Room Temperature Diels-Alder Reactions of 4-Vinylimidazoles.

In the course of studying Diels-Alder reactions of 4-vinylimidazoles with N-phenylmaleimide, it was discovered that they engage in cycloaddition at room temperature to give high yields of the initial cycloadduct as a single stereoisomer. In certain cases, the product precipitated out of the reaction mixture and could be isolated by simple filtration, thereby avoiding issues with aromatization observed during chromatographic purification. Given these results, intramolecular variants using doubly activated dienophiles were also investigated at room temperature. Amides underwent cycloaddition at room temperature in modest yields, but the initial adducts were not isolable with Nimid-benzyl-protected systems. Attempts to extend these results to the corresponding esters and hydroxamate were less successful with these substrates only undergoing cycloaddition at elevated temperatures in lower yields. Density functional theory calculations were performed to evaluate the putative transition states for both the inter- and intramolecular variants to rationalize experimental observations.

Total synthesis of haploscleridamine, villagorgin A and an approach towards lissoclin C.

An investigation of asymmetric total syntheses of three indole-imidazole alkaloids from histidine are described. A common advanced piperidinone was contructed via a ring-closing metathesis which was then subjected to a modified Fischer indole synthesis. Deprotection of an N-tosyl group via a dissolving metal reduction affords haploscleridamine which upon reaction with aqueous formaldehyde in trifluoroethanol provided villagorgin A. On closer examination, it was found that villagorgin A was produced as a byproduct during the reductive detosylation in the presence of magnesium and methanol. Attempts to obtain the brominated haploscleridamine congener, lissoclin C through use of bromophenyl hydrazone were thwarted by reductive debromination during deprotection efforts. Investigation of the enantiopurity of the synthetic natural products revealed production of almost racemic materials in some batches as the result of partial racemization of an early stage intermediate. A revised approach routinely provided scalemic haploscleridamine and villagorgin in 30% ee. Analysis of the enantiomer composition of all intermediates by HPLC using columns with chiral stationary phases; this analysis revealed several steps where erosion of enantiomer composition occurred.

Liquid chromatography enantiomeric separation of chiral ethanolamine substituted compounds.

Eleven racemic ethanolamine derivatives were prepared, and their enantiomers were separated using liquid chromatography with various chiral columns. These derivatives included chiral vicinal amino alcohols, ß-hydroxy ureas, ß-hydroxy thioureas, and ß-hydroxy guanidines, all of which are present in many active pharmaceutical ingredients. The screening study was performed with six chiral stationary phase containing columns, including four recently introduced superficially porous particles bonded with two macrocyclic glycopeptides, a cyclodextrin derivative and a cyclofructan derivative. The two remaining columns contained chiral stationary phases, based on either a cellulose derivative or derivatized amylose, both bonded to fully porous particles. The cyclodextrin and cellulose-based chiral stationary phases proved to be the most broadly effective selectors and were able to separate 8 and 7 of the 11 tested compounds, respectively. With respect to analyte structural features, marked differences in enantiorecognition were observed between compounds containing phenyl and cyclohexyl groups adjacent to the stereogenic center. Additionally, replacing a small electronegative oxygen atom by a larger and less electronegative sulfur atom induced a significant difference in chiral recognition by the cellulose derivative as well as by the vancomycin-based chiral selectors.

One-Pot Synthesis of Novel 2-Imino-5-Arylidine-Thiazolidine Analogues and Evaluation of Their Anti-Proliferative Activity against MCF7 Breast Cancer Cell Line.

An efficient surface-mediated synthetic method to facilitate access to a novel class of thiazolidines is described. The rationale behind the design of the targeted thiazolidines was to prepare stable thiazolidine analogues and evaluate their anti-proliferative activity against a breast cancer cell line (MCF7). Most of the synthesized analogues exhibited increased potency ranging from 2-15-fold higher compared to the standard reference, cisplatin. The most active thiazolidines contain a halogenated or electron withdrawing group attached to the N-phenyl ring of exocyclic 2-imino group. However, combination of the two substituents did not enhance the activity. The anti-proliferative activity was measured in terms of IC50 values using an MTT assay.

Pyrrole carboxamide introduction in the total synthesis of pyrrole-imidazole alkaloids.

In this review various strategies for the incorporation of the signature pyrrole carboxamide moiety in the total syntheses of pyrrole-imidazole alkaloids (PIA) are discussed. These so-called oroidin alkaloids have a broad range of biological activities and display interesting skeletal diversity and complexity. These alkaloids are sponge-derived secondary metabolites and thus far more than 200 members of the PIA family have been isolated over the past few decades. Methods range from classical amide bond forming processes to non-traditional bond formation including the de novo synthesis of the pyrrole itself.

Enantiomeric Separation of New Chiral Azole Compounds.

Twelve new azole compounds were synthesized through an ene reaction involving methylidene heterocycles and phenylmaleimide, producing four oxazoles, five thiazoles, and one pyridine derivative, and ethyl glyoxal for an oxazole and a thiazole compound. The twelve azoles have a stereogenic center in their structure. Hence, a method to separate the enantiomeric pairs, must be provided if any further study of chemical and pharmacological importance of these compounds is to be accomplished. Six chiral stationary phases were assayed: four were based on macrocyclic glycopeptide selectors and two on linear carbohydrates, i.e., derivatized maltodextrin and amylose. The enantiomers of the entire set of new chiral azole compounds were separated using the three different mobile phase elution modes: normal phase, polar organic, and reversed phase. The most effective chiral stationary phase was the MaltoShell column, which was able to separate ten of the twelve compounds in one elution mode or another. Structural similarities in the newly synthesized oxazoles provided some insights into possible chiral recognition mechanisms.

Total Synthesis of the Nagelamides - Synthetic Studies toward the Reported Structure of Nagelamide D and Nagelamide E Framework.

The nagelamides are a small subset of the oroidin family of marine sponge-derived alkaloids and are, for the most part, dimeric in nature. As part of our efforts to develop synthetic access to this family, a Stille cross-coupling strategy is used to construct the bis-imidazolyl core skeleton. Reduction of the bis-vinylimidazole delivered the core framework of nagelamide D. Introduction of the 2-amino groups via the corresponding azides and introduction of the pyrrolecarboxamides through a double Mitsunobu reaction using a pyrrole hydantoin provided the putative structure of nagelamide D. The spectroscopic data for the synthetic and sponge-derived materials did not match well, whereas the spectroscopic data were a good match for closely related oroidin alkaloids, supporting the structure of the synthetic material. The structure of the synthetic material was further corroborated by obtaining an X-ray crystal structure of a derivative. Electrocyclization of an advanced precursor affords a dihydrobenzimidazole, which is expected to serve as a key intermediate en route to nagelamide E and ageliferin.

Novel thiazolidines: Synthesis, antiproliferative properties and 2D-QSAR studies.

A series of N-substituted (Z)-2-imino-(5Z)-ylidene thiazolidines/thiazolidin-4-ones were synthesized and their antiproliferative activities against colon (HCT-116) and breast (MCF7) cancer cell lines were evaluated utilizing an MTT growth assay. A 2D-QSAR investigation was conducted to probe and validate the obtained antiproliferative properties for the thiazolidine derivatives. The majority of the thiazolidines exhibit higher potency against a colon cancer cell line relative to the standard reference. The p-halophenylimino p-anisylidene derivatives exhibited the highest anti-proliferative activity against HCT116 relative to control (IC50 = 8.9-10.0 µM compared to 20.4 µM observed for 5-fluorouracil as positive control). An X-ray study confirmed the Z, Z'-configurations for two examples of the synthesized compounds.

Total synthesis and cytotoxicity of Leucetta alkaloids.

The total synthesis of a number of representative natural products isolated from Leucetta and Clathrina sponges containing a polysubstituted 2-aminoimidazole are described. These syntheses take advantage of the site specific metallation reactions of 4,5-diiodoimidazoles resulting in the syntheses of three different classes of Leucetta derived natural products. The cytotoxicities of these natural products, along with several precursors in MCF7 cells were determined through an MTT growth assay. For comparative purposes a series of naphthimidazole-containing family members are included.

Thio acid-mediated conversion of azides to amides - exploratory studies en route to oroidin alkaloids.

The utility of the thio acid-azide coupling reaction to afford amides is explored in imidazole-containing substrates for application in the total synthesis of examples of oroidin alkaloids. Good yields of the expected amides are obtained in both monomeric and dimeric substrates. Bis azides react preferentially at the 2-azido position but hydrosulfenylation and reduction interfere. 2-Thiophenyl and 2-oxo groups were evaluated as 2-amino surrogates, the thioether delivered the expected amide, whereas 2-imidazolone gave a mixture of the expected amide and the hydrosulfenylation product.

Preparation and Diels-Alder reactions of 1'-heterosubsituted vinylimidazoles.

A Diels-Alder/rearrangement sequence has been pursued in our lab en route to a number of oroidin dimers. In order to access the fully substituted core of these molecules, 1',2'-disubstituted 4-vinylimidazoles were required as dienes. The preparation of a series of a 4-vinylimidazoles containing substituents on the vinyl moiety via hydroalumination/electrophilic trapping or hydrosilylation are described. These derivatives undergo Diels-Alder reactions with N-phenylmaleimide to provide the tetrahydrobenzimidazole derivatives. The cycloadducts derived from halosubstituted systems generally undergo elimination, leading to the corresponding dihydrobenzimidazole, whereas the silyl and stannyl derivatives provide the corresponding 4-substituted tetrahydrobenzimidazole.

Studies Towards the Leucetta-derived Alkaloids Spirocalcaridine A and B - Possible Biosynthetic Implications.

An exploration of an abiotic approach to spirocalcaridines A and B is described centered on electrophile-induced dearomatizing spirocyclization of aryl enyne derivatives. Elaboration of the α-iodoenone via an Ullmann-like, copper-catalyzed amidation provided a formamide which upon treatment with methylamine undergoes a dienol-arene rearrangement, providing the corresponding kealiinine-like framework. This observation suggests a possible biosynthetic links between the spirocalcaridines and the naphthimidazole group of Leucetta alkaloids.

Total synthesis of 7'-desmethylkealiiquinone, 4'-desmethoxykealiiquinone, and 2-deoxykealiiquinone.

Synthetic approaches to the imidazonaphthoquinone core of kealiiquinone and related Leucetta-derived alkaloids are described. The polysubstituted benzimidazole framework can be constructed through intramolecular Diels-Alder reactions of propiolate-derived enynes followed by oxidation. Adjustment of the oxidation state of the thus formed lactone allows introduction of the 2,3-dihydroxybenzoquinone moiety through a presumed benzoin-like condensation between a phthaldehyde derivative and a masked glyoxal equivalent catalyzed by a cyanide ion. Oxidation of the C2-position can be accomplished through application of an operationally simple treatment of an imidazolium salt with bleach, thus producing the corresponding 2-imidazolone. Debenzylation of a late stage intermediate en route to kealiiquinone was compromised by concomitant O-demethylation upon treatment with triflic acid resulting in the formation of non-natural 7'-desmethylkealiiquinone. Other endgame strategies were evaluated; however, these efforts did not lead to completion of a synthesis of kealiiquinone but did provide access to other closely related analogues.

Total syntheses and cytotoxicity of kealiiquinone, 2-deoxy-2-aminokealiiquinone and analogs.

Concise syntheses of two Leucetta-derived naphthimidazole alkaloids, kealiiquinone and 2-deoxy-2-aminokealiiquinone, are described based on a biosynthetic-guided hypothesis. Advanced intermediates containing the full naphthimidazole framework are constructed through Friedel-Crafts chemistry followed by oxidation of the electron rich C-ring with hydrogen peroxide. The cytotoxicity of these alkaloids in a breast cancer cell line along with several closely related marine-derived natural products kealiinines A-C and analogs are reported.

Total syntheses of oroidin, hymenidin and clathrodin.

The total syntheses of oroidin, hymenidin and clathrodin are reported via the intermediacy of an imidazo[1,2-a]pyrimidine derivative. The chemistry described herein obviates the need for expensive guanidine reagents, multiply protected prefunctionalized 2-aminoimidazole synthons, or the need for laborious olefinations thereby achieving synthetic efficiency amenable to scale-up. The approach outlined in this manuscript provides an opportunity for further functionalizations through the imidazo[1,2-a]pyrimidine core and through functional groups placed strategically on the side chain.

The enantiomeric separation of tetrahydrobenzimidazoles cyclodextrins- and cyclofructans.

High performance liquid chromatography (HPLC) and capillary electrophoresis (CE) were used to examine the enantiomeric separation of a series of 17 racemic tetrahydrobenzimidazole analytes. These compounds were prepared as part of a synthetic program directed towards a select group of pyrrole-imidazole alkaloids. This group of natural products has a unique framework of pyrrole- and guanidine-containing fused rings which can be constructed through the intermediacy of a tetrahydrobenzimidazole scaffold. Several bonded cyclodextrin- (both native and derivatized) and derivatized cyclofructan-based chiral stationary phases were evaluated for their ability to separate these racemates via HPLC. Similarly, several cyclodextrin derivatives and derivatized cyclofructan were evaluated for their ability to separate this set of chiral compounds via CE. Enantiomeric selectivity was observed for the entire set of racemic compounds using HPLC with resolution values up to 3.0. Among the 12 different CSPs, enantiomeric recognition was most frequently observed with the Cyclobond RN and LARIHC CF6-P, while the Cyclobond DMP yielded the greatest number of baseline separations. Fifteen of the analytes showed enantiomeric recognition in CE with resolution values as high as 5.0 and hydroxypropyl-γ-cyclodextrin was the most effective chiral additive.

Novel thiazolidines of potential anti-proliferation properties against esophageal squamous cell carcinoma via ERK pathway.

The discovery of a new class of extracellular-signal-regulated kinase (ERK) inhibitors has been achieved via developing novel 2-imino-5-arylidene-thiazolidine analogues. A novel synthetic method employing a solid support-mediated reaction was used to construct the targeted thiazolidines through a cascade reaction with good yields. The chemical and physical stability of the new thiazolidine library has successfully been achieved by blocking the labile C5-position to aerobic oxidation. A cell viability study was performed using esophageal squamous cell carcinoma cell lines (KYSE-30 and KYSE-150) and non-tumorous esophageal epithelial cell lines (HET-1A and NES-G4T) through utilization of an MTT assay, revealing that (Z)-5-((Z)-4-bromobenzylidene)-N-(4-methoxy-2-nitrophenyl)-4,4-dimethylthiazolidin-2-imine (6g) was the best compound among the synthesized library in terms of selectivity. DAPI staining experiments were performed to visualize the morphological changes and to investigate the apoptotic activity. Moreover, western blots were used to probe the mechanism/pathway behind the observed activity/selectivity of thiazolidine 6g which established selective inhibition of phosphorylation in the ERK pathway. Molecular modeling techniques have been utilized to confirm the observed activity. A molecular docking study revealed similar binding interactions between the synthesized thiazolidines and reported co-crystalized inhibitors with ERK proteins. Thus, the present study provides a starting point for the development of interesting bioactive 2-imino-5-arylidene-thiazolidines.

Self-assembling Peptides with Internal Ionizable Unnatural Amino Acids: A General Approach to pH-responsive Peptide Materials.

Self-assembled peptides are an emerging family of biomaterials that show great promise for a range of biomedical and biotechnological applications. Introducing and tuning the pH-responsiveness of the assembly is highly desirable for improving their biological activities. Inspired by proteins with internal ionizable residues, we report a simple but effective approach to constructing pH-responsive peptide assembly containing unnatural ionic amino acids with an aliphatic tertiary amine side chain. Through a combined experimental and computational investigation, we demonstrate that these residues can be accommodated and stabilized within the internal hydrophobic compartment of the peptide assembly. The hydrophobic microenvironment shifts their pKa significantly from a basic pH typically found for free amines to a more biologically relevant pH in the weakly acidic range. The pH-induced ionization and ionization-dependent self-assembly and disassembly are thoroughly investigated and correlated with the biological activity of the assembly. This new approach has unique advantages in tuning the pH-responsiveness of self-assembled peptides across a large pH range in a complex biological environment. We anticipate the ionizable amino acids developed here can be widely applicable to the synthesis and self-assembly of many amphiphilic peptides with endowed pH-responsive properties to enhance their biological activities toward applications ranging from targeted therapeutic delivery to proton transport.

Structure and synthesis of 2-aminoimidazole alkaloids from Leucetta and Clathrina sponges.

Marine sponges belonging to the Calcarea family have been studied for some time and have yielded a wide variety of structurally unique secondary metabolites. In particular, these sponges have produced a large number of bioactive alkaloids containing an imidazole heterocycle, typically substituted with two benzylic fragments at various locations around the azole nucleus and at various oxidation states. This review will describe the isolation, structural determination and synthetic studies towards this growing class of natural products.