A Practical Entry to the Crambescidin Family of Guanidine Alkaloids. Enantioselective Total Syntheses of Ptilomycalin A, Crambescidin 657 and Its Methyl Ester (Neofolitispates 2), and Crambescidin 800

Convenient Solid-Phase Attachment of Small-Molecule Ligands to Oligonucleotides via a Biodegradable Acid-Labile P-N-Bond

One of the key problems in the design of therapeutic and diagnostic oligonucleotides is the attachment of small-molecule ligands for targeted deliveries in such a manner that provides the controlled release of the oligonucleotide at a certain moment. Here, we propose a novel, convenient approach for attaching ligands to the 5'-end of the oligonucleotide via biodegradable, acid-labile phosphoramide linkage. The method includes the activation of the 5'-terminal phosphate of the fully protected, support-bound oligonucleotide, followed by interaction with a ligand bearing the primary amino group. This technique is simple to perform, allows for forcing the reaction to completion by adding excess soluble reactant, eliminates the problem of the limited solubility of reagents, and affords the possibility of using different solvents, including water/organic media. We demonstrated the advantages of this approach by synthesizing and characterizing a wide variety of oligonucleotide 5'-conjugates with different ligands, such as cholesterol, aliphatic oleylamine, and p-anisic acid. The developed method suits different types of oligonucleotides (deoxyribo-, 2'-O-methylribo-, ribo-, and others).

Discovery of potent Guanidine derivative that selectively binds and stabilizes the human BCL-2 G-quadruplex DNA and downregulates the transcription

Small molecules that interact with quadruplexes offer a wide range of potential applications, including not just as medications but also as sensors for quadruplexes structures. The BCL-2 is a proto-oncogene that often gets mutated in lethal cancer and could be an interesting target for developing an anti-cancer drug. In the present study, we have employed various biophysical techniques such as fluorescence, CD, Isothermal calorimeter, gel retardation, and PCR stop assay, indicating that Guanidine derivatives GD-1 and GD-2 selectively interact with high affinity with BCL-2 G-quadruplex over other G-quadruplex DNA and duplex DNA. The most promising small molecule GD-1 increases the thermostability of the BCL-2 GQ structure by 12°C. Our biological experiments such as ROS generation, qRT-PCR, western blot, TFP based Reporter assay, show that the GD-1 ligand causes a synthetic lethal interaction by suppressing the expression BCL-2 genes via interaction and stabilization of its the promoter G-quadruplexes in HeLa cells and act as a potential anti-cancer agent.

Guanidine–Curcumin Complex-Loaded Amine-Functionalised Hollow Mesoporous Silica Nanoparticles for Breast Cancer Therapy

The current study focuses on developing a tumour-targeted functionalised nanocarrier that wraps hollow mesoporous silica nanoparticles. The guanidine carbonate and curcumin are immobilised on the surface of 3-aminopropyl-triethoxy silane (APTES)-decorated hollow mesoporous silica nanoparticles (HMSNP), as confirmed through XPS and NMR analysis. XPS analysis demonstrates that the shape of the hysteresis loops is modified and that pore volume and pore diameter are consequently decreased compared to control. Guanidine (85%) and guanidine–curcumin complex (90%) were successfully encapsulated in HMSNAP and showed a 90% effective and sustained release at pH 7.4 for up to 72 h. Acridine orange/ethidium bromide dual staining determined that GuC-HMNSAP induced more late apoptosis and necrosis at 48 and 72 h compared with Gu-HMNSAP-treated cells. Molecular investigation of guanidine-mediated apoptosis was analysed using western blotting. It was found that cleaved caspases, c-PARP, and GSK-3β (Ser9) had increased activity in MCF-7 cells. GuC-HMSNAP increased the activity of phosphorylation of oncogenic proteins such as Akt (Ser473), c-Raf (Ser249), PDK1 (Ser241), PTEN (Ser380), and GSK-3β (Ser9), thus inducing cell death in MCF-7 cells. Altogether, our findings confirm that GuC-HMNSAP induces cell death by precisely associating with tumour-suppressing proteins, which may lead to new therapeutic approaches for breast cancer therapy.

Synthesis of pyrimidine-containing alkaloids

This review deals with the synthesis of naturally occurring alkaloids containing partially or completely saturated pyrimidine nuclei. The interest in these compounds is associated with their structural diversity, high biological activity and toxicity. The review is divided into four parts, each of which describes a number of synthetic methodologies toward structurally different naturally occurring alkaloids containing saturated cyclic six-membered amidine, guanidine, aminal and urea (thiourea) moieties, respectively. The development of various synthetic strategies for the preparation of these compounds has remarkably increased during the past few decades. This is primarily due to the fact that some of these compounds are isolated only in limited quantities, which makes it practically impossible to study their full structural characteristics and biological activity.

New Crambescidin-Type Alkaloids from the Indonesian Marine Sponge Clathria bulbotoxa

A crude methanolic extract of the Indonesian sponge Clathria bulbotoxa showed a potent cytotoxic activity against the human epidermoid carcinoma A431 cells. An investigation of the active components led to the isolation of three new compounds named crambescidins 345 (1), 361 (2), and 373 (3), together with the known related metabolites crambescidins 359 (4), 657 (5), and 800 (6). The structures of the compounds were determined by spectroscopic analysis. These compounds 1–4 that possess a simple pentacyclic guanidine core exhibited moderate cytotoxicity against the A431 cells with the IC₅₀ values of 7.0, 2.5, 0.94, and 3.1 μM, respectively, while the known compounds 5 and 6 that possess a long aliphatic side chain were found to be significantly cytotoxic. On the other hand, in an anti-oomycete activity test against the fungus-like plant pathogen Phytophthora capsici, 1–4 showed a higher activity than that of 5 and 6, suggesting that the long aliphatic side chain plays a significant role for cytotoxicity, but is not effective or suppressive for anti-oomycete activity.

The Oxepane Motif in Marine Drugs

Oceans have shown to be a remarkable source of natural products. The biological properties of many of these compounds have helped to produce great advances in medicinal chemistry. Within them, marine natural products containing an oxepanyl ring are present in a great variety of algae, sponges, fungus and corals and show very important biological activities, many of them possessing remarkable cytotoxic properties against a wide range of cancer cell lines. Their rich chemical structures have attracted the attention of many researchers who have reported interesting synthetic approaches to these targets. This review covers the most prominent examples of these types of compounds, focusing the discussion on the isolation, structure determination, medicinal properties and total synthesis of these products.

An efficient synthesis of indolo[2,3-b]quinoline guanidine derivatives with their in vitro and in vivo study

An optimization of the guanidylation process by verifying the efficacy of common guanylation reagents in order to obtain the guanidine derivatives of indolo[2,3-b]quinoline has been performed. As a result, a high-yield procedure using N,N′-di-Boc-N′′-triflylguanidine was applied to synthesize the guanidine derivative of indolo[2,3-b]quinoline 1 in a gram scale for specific in vitro and in vivo biological research. Extensive studies on the antiproliferative activity against eight human tumor cell lines were completed. Compound 1 revealed the highest activity against A549 lung adenocarcinoma and MCF7 breast cancer cell lines. Thus, 1 was evaluated for the in vivo anticancer activity against 4T1 mammary gland carcinoma and KLN205 murine lung carcinoma in mouse models. The anticancer effect was observed in the KLN205 model with a 37% tumor growth inhibition at the 20 mg/kg dose. This anticancer activity of 1 was comparable to that of cyclophosphamide which inhibited murine lung tumor growth in the range of 27–43% at the dose of 100 mg/kg. The biochemistry research after 1 admission, including measurements of blood parameters like alanine aminotransferase, aspartate aminotransferase, lactate dehydrogenase, and urea and creatinine, were also performed.

Structure, synthesis and biological properties of the pentacyclic guanidinium alkaloids

The pentacyclic guanidinium alkaloids (PGAs) are a family of marine natural products that possess a polycyclic guanidine-containing core and a long alkyl chain tethered spermidine-derived tail that is rarely observed in other natural products. These natural products exhibit potent activities on a wide range of organisms and therefore have attracted the attention of many synthetic chemists; however, the structure-activity relationships and mechanisms of action of PGAs remain largely elusive. Herein we summarize the structure, synthesis, toxicity and mechanisms of action of PGAs and highlight their potential as chemical probes and/or therapeutic leads.

Cytotoxic Guanidine Alkaloids from a French Polynesian Monanchora n. sp. Sponge

Four bicyclic and three pentacyclic guanidine alkaloids (1-7) were isolated from a French Polynesian Monanchora n. sp. sponge, along with the known alkaloids monalidine A (8), enantiomers 9-11 of known natural product crambescins, and the known crambescidins 12-15. Structures were assigned by spectroscopic data interpretation. The relative and absolute configurations of the alkaloids were established by analysis of (1)H NMR and NOESY spectra and by circular dichroism analysis. The new norcrambescidic acid (7) corresponds to interesting biosynthetic variation within the pentacyclic core. All compounds exhibited antiproliferative and cytotoxic efficacy against KB, HCT116, HL60, MRC5, and B16F10 cancer cells, with IC50 values ranging from 4 nM to 10 μM.

Synthesis of the Paralytic Shellfish Poisons (+)-Gonyautoxin 2, (+)-Gonyautoxin 3, and (+)-11,11-Dihydroxysaxitoxin

The paralytic shellfish poisons are a collection of guanidine-containing natural products that are biosynthesized by prokaryote and eukaryote marine organisms. These compounds bind and inhibit isoforms of the mammalian voltage-gated Na(+) ion channel at concentrations ranging from 10(-11) to 10(-5) M. Here, we describe the de novo synthesis of three paralytic shellfish poisons, gonyautoxin 2, gonyautoxin 3, and 11,11-dihydroxysaxitoxin. Key steps include a diastereoselective Pictet-Spengler reaction and an intramolecular amination of an N-guanidyl pyrrole by a sulfonyl guanidine. The IC50's of GTX 2, GTX 3, and 11,11-dhSTX have been measured against rat NaV1.4, and are found to be 22 nM, 15 nM, and 2.2 μM, respectively.

Polycyclic Guanidine Alkaloids from Poecilosclerida Marine Sponges

Sessile marine sponges provide an abundance of unique and diversified scaffolds. In particular, marine guanidine alkaloids display a very wide range of biological applications. A large number of cyclic guanidine alkaloids, including crambines, crambescins, crambescidins, batzelladines or netamins have been isolated from Poecilosclerida marine sponges. In this review, we will explore the chemodiversity of tri- and pentacyclic guanidine alkaloids. NMR and MS data tools will also be provided, and an overview of the wide range of bioactivities of crambescidins and batzelladines derivatives will be given.

The synthesis of indolo[2,3-b]quinoline derivatives with a guanidine group: highly selective cytotoxic agents

The synthesis of indolo[2,3-b]quinoline derivatives containing guanidine, amino acid or guanylamino acid substituents as well as their in vitro evaluation for the cytotoxic and antifungal activity are reported. The influence of the guanidine group on the selective cytotoxic and hemolytic properties of indolo[2,3-b]quinoline was investigated. Most of the compounds displayed a high cytotoxic activity in vitro and two of the most promising compounds (3 and 12) exhibited a high selectivity between normal and cancer cell-lines. The cytotoxic activity of compound 3 was about 600-fold lower against normal fibroblasts than against A549 and MCF-7 cancer cell lines. Novel entities acted as the DNA-intercalators when tested using a DNA-methyl green assay but demonstrated zero or low hemolytic activity in comparison to their unsubstituted analogs. The mechanism of action was studied for guanidine derivatives 3 and 12 and both compounds were found to be very effective inducers of apoptosis.

Syntheses of Cyclic Guanidine-Containing Natural Products

Naturally occurring guanidine derivatives frequently display medicinally useful properties. Among them, the higher order pyrrole-imidazole alkaloids, the dragmacidins, the crambescidins/batzelladines, and the saxitoxins/tetradotoxins have stimulated the development of many new synthetic methods over the past decades. We provide here an overview of the syntheses of these cyclic guanidine-containing natural products.

Desymmetrization of meso-2,5-diallylpyrrolidinyl ureas through asymmetric palladium-catalyzed carboamination: stereocontrolled synthesis of bicyclic ureas

The stereoselective synthesis of fused bicyclic ureas 8 is accomplished via enantioselective Pd-catalyzed desymmetrizing carboamination reactions of meso -2,5-diallylpyrroldinyl urea 7c . The reactions generate a C–N bond, a C–C bond, and afford products bearing three stereocenters with good diastereoselectivity (6–12:1 dr) and enantioselectivity (up to 95:5 er). The N -(p -chlorophenyl) group can be cleaved in good yield using a two step sequence. In addition, 8c was transformed to a tricyclic guanidine product using a four-step (two pot) procedure and was converted to 9-epi -batzelladine k in seven steps.

Enantioconvergent cross-couplings of racemic alkylmetal reagents with unactivated secondary alkyl electrophiles: catalytic asymmetric Negishi α-alkylations of N-Boc-pyrrolidine

Although enantioconvergent alkyl-alkyl couplings of racemic electrophiles have been developed, there have been no reports of the corresponding reactions of racemic nucleophiles. Herein we describe Negishi cross-couplings of racemic α-zincated N-Boc-pyrrolidine with unactivated secondary halides, thus providing a one-pot, catalytic asymmetric method for the synthesis of a range of 2-alkylpyrrolidines (an important family of target molecules) from N-Boc-pyrrolidine, a commercially available precursor. Preliminary mechanistic studies indicated that two of the most straightforward mechanisms for enantioconvergence (dynamic kinetic resolution of the organometallic coupling partner and a simple β-hydride elimination/β-migratory insertion pathway) are unlikely to be operative.

3,6-bis(3-alkylguanidino)acridines as DNA-intercalating antitumor agents

A series of 3,6-bis(3-alkylguanidino) acridines was prepared and the interaction of these novel compounds with calf thymus DNA was investigated with UV-vis, fluorescence and circular dichroism spectroscopy, in addition to DNA melting techniques. The binding constants K were estimated to range from 1.25 to 5.26 × 10(5) M(-1), and the percentage of hypochromism was found to be 17-42% (from spectral titration). UV-vis, fluorescence and circular dichroism measurements indicated that the compounds act as effective DNA-intercalating agents. Electrophoretic separation proved that ligands 6a-e relaxed topoisomerase I at a concentration of 60 μM, although only those with longer alkyl chains were able to penetrate cell membranes and suppress cell proliferation effectively. The biological activity of novel compounds was assessed using different techniques (cell cycle distribution, phosphatidylserine externalization, caspase-3 activation, changes in mitochondrial membrane potential) and demonstrated mostly transient cytostatic action of the ethyl 6c and pentyl 6d derivatives. The hexyl derivative 6e proved to be the most cytotoxic. Different patterns of cell penetration were also observed for individual derivatives. Principles of molecular dynamics were applied to explore DNA-ligand interactions at the molecular level.

Is there no end to the total syntheses of strychnine? Lessons learned in strategy and tactics in total synthesis

From the 19th century to the present, the complex indole alkaloid strychnine has engaged the chemical community. In this Review, we examine why strychnine has been and remains today an important target for directed synthesis efforts. A selection of the diverse syntheses of strychnine is discussed with the aim of identifying their influence on the evolution of the strategy and tactics of organic synthesis.

Highlights of marine invertebrate-derived biosynthetic products: their biomedical potential and possible production by microbial associants

Coral reefs are among the most productive marine ecosystems and are the source of a large group of structurally unique biosynthetic products. Annual reviews of marine natural products continue to illustrate that the most prolific source of bioactive compounds consist of coral reef invertebrates-sponges, ascidians, mollusks, and bryozoans. This account examines recent milestone developments pertaining to compounds from invertebrates designated as therapeutic leads for biomedical discovery. The focus is on the secondary metabolites, their inspirational structural scaffolds and the possible role of micro-organism associants in their biosynthesis. Also important are the increasing concerns regarding the collection of reef invertebrates for the discovery process. The case examples considered here will be useful to insure that future research to unearth bioactive invertebrate-derived compounds will be carried out in a sustainable and environmentally conscious fashion. Our account begins with some observations pertaining to the natural history of these organisms. Many still believe that a serious obstacle to the ultimate development of a marine natural product isolated from coral reef invertebrates is the problem of compound supply. Recent achievements through total synthesis can now be drawn on to forcefully cast this myth aside. The tools of semisynthesis of complex natural products or insights from SAR efforts to simplify an active pharmacophore are at hand and demand discussion. Equally exciting is the prospect that invertebrate-associated micro-organisms may represent the next frontier to accelerate the development of high priority therapeutic candidates. Currently in the United States there are two FDA approved marine-derived therapeutic drugs and two others that are often cited as being marine-inspired. This record will be examined first followed by an analysis of a dozen of our favorite examples of coral reef invertebrate natural products having therapeutic potential. The record of using complex scaffolds of marine invertebrate products as the starting point for development will be reviewed by considering eight case examples. The potential promise of developing invertebrate-derived micro-organisms as the starting point for further exploration of therapeutically relevant structures is considered. Also significant is the circumstance that there are some 14 sponge-derived compounds that are available to facilitate fundamental biological investigations.

Ptilomycalin A inhibits laccase and melanization in Cryptococcus neoformans

The antifungal spirocyclic guanidine alkaloid, ptilomycalin A, from marine sponge Monanchora arbuscula, inhibits melanogenesis of Cryptococcus neoformans in vitro through inhibition of biosynthesis of laccase in the melanin biosynthetic pathway with an IC(50) of 7.3 μM.

Annulation of thioimidates and vinyl carbodiimides to prepare 2-aminopyrimidines, competent nucleophiles for intramolecular alkyne hydroamination. Synthesis of (-)-crambidine

A convergent synthesis of (-)-crambidine is reported. The sequence capitalizes on two novel key transformations, including a [4+2] annulation of thioimidates with vinyl carbodiimides and an alkyne hydroamination employing 2-aminopyrimidine nucleophiles.

Marine-derived metabolites of S-adenosylmethionine as templates for new anti-infectives

S-Adenosylmethionine (AdoMet) is a key biochemical co-factor whose proximate metabolites include methylated macromolecules (e.g., nucleic acids, proteins, phospholipids), methylated small molecules (e.g., sterols, biogenic amines), polyamines (e.g., spermidine, spermine), ethylene, and N-acyl-homoserine lactones. Marine organisms produce numerous AdoMet metabolites whose novel structures can be regarded as lead compounds for anti-infective drug design.

Marine natural products: synthetic aspects

An overview of marine natural products synthesis during 2005 is provided. In a similar vein to earlier installments in this series, the emphasis is on total syntheses of molecules of contemporary interest, new total syntheses, and syntheses that have resulted in structure confirmation or stereochemical assignments.

Enantioselective total synthesis of (-)-dehydrobatzelladine C. [structure: see text]

The oxidation of two tethered Biginelli adducts was examined as a potential key step in total syntheses of highly oxidized batzelladine and crambescidin alkaloids. Although angular hydroxyl substitution could not be introduced, dehydrogenation was readily accomplished. This latter conversion is a key step in the first total synthesis of dehydrobatzelladine C. [structure: see text]

Crystallographic and NMR studies of antiinfective tricyclic guanidine alkaloids from the sponge Monanchora unguifera

Three tricyclic guanidine alkaloids, including 1,8a;8b,3a-didehydro-8beta-hydroxyptilocaulin (1), 1,8a;8b,3a-didehydro-8alpha-hydroxyptilocaulin (2) and mirabilin B (3), were identified from the marine sponge Monanchora unguifera. 1,8a;8b,3a-Didehydro-8alpha-hydroxyptilocaulin (2) is a new stereoisomer of 1, the structure of which was elucidated by spectroscopic analysis, comparison of its spectral data with those of 1, and confirmed by X-ray analysis. Compounds 1 and 2 co-crystallized in an unusual perfect order and packed around an approximate inversion center. A mixture of 1 and 2 is active against the malaria parasite Plasmodium falciparum with an IC50 value of 3.8 microg/mL while mirabilin B (3) exhibited antifungal activity against Cryptococcus neoformans with an IC50 value of 7.0 microg/mL and antiprotozoal activity against Leishmania donovani with an IC50 value of 17 microg/mL.

Guanidine alkaloid analogs as inhibitors of HIV-1 Nef interactions with p53, actin, and p56lck

With current anti-HIV treatments targeting only 4 of the 15 HIV proteins, many potential viral vulnerabilities remain unexploited. We report small-molecule inhibitors of the HIV-1 protein Nef. In addition to expanding the anti-HIV arsenal, small-molecule inhibitors against untargeted HIV proteins could be used to dissect key events in the HIV lifecycle. Numerous incompletely characterized interactions between Nef and cellular ligands, for example, present a challenge to understanding molecular events during HIV progression to AIDS. Assays with phage-displayed Nef from HIV(NL4-3) were used to identify a series of guanidine alkaloid-based inhibitors of Nef interactions with p53, actin, and p56(lck). The guanidines, synthetic analogs of batzellidine and crambescidin natural products, inhibit the Nef-ligand interactions with IC(50) values in the low micromolar range. In addition, sensitive in vivo assays for Nef inhibition are reported. Although compounds that are effective in vitro proved to be too cytotoxic for cellular assays, the reported Nef inhibitors provide proof-of-concept for disrupting a new HIV target and offer useful leads for drug development.

Synthesis and anticancer activity of side chain analogs of the crambescidin alkaloids

Twenty three side chain analogs of the crambescidin alkaloids were prepared from the corresponding pentacyclic zwitterionic core acid. In the crambescidin 800 and 657 series, potency increased with increasing chain length. In addition, substantial variations in tumor selectivity with structure were seen. Crambescidin analogs having short, nonpolar side chains were identified for the first time as promising anticancer agents.