive understanding of these diagnostic NMR parameters, specifically interrogating 1JCH, 2JCH, and 2JHH, to differentiate 3- and 4-monosubstituted β-lactams. Further investigation using computational chemistry methods was employed to explore the geometric and electronic origins for the observed and calculated differences between the two substitution patterns.

Orthogonal Deprotection Strategy of Fmoc Provides Improved Synthesis of Sensitive Peptides: Application to Z-Arg-Lys-AOMK

Protecting groups (PGs) in peptide synthesis have inspired advanced design principles that incorporate "orthogonality" for selective C- and N-terminus and side-chain deprotections. The conventionally acid-stable 9-fluorenylmethoxycarbonyl (Fmoc) group is one of the most widely used N-protection groups in solid- and solution-phase synthesis. Despite the versatility of Fmoc, deprotection by the removal of the Fmoc group to unmask primary amines requires the use of a basic secondary amine nucleophile, but this stratagem poses challenges in sensitive molecules that bear reactive electrophilic groups. An expansion of PG versatility, a tunable orthogonality, in the late-stage synthesis of peptides would add flexibility to the synthetic design and implementation. Here, we report a novel Fmoc deprotection method using hydrogenolysis under mildly acidic conditions for the synthesis of Z-Arg-Lys-acyloxymethyl ketone (Z-R-K-AOMK). This new method is not only valuable for Fmoc deprotection in the synthesis of complex peptides that contain highly reactive electrophiles, or other similar sensitive functional groups, that are incompatible with traditional Fmoc deprotection conditions but also tolerant of N-Boc groups present in the substrate.

Petrosamine Revisited. Experimental and Computational Investigation of Solvatochromism, Tautomerism and Free Energy Landscapes of a Pyridoacridinium Quaternary Salt

Petrosamine (1)-a colored pyridoacridine alkaloid from the Belizean sponge, Petrosia sp., that is also a potent inhibitor of acetylcholine esterase (AChE)-was investigated by spectroscopic and computational methods. Analysis of the petrosamine-free energy landscapes, pKa and tautomerism, revealed an accurate electronic depiction of the molecular structure of 1 as the di-keto form, with a net charge of q = +1, rather than a dication (q = +2) under ambient conditions of isolation-purification. The pronounced solvatochromism (UV-vis) reported for 1, and related analogs were investigated in detail and is best explained by charge delocalization and stabilization of the ground state (HOMO) of 1 rather than an equilibrium of competing tautomers. Refinement of the molecular structure 1 by QM methods complements published computational docking studies to define the contact points in the enzyme active site that may improve the design of new AChE inhibitors based on the pyridoacridine alkaloid molecular skeleton.

Small Molecule in situ Resin Capture - A Compound First Approach to Natural Product Discovery

Microbial natural products remain an important resource for drug discovery. Yet, commonly employed discovery techniques are plagued by the rediscovery of known compounds, the relatively few microbes that can be cultured, and laboratory growth conditions that do not elicit biosynthetic gene expression among myriad other challenges. Here we introduce a culture independent approach to natural product discovery that we call the Small Molecule In situ Resin Capture (SMIRC) technique. SMIRC exploits in situ environmental conditions to elicit compound production and represents a new approach to access poorly explored chemical space by capturing natural products directly from the environments in which they are produced. In contrast to traditional methods, this compound-first approach can capture structurally complex small molecules across all domains of life in a single deployment while relying on Nature to provide the complex and poorly understood environmental cues needed to elicit biosynthetic gene expression. We illustrate the effectiveness of SMIRC in marine habitats with the discovery of numerous new compounds and demonstrate that sufficient compound yields can be obtained for NMR-based structure assignment. Two new compound classes are reported including one novel carbon skeleton that possesses a functional group not previously observed among natural products and a second that possesses potent biological activity. We introduce expanded deployments, in situ cultivation, and metagenomics as methods to facilitate compound discovery, enhance yields, and link compounds to producing organisms. This compound first approach can provide unprecedented access to new natural product chemotypes with broad implications for drug discovery.

Geobarrettin D, a Rare Herbipoline-Containing 6-Bromoindole Alkaloid from Geodia barretti

Geobarrettin D (1), a new bromoindole alkaloid, was isolated from the marine sponge Geodia barretti collected from Icelandic waters. Its structure was elucidated by 1D, and 2D NMR (including 1H-15N HSQC, 1H-15N HMBC spectra), as well as HRESIMS data. Geobarrettin D (1) is a new 6-bromoindole featuring an unusual purinium herbipoline moiety. Geobarrettin D (1) decreased secretion of the pro-inflammatory cytokine IL-12p40 by human monocyte derived dendritic cells, without affecting secretion of the anti-inflammatory cytokine IL-10. Thus, compound 1 shows anti-inflammatory activity.

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.

Chlororesistoflavins A and B, Chlorinated Benzopyrene Antibiotics Produced by the Marine-Derived Actinomycete Streptomyces sp. Strain EG32

As part of a collaborative biomedical investigation of actinomycete bacteria isolated from sediments collected along the northern coast of Egypt (Mediterranean Sea), we explored the antibacterial metabolites from a bacterium identified as a Streptomyces sp., strain EG32. HPLC analysis and antibacterial testing against methicillin-resistant Staphylococcus aureus (MRSA) resulted in the identification of six compounds related to the resistoflavin and resistomycin class. Two of these metabolites were the chlorine-containing analogues chlororesistoflavins A (1) and B (2). The absolute configurations of the lone stereogenic center (C-11b) in these metabolites were assigned by analysis of their ECD spectra. Interestingly, the ECD spectrum of chlororesistoflavin A (1) shows a Cotton effect of the n-π* transition antipodal to that of the parent natural product, a consequence of 1,3-allylic strain induced by the adjacent bulky chlorine atom that distorts the coplanarity of the carbonyl group with the π-system. The chiroptical analysis thus resolves the paradox and uniformly aligns the configuration of all analogues as identical to that reported for natural resistoflavin. Chlororesistoflavins A (1) and B (2) exhibited antibacterial activity against MRSA with a minimum inhibitory concentration of 0.25 and 2.0 μg/mL, respectively.

Oceanapiside, a Marine Natural Product, Targets the Sphingolipid Pathway of Fluconazole-Resistant Candida glabrata

Oceanapiside (OPS), a marine natural product with a novel bifunctional sphingolipid structure, is fungicidal against fluconazole-resistant Candida glabrata at 10 μg/mL (15.4 μM). The fungicidal effect was observed at 3 to 4 h after exposure to cells. Cytological and morphological studies revealed that OPS affects the budding patterns of treated yeast cells with a significant increase in the number of cells with single small buds. In addition, this budding morphology was found to be sensitive in the presence of OPS. Moreover, the number of cells with single medium-sized buds and cells with single large buds were decreased significantly, indicating that fewer cells were transformed to these budding patterns, suggestive of inhibition of polarized growth. OPS was also observed to disrupt the organized actin assembly in C. glabrata, which correlates with inhibition of budding and polarized growth. It was also demonstrated that phytosphingosine (PHS) reversed the antifungal activity of oceanapiside. We quantified the amount of long chain-bases (LCBs) and phytoceramide from the crude extracts of treated cells using LC-ESI-MS. PHS concentration was elevated in extracts of cells treated with OPS when compared with cells treated with miconazole and amphotericin B. Elevated levels of PHS in OPS-treated cells confirms that OPS affects the pathway at a step downstream of PHS synthesis. These results also demonstrated that OPS has a mechanism of action different to those of miconazole and amphotericin B and interdicts fungal sphingolipid metabolism by specifically inhibiting the step converting PHS to phytoceramide.

Bromotryptamine and Imidazole Alkaloids with Anti-inflammatory Activity from the Bryozoan Flustra foliacea

Chemical investigation of the marine bryozoan Flustra foliacea collected in Iceland resulted in isolation of 13 new bromotryptamine alkaloids, flustramines Q-W (1-7) and flustraminols C-H (8-13), and two new imidazole alkaloids, flustrimidazoles A and B (14 and 15), together with 12 previously described compounds (16-27). Their structures were established by detailed spectroscopic analysis using 1D and 2D NMR and HRESIMS. Structure 2 was verified by calculations of the 13C and 1H NMR chemical shifts using density functional theory. The relative and absolute configurations of the new compounds were elucidated on the basis of coupling constant analysis, NOESY, [α]D, and ECD spectroscopic data, in addition to chemical derivatization. The compounds were tested for in vitro anti-inflammatory activity using a dendritic cell model. Eight compounds (1, 3, 5, 13, 16, 18, 26, and 27) decreased dendritic cell secretion of the pro-inflammatory cytokine IL-12p40, and two compounds (4 and 14) increased secretion of the anti-inflammatory cytokine IL-10. Deformylflustrabromine B (27) showed the most potent anti-inflammatory effect (IC50 2.9 μM). These results demonstrate that F. foliacea from Iceland expresses a broad range of brominated alkaloids, many without structural precedents. The potent anti-inflammatory activity in vitro of metabolite 27 warrants further investigations into its potential as a lead for inflammation-related diseases.

Synchronous bond molecular dynamics of conjugated chlorocyclopropyl alk-yn-enes revealed by ECD and UV-vis

Chlorocyclopropanes (CCPs) conjugated to alk-yn-enes occur in a unique family of polyketide natural products from marine sponges. Synthesis of several optically enriched analogs of CCPs and measurement of their UV-vis spectra and electronic circular dichroism (ECD) spectra reveal unusually strong hyperconjugation that constrains and aligns the cyclopropyl C-C bond with the π-plane of the distal ene-bond. This alignment imposes a barrier to rotation of at least 5.0 kcal·mol-1 . Comparison of red-shifted Cotton effects in chiral CCPs show the barrier is independent of alkene substituent and establishes an empirical rule for assignment of other CCP-containing natural products.

Bromo-spiroisoxazoline Alkaloids, Including an Isoserine Peptide, from the Caribbean Marine Sponge Aplysina lacunosa

Three new bromotyrosine spiroisoxazoline alkaloids, lacunosins A and B (1 and 2) and desaminopurealin (3), were isolated from a MeOH extract of the marine sponge Aplysina lacunosa that showed modest α-chymotrypsin inhibitory activity. The structures of 1-3 share the spirocyclohexadienyl-isoxazoline ring system found in purealidin-R and several other Verongid sponge secondary metabolites. Compounds 1 and 2 are coupled to a glycine and an isoserine methyl ester, respectively. Alkaloid 3 is linked, contiguously, to an O-1-aminopropyl 3,5-dibromotyrosyl ether and, finally, to histamine through an amide bond. The planar structures of all three compounds were obtained from analysis of MS and 1D and 2D NMR data. The absolute configuration of the SIO unit of 1-3 was assigned by electronic circular dichroism (ECD). The isoserine amino acid residue in 2 was found to be a 1:1 mixture of epimers using a new Marfey's type reagent, derived from Trp-NH2. Allylic O-naphthoylation of the SIO subunit enhances the ECD spectrum of SIOs and improves discrimination of enantiomorphs. A unifying hypothesis is proposed that links the biosynthesis of several of the new compounds with previously reported analogues.

The Configuration of Distaminolyne A is S: Quantitative Evaluation of Exciton Coupling Circular Dichroism of N, O- Bis-arenoyl-1-amino-2-alkanols

The 2 S configuration of the marine natural product distaminolyne A was recently disputed based upon total synthesis, yet paradoxically supported by a second independent total synthesis from a different research group. We now verify the 2 S configuration of distaminolyne A by extensive chiroptical studies and support the veracity of the EC ECD method originally used to prove it. The origin of the apparent paradox appears to lie in the limits of precision of polarimetry in the context of weakly rotatory molecules, which strikes a cautionary note on the reliability of "reassignment" of natural product configurations based solely on specific rotation.

Synergistic Anti-Candida Activity of Bengazole A in the Presence of Bengamide A †

Bengazoles A⁻G from the marine sponge Jaspis sp. exhibit potent in vitro antifungal activity against Candida spp. and other pathogenic fungi. The mechanism of action (MOA) of bengazole A was explored in Candida albicans under both liquid culture and surface culture on Mueller-Hinton agar. Pronounced dose-dependent synergistic antifungal activity was observed with bengazole A in the presence of bengamide A, which is also a natural product from Jaspis sp. The MOA of bengazole A was further explored by monitoring the sterol composition of C. albicans in the presence of sub-lethal concentrations of bengazole A. The GCMS of solvent extracts prepared from liquid cultures of C. albicans in the presence of clotrimazole-a clinically approved azole antifungal drug that suppresses ergosterol biosynthesis by the inhibition of 14α-demethylase-showed reduced cellular ergosterol content and increased concentrations of lanosterol and 24-methylenedihydrolanosterol (a shunt metabolite of ergosterol biosynthesis). No change in relative sterol composition was observed when C. albicans was cultured with bengazole A. These results eliminate an azole-like MOA for the bengazoles, and suggest that another as-yet unidentified mechanism is operative.

The value of universally available raw NMR data for transparency, reproducibility, and integrity in natural product research

Covering: up to 2018With contributions from the global natural product (NP) research community, and continuing the Raw Data Initiative, this review collects a comprehensive demonstration of the immense scientific value of disseminating raw nuclear magnetic resonance (NMR) data, independently of, and in parallel with, classical publishing outlets. A comprehensive compilation of historic to present-day cases as well as contemporary and future applications show that addressing the urgent need for a repository of publicly accessible raw NMR data has the potential to transform natural products (NPs) and associated fields of chemical and biomedical research. The call for advancing open sharing mechanisms for raw data is intended to enhance the transparency of experimental protocols, augment the reproducibility of reported outcomes, including biological studies, become a regular component of responsible research, and thereby enrich the integrity of NP research and related fields.

Enantiomeric Variability of Distaminolyne A. Refinement of ECD and NMR Methods for Determining Optical Purity of 1-Amino-2-Alkanols

Sample configurations of distaminolyne A (1a); isolated from the ascidians Pseudodistoma opacum and P. cereum, and collected at different sites in New Zealand, were investigated by two methods: Exciton coupled electronic circular dichroism (EC ECD) of the corresponding N,O-dibenzoyl derivative 1b; and chiral reagent derivatization of 1a with (S)- and (R)-α-methoxyphenylacetic acid (MPA), followed by ¹H-NMR analysis. Configuration and optical purity of 1a (%ee) was found to vary depending on the geographic distribution of ascidian colonies. An improved method for preparing N,O-diarenoyl derivatives of 1a was optimized. The EC ECD method was found to be complementary to the MPA-NMR method at different ranges of %ee.

Correction: The value of universally available raw NMR data for transparency, reproducibility, and integrity in natural product research

Correction for 'The value of universally available raw NMR data for transparency, reproducibility, and integrity in natural product research' by James B. McAlpine et al., Nat. Prod. Rep., 2018, DOI: .

Lepadins I-K, 3- O-(3'-Methylthio)acryloyloxy-decahydroquinoline Esters from a Bahamian Ascidian Didemnum sp. Assignment of Absolute Stereostructures

Three decahydroisoquinoline alkaloids, lepadins I-K, were isolated from a specimen of Didemnum sp. collected in the Bahamas. The structures of the new compounds were assigned by an integrated analysis of MS, IR, and ¹H, ¹³C, and 2D NMR spectra. Like previously reported lepadins, the structures of the new compounds contain a decahydroquinoline heterocyclic core in lepadin I, and a new variation, an octahydroquinoline in lepadin J, but differ from earlier reported compounds by acylation of the 3-hydroxyl group by a rare 3'-methylthioacrylate. The absolute configuration of lepadin I was solved by interpretation of NOE measurements, and exciton coupled circular dichroism (ECCD) of the corresponding N- p-bromobenzoyl derivative. The latter constitutes a general method for determination of absolute configuration of the entire lepadin family. The configuration of the remote side-chain secondary carbinol was solved by the modified Mosher's esters method. Lepadin I inhibited butyrylcholineesterase (BuChE, IC₅₀ 3.1 μM), but only weakly inhibited acetylcholineesterase (AChE) (10% at 100 μM).

Bromopyrrole Alkaloid Inhibitors of the Proteasome Isolated from a Dictyonella sp. Marine Sponge Collected at the Amazon River Mouth

The new pyrrole-imidazole and pyrrole-guanidine alkaloids 4-debromooroidin (1), 4-debromougibohlin (2), 5-debromougibohlin (3), and 5-bromopalau'amine (4), along with the known hymenidin (5) and (+)-monobromoisophakellin (6), have been isolated from a Dictyonella sp. marine sponge, collected at the Amazon River mouth. The bromine-substitution pattern observed for compounds 1, 2 and 4 is unusual among bromopyrrole alkaloids isolated from marine sponges. The 20S proteasome inhibitory activities of compounds 1-6 have been recorded, with 5-bromopalau'amine (4) being the most active in this series.

Resolution of Atropisomeric Cyclic Catechol Monoether O-Sulfate Esters by a Molluscan Sulfatase

Atropisomeric cyclic catechol ethers are notoriously difficult to resolve by classical chiral phase high-performance liquid chromatography. Here, we show the first application of sulfatase enzymes for the kinetic resolution of O-sulfato-catechol ethers with enantioselectivities ranging from 30 to 65% ee, as determined by preparation of their Marfey's ether derivatives. Substrate-structure dependence was briefly explored.

Cyclic azole-homologated peptides from Marine sponges

This review discusses the chemistry of cyclic azole-homologated peptides (AHPs) from the marine sponges, Theonella swinhoei, other Theonella species, Calyx spp. and Plakina jamaicensis. The origin, distribution of AHPs and molecular structure elucidations of AHPs are described followed by their biosynthesis, bioactivity, and synthetic efforts towards their total synthesis. Reports of partial and total synthesis of AHPs extend beyond peptide coupling reactions and include creative construction of the non-proteinogenic amino acid components, mainly the homologated heteroaromatic and α-keto-β-amino acids. A useful conclusion is drawn regarding AHPs: despite their rarity, exotic structures and the potent protease inhibitory properties of some members, their synthesis is under-developed and beckons solutions for outstanding problems towards their efficient assembly.

Six Trikentrin-like Cyclopentanoindoles from Trikentrion flabelliforme. Absolute Structural Assignment by NMR and ECD

Six new cyclopenta[g]indoles were isolated from a West Australian sponge, Trikentrion flabelliforme Hentschel, 1912, and their structures elucidated by integrated spectroscopic analysis. The compounds are analogues of previously described trikentrins, herbindoles, and trikentramides from related Axinellid sponges. The assignment of absolute configuration of the new compounds was carried out largely by comparative analysis of specific rotation, calculated and measured ECD, and exploiting van't Hoff's principle of optical superposition. Five of the new compounds were chemically interconverted to establish their stereochemical relationships, leading to a simple chiroptical mnemonic for assignment of the this family of chiral indoles. The first biosynthetic hypothesis is advanced to explain the origin of the trikentrin-herbinole family and proposes a pyrrole-carboxylic thioester-initiated polyketide synthase mechanism.

Liposomal Circular Dichroism (L-CD) of Arenoyl Derivatives of Sphingolipids. Amplification of Cotton Effects in Ordered Lipid Bilayers

Liposomal circular dichroism (L-CD) of acyclic amino alcohols exhibit amplification of Cotton effects when measured in highly uniform, unilamellar liposomes. The effect is likely due to intermolecular associations-H-aggregates-that self-assemble spontaneously within the lipid bilayer, and persists over long time scales. L-CD spectra of N,O,O'-tri-(6'methoxy-2'-naphthoyl)-d-erythro-sphingosine, or the corresponding dihydro-derivative (sphinganine), shows ~10-fold amplification of magnitudes of Cotton effects over conventional CD spectra recorded in isotropic solution.

Isolation of bastadin-6-O-sulfate and expedient purifications of bastadins-4, -5 and -6 from extracts of Ianthella basta

Bastadin-6-34-O-sulfate ester (8) was isolated from methanol extracts of Ianthella basta. The structure of 8 was characterized by analysis of MS and NMR data, and conversion through acid hydrolysis, to the parent compound, bastadin-6, which was identical by HPLC, MS and NMR with an authentic sample. An improved procedure for procurement of pure samples of bastadins-4 (4), -5 (5) and -6 (6) is described.

Cyclic Hexapeptide Dimers, Antatollamides A and B, from the Ascidian Didemnum molle. A Tryptophan-Derived Auxiliary for l- and d-Amino Acid Assignments

Two dimerized cyclic hexapeptides, antatollamides A (1) and B (2), were isolated from the colonial ascidian Didemnum molle collected in Pohnpei. The amino acid compositions and sequences were determined by interpretation of MS and 1D and 2D NMR data. Raney Ni reduction of antatollamide A cleaved the dimer to the corresponding monomeric cyclic hexapeptide with replacement of Cys by Ala. The amino acid configuration of 1 was established, after total hydrolysis, by derivatization with a new chiral reagent, (5-fluoro-2,4-dinitrophenyl)-N^α-l-tryptophanamide (FDTA), prepared from l-Trp, followed by LCMS analysis; all amino acids were found to be l-configured except for d-Ala.

Ultra-high resolution band-selective HSQC for nanomole-scale identification of chlorine-substituted 13 C in natural products drug discovery

Ultra-high resolution band-selective HSQC (bsHSQC) has been employed for detection of 35 Cl-37 Cl isotope shifted 13 C NMR signals for assignment of regioisomerism in bromo-chloro natural products. Optimum pulse sequence and instrumental parameters for maximization of detection of the isotope shifts were explored. The chlorine isotope shifts (Δδ) were detected within crosspeaks and were shown to vary with hybridization of 13 C, substitution of 13 C, presence of β-chloro substituents, and their relative configuration. Deconvolution of Cl-substituted CH bsHSQC crosspeaks may provide other useful information, including a potentially MS-independent method for quantitating 37 Cl/35 C isotopic fractionation during the biosynthesis of halogenated natural products. Copyright © 2016 John Wiley & Sons, Ltd.

Synthesis and Structure Revision of Symplocin A

Symplocin A, a linear peptide possessing N-terminal N,N-dimethylisoleucine, statine, and valic acid residues, has been synthesized for the first time employing our previously established 'one-pot intramolecular tandem protocol'. Moreover, the stereochemistry of natural symplocin A was unambiguously revised through the confirmation by 1D NMR, 2D NMR, and HPLC comparisons with authentic natural product.

Jamaicensamide A, a Peptide Containing β-Amino-α-keto and Thiazole-Homologated η-Amino Acid Residues from the Sponge Plakina jamaicensis

A new cyclic peptide, jamaicensamide A, composed of six amino acids, including a thiazole-homologated amino acid, was isolated from the Bahamian sponge Plakina jamaicensis, along with known compounds bitungolide A and franklinolide A. The structure of the title peptide was solved by integrated analysis of MS, 1D and 2D NMR data, oxidation-hydrolyses to α-amino acids, and their stereodetermination by Marfey's method. The close structural resemblance of Western Atlantic-derived jamaicensamide A to known Western Pacific-derived peptides of lithistid sponges in the genus Theonella and Discodermia suggests a common origin: the symbiotic bacterium Entotheonella sp., a so-called "talented producer" responsible for biosynthesis of most Theonella-associated peptides. Similar natural products from sponges of disparate genera evince the likelihood that these invertebrates harbor the same or a very similar symbiont.

Structure-activity of antifungal compounds inspired by aminobisabolenes from the sponge Halichondria sp

Structure-activity relationships of the antifungal aminobisabolene natural product, 1 isolated from Halichondria sp., and synthetic analogs were explored, in parallel with the antidermatophytic allylamine, Terbinafine®, against a panel of pathogenic fungi: Candida spp., Cryptococcus spp. and Trichophyton rubrum. Interpretation of the results suggest different modes of action in antifungal activity for the two classes of compounds.

Peroxide Natural Products from Plakortis zyggompha and the Sponge Association Plakortis halichondrioides-Xestospongia deweerdtae: Antifungal Activity against Cryptococcus gattii

Cryptococcus gattii is a human pathogen and causative agent of a pernicious, sometimes fatal, disseminated fungal disease. Investigation of antifungal extracts of the marine sponge association Plakortis halichondrioides-Xestospongia deweerdtae and the sponge Plakortis zyggompha from the Bahamas led to the discovery and isolation of 6-epi-7,8-dihydroplakortide K (1), plakortide AA (2), and three new plakinic acids, N-P (4-6; unstable 1,2-dioxolanes bearing benzyl-substituted conjugated dienes), along with known plakinic acids L, K, and M.5 Chiroptical comparisons and DFT calculations of (13)C NMR chemical shifts were used to assign the absolute stereostructure of 4. The stereospecific base-promoted rearrangement-saponification of 1 to 10 was briefly investigated and showed tight kinetic control and stereospecific formation of the new C-2 stereocenter with inversion at C-3. Plakinic acid M and plakortides 9 and 11 exhibited antifungal activity against C. gattii (MIC90 = 2.4 to 36 μM), but plakinic acids N-P were inactive under the same conditions.

Mollenynes B-E from the marine sponge Spirastrella mollis. Band-selective heteronuclear single quantum coherence for discrimination of bromo-chloro regioisomerism in natural products

Four new chlorobromohydrins, mollenynes B-E, were isolated from the marine sponge Spirastrella mollis collected from Hogsty Reef, Bahamas. Their structures were elucidated by integrated analysis of NMR, MS, and computational methods. A high-resolution band-selective HSQC experiment was developed to identify (13)C NMR signals in samples at the nanomole-scale that arise from Cl-substituted (13)C by exploiting the (35)Cl/(37)Cl isotope shift.

Antipodal crambescin A2 homologues from the marine sponge Pseudaxinella reticulata. Antifungal structure-activity relationships

Investigation of antifungal natural products from the marine sponge Pseudaxinella reticulata from the Bahamas led to the discovery of new crambescin homologues (1, 2) and enantiomers (3, 4) of known natural products. The cyclic-guanidine structures were solved through analysis of 2D NMR, MS-MS, and CD data. The absolute configurations of 1-4 were established as 13R-opposite of known homologues reported from Crambe crambe obtained from the Mediterranean Sea-by comparison of their CD spectra with predicted Cotton effects obtained from DFT calculations. Antifungal activities of 1-4 against the pathogenic strains Candida albicans and Cryptococcus sp. were observed to correlate potency (MIC50 and MIC90) with the length of the alkyl side chain.

Cytotoxic effects on tumour cell lines of fatty acids from the marine sponge Scopalina ruetzleri

OBJECTIVES

Marine sponges are among the most promising sources of chemically diversified fatty acids (FAs). In addition, several studies have shown the effect of polyunsaturated FAs on cancer therapy. This research carried out a biological and chemical evaluation of the sponge Scopalina ruetzleri collected on the South Brazilian coastline.

METHODS

Bioassay-guided fractionation of S. ruetzleri was performed in human glioma (U87) and neuroblastoma (SH-SY5Y) cell lines, and the in-vitro effects on free radicals were evaluated.

KEY FINDINGS

The ethyl acetate fraction of S. ruetzleri showed promising cytotoxic effects in cancer cell lines, with IC50  < 20 μg/ml. Fingerprint (1) H Nuclear Magnetic Resonance (NMR) analysis showed that this fraction is mainly constituted of FAs. Through FA methyl ester analysis, it was possible to identify 32 FAs. In addition, some minor unusual FAs for the marine biosphere were identified. The results of conjugated dienes method showed that FAs fraction, at concentrations above 50 μg/ml, has a pro-oxidant effect, indicating that lipid peroxidation may be partially responsible for the mechanism of cytotoxicity on cancer cells.

CONCLUSION

This work also contributes to studies that focus on the application of FAs on cancer therapy as a new adjuvant to radio or chemotherapy, or as a chemotherapeutic agent.

Salvadenosine, a 5'-deoxy-5'-(methylthio) nucleoside from the Bahamian tunicate Didemnum sp

Salvadenosine, (1) a rare 5′-deoxy-5′-(methylthio) nucleoside, was isolated from the deep-water Bahaman tunicate Didemnum sp. The structure was solved by integrated analysis of MS and 1D and 2D NMR data. We revise the structure of the known natural product, hamiguanosinol, which is a constitutional isomer of 1, to 5 by interpretation of the spectroscopic data and comparison with synthesized nucleosides.

Potent fluorinated agelastatin analogues for chronic lymphocytic leukemia: design, synthesis, and pharmacokinetic studies

Chronic lymphocytic leukemia (CLL) is the most common lymphoid neoplasia in Western societies and is currently incurable. Multiple treatment options are practiced, but the available small molecule drugs suffer from dose-limiting toxicity and undesirable side effects. The need for new, less toxic treatments is a pressing concern. Here, we demonstrate that (-)-agelastatin A (1a), a pyrrole-imidazole alkaloid obtained from a marine sponge, exhibits potent in vitro activity against primary cell lines of CLL and disclose the synthesis of several analogues that are equipotent or exceed the potency of the natural product. The novel synthetic analogue, 13-debromo-13-trifluoromethyl agelastatin A (1j), showed higher activity than the natural product when tested against the same cell lines and is the most potent agelastatin derivative reported to date. A detailed in vitro structure-activity relationship of 1a in CLL compared to that of 22 synthetic analogues is described along with preliminary in vivo pharmacokinetic and metabolism studies on the most potent compounds.

Structures and solution conformational dynamics of stylissamides G and H from the Bahamian sponge Stylissa caribica

Two new peptides, stylissamides G and H, were isolated from extracts of a sample of Stylissa caribica collected in deep waters of the Caribbean Sea. A single sample of S. caribica among a collection of 10 samples that were examined by LC-MS appeared to be a different chemotype from the others in that it lacked the familiar pyrrole-2-aminoimidazole alkaloids, stevensine and oroidin, and contained peptides of the stylissamide class. The structures of the title compounds were solved by integrated analysis of the MS and NMR spectra and chemical degradation. The solution conformation of stylissamide G was briefly examined by electronic circular dichroism and temperature-dependent (1)H NMR chemical shifts of amide NH signals, which supported a conformationally rigid macrocycle.

Temperature-dependent conformations of a membrane supported zinc porphyrin tweezer by 2D fluorescence spectroscopy

We studied the equilibrium conformations of a zinc porphyrin tweezer composed of two carboxylphenyl-functionalized zinc tetraphenyl porphyrin subunits connected by a 1,4-butyndiol spacer, which was suspended inside the amphiphilic regions of 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) liposomes. By combining phase-modulation two-dimensional fluorescence spectroscopy (2D FS) with linear absorbance and fluorimetry, we determined that the zinc porphyrin tweezer adopts a mixture of folded and extended conformations in the membrane. By fitting an exciton-coupling model to a series of data sets recorded over a range of temperatures (17-85 °C) and at different laser center wavelengths, we determined that the folded form of the tweezer is stabilized by a favorable change in the entropy of the local membrane environment. Our results provide insights toward understanding the balance of thermodynamic factors that govern molecular assembly in membranes.

Acremolin from Acremonium strictum is N(2),3-etheno-2'-isopropyl-1-methylguanine, not a 1H-azirine. Synthesis and structural revision

The first synthesis of the heterocyclic marine natural product, acremolin, is reported along with the revision of the structure from a 1H-azirine to a substituted N(2),3-ethenoguanine (5-methyl-7-isopropyl-4,5-dihydroimidazo[2,1-b]purine). Additional properties of acremolin are also described including its (1)H-(15)N-HMBC and fluorescence spectra.

Halisphingosines A and B, modified sphingoid bases from Haliclona tubifera. Assignment of configuration by circular dichroism and van't Hoff's principle of optical superposition

Halisphingosines A (1) and B (2), modified long-chain sphingoid bases, from the marine sponge Haliclona tubifera collected in Brazil, were characterized after conversion to their N-Boc derivatives. The 2R,3R,6R configuration of halisphingosine A, a compound first reported from Haliclona sp. from South Korea, was confirmed using a novel CD approach: deconvolution of exciton coupling from mono- and trinaphthoyl derivatives obtained by derivatization of the natural product. The sensitive CD deconvolution method, applicable to submilligram samples, simultaneously predicted the relative and absolute configuration of three stereocenters in halisphingosine A with precision and accuracy. Halisphingosine B was assigned by correlation to halisphingosine A.

D-Glucosamine-derived synthons for assembly of L-threo-sphingoid bases. Total synthesis of rhizochalinin C

A five-step transformation of D-glucosamine, commencing with indium-mediated Barbier reaction without isolation of intermediates, into (R,R)-2-aminohex-5-ene-1,3-diol in 45-51% is described. The latter is a useful synthon for assembly of L-threo-sphingoid bases: long-chain aminoalkanols and aminoalkanediols with configurations antipodal to that found in mammalian D-erythro-sphingosine but prevalent among invertebrate-derived sphingolipids. The utility of the method is demonstrated by the first total synthesis of rhizochalinin C, the long-chain, "two-headed" sphingoid base aglycon of the natural product rhizochalin C from the marine sponge Rhizochalina incrustata.

Antifungal Diterpene Alkaloids from the Caribbean Sponge Agelas citrina: Unified Configurational Assignments of Agelasidines and Agelasines

Three new diterpene alkaloids - the hypotaurocyamines, (-)-agelasidines E and F (5-6), and the adeninium salt, agelasine N (9) - were isolated from the Caribbean sponge Agelas citrina along with six known natural products agelasines B-E (7, 10-12), 2-oxo-agelasine B (8), and (-)-agelasidine C (3). The chemical structures of 5, 6 and 9 were elucidated by analysis of NMR spectra and mass spectrometry. This represents the first report of natural products from the sponge A. citrina. Unified assignment of absolute configurations of the new compounds and known compounds were achieved by chemical correlation, quantitative measurements of molar rotations, and comparative analysis by van't Hoff's principle of optical superposition. (-)-Agelasidine C (3) exhibited potent antifungal and modest cytotoxic activity against human chronic lymphocytic leukemia (CLL) cells.

Pyrrole aminoimidazole alkaloid metabiosynthesis with marine sponges Agelas conifera and Stylissa caribica

Game-SET-match: Pyrrole aminoimidazole alkaloids (PAIs) are metabiosynthesized from chlorinated analogues of oroidin by cell-free enzyme preparations from PAI-producing sponges. Evidence and implications for the biosynthesis of PAIs include putative single-electron transfers (SETs) that promote C-C bond-forming reactions of precursors.

De novo synthesis of benzosceptrin C and nagelamide H from 7-15N-oroidin: implications for pyrrole-aminoimidazole alkaloid biosynthesis

De novo synthesis of the natural products benzosceptrin C (7) and nagelamide H (8) was achieved using cell-free enzyme preparations from the marine sponges Agelas sceptrum and Stylissa caribica employing synthetic 7-(15)N-oroidin. These studies provide direct experimental evidence to support the long-standing, but untested, hypothesis that oroidin is a precursor to more complex pyrrole-aminoimidazole alkaloids, such as the sceptrins, benzosceptrins, and nagelamides. In addition, a new nagelamide, didebromonagelamide A (5b), was isolated from S. caribica, representing the first report of a nagelamide-like compound from the Caribbean.

Symplocin A, a linear peptide from the Bahamian cyanobacterium Symploca sp. Configurational analysis of N,N-dimethylamino acids by chiral-phase HPLC of naphthacyl esters

The absolute stereostructures of the components of symplocin A (3), a new N,N-dimethyl-terminated peptide from the Bahamian cyanobacterium Symploca sp., were assigned from spectroscopic analysis, including MS, 2D NMR, and Marfey's analysis. The complete absolute configuration of symplocin A, including the unexpected D-configurations of the terminal N,N-dimethylisoleucine and valic acid residues, was assigned by chiral-phase HPLC of the corresponding 2-naphthacyl esters, a highly sensitive, complementary strategy for assignment of N-blocked peptide residues where Marfey's method is ineffectual or other methods fall short. Symplocin A exhibited potent activity as an inhibitor of cathepsin E (IC(50) 300 pM).

Mollenyne A, a long-chain chlorodibromohydrin amide from the sponge Spirastrella mollis

The structure of mollenyne A, a cytotoxic nitrogenous halogenated long-chain carboxamide from the sponge Spirastrella mollis, was elucidated by integrated spectroscopic analysis, including CD, and chemical conversion.

Assembly of the isoindolinone core of muironolide A by asymmetric intramolecular Diels-Alder cycloaddition

The hexahydro-1H-isoindolin-1-one core of muironolide A was prepared by asymmetric intramolecular Diels-Alder cycloaddition using a variant of the MacMillan organocatalyst which sets the C4,C5 and C11 stereocenters.