Imidazole, oxazole and thiazole alkaloids

Two-step Hantzsch based macrocyclization approach for the synthesis of thiazole-containing cyclopeptides

Macrocyclization via an efficient high-yielding solid-phase intramolecular thioalkylation reaction is described. The reaction of S-nucleophiles with newly generated N-terminal 4-chloromethyl thiazoles led to the desired macrocyclization products 5 in high purities and good overall yields.

In vitro antituberculosis activities of the constituents isolated from Haloxylon salicornicum

In vitro antituberculosis activities of fractions and pure compounds (1-20) including seven triterpenes, two alkaloids, two cycloheximide derivatives, two coumarins six sterol derivatives and a long chain alcohol, respectively, isolated from Haloxylon salicornicum were determined against Mycobacterium tuberculosis H37Rv. Actively growing cultures were tested by rapid colorimetric method while the stationary phase cultures were tested by drug exposure methods for bactericidal activity. The MIC values were found to be 50 microg/ml for compounds 15, 19 and 20 where as rest of the compounds invariably showed MIC value of 100 microg/ml against the logarithmic phase culture. These were compare to Isoniazid as a control drug. The compounds exhibited no activity against the stationary phase culture of M. tuberculosis H37Rv up to 200 microg/ml. Further studies are required to investigate the in vivo efficacies and activities of the compounds in combination with antimicrobials that are already being used for TB therapy.

Development of an oxazole conjunctive reagent and application to the total synthesis of siphonazoles

The preparation of 4-carbethoxy-5-methyl-2-(phenylsulfonyl)methyloxazole and its use in the elaboration of more complex oxazoles are described. A total synthesis of the unique natural products siphonazoles A and B, illustrates an application of this building block. A discussion of the biological activity of the siphonazoles is also presented.

Enamides and enesulfonamides as nucleophiles: formation of complex ring systems through a platinum(II)-catalyzed addition/Friedel-Crafts pathway

Cyclic enamine derivatives (enesulfonamides and enamides) tethered to an 1-arylalkynyl fragment undergo a platinum(II)-catalyzed tandem alkyne addition/Friedel-Crafts ring closure to form nitrogen-containing polycyclic structures. Regioselectivity in the initial addition of the enesulfonamide or enamide nucleophile to the platinum(II)-alkyne complex is important. Electron-rich arenes and heterocycles led to the formation of products resulting from an initial 6-endo cyclization. Twenty-three examples of this process are presented.

A submarine journey: the pyrrole-imidazole alkaloids

In his most celebrated tale "The Picture of Dorian Gray", Oscar Wilde stated that "those who go beneath the surface do so at their peril". This sentence could be a prophetical warning for the practitioner who voluntarily challenges himself with trying to synthesize marine sponge-deriving pyrrole-imidazole alkaloids. This now nearly triple-digit membered community has been growing exponentially in the last 20 years, both in terms of new representatives and topological complexity--from simple, achiral oroidin to the breathtaking 12-ring stylissadines A and B, each possessing 16 stereocenters. While the biosynthesis and the role in the sponge economy of most of these alkaloids still lies in the realm of speculations, significant biological activities for some of them have clearly emerged. This review will account for the progress in achieving the total synthesis of the more biologically enticing members of this class of natural products.

Toward palau'amine: Hg(OTf)2-catalyzed synthesis of the cyclopentane core

Catalytic cyclization: The Hg(OTf)2‐catalyzed N‐selective cyclization of amide carbonyl moieties for the construction of a quaternary carbon center was developed. The Hg(OTf)2‐catalyzed cyclization of cyclopentylidene alcohol with acylhydrazide afforded the desired cyclopenta[c]pyridazinone in good yield. The subsequent eight steps gave the functionalized cyclopentane with the correct stereochemistry that corresponds to the E ring of palau′amine (see scheme).magnified image

1,3-Bis(4-chlorophenyl)-4,5-diethoxyimidazolidine

In the mol­ecule of the title compound, C₁₉H₂₂Cl₂N₂O₂, the two benzene rings are oriented at a dihedral angle of 3.70 (3)°. The five-membered ring adopts an envelope conformation.

Synthesis and antitumor activity of mechercharmycin A analogues

Several analogues of the cytotoxic thiopeptide IB-01211 or mechercharmycin A (1) have been synthesized. The cytotoxicity of 1 and the synthesized analogues were evaluated against a panel of three human tumor cell lines. Thiopeptide 1 and the most active derivatives 2 and 3c were chosen for further studies on effects on cell cycle progression and induction of apoptosis. Interestingly, the inhibition of cell division and activation of a programmed cell death by apoptosis were detected.

Convergent total syntheses of fluvibactin and vibriobactin using molybdenum(VI) oxide-catalyzed dehydrative cyclization as a key step

Efficient total syntheses of fluvibactin and vibriobactin have been achieved via molybdenum(VI) oxide-catalyzed dehydrative cyclization, Sb(OEt)(3)-catalyzed ester-amide transformation, and WSCI and HOAt-promoted dehydrative amide formation.

Marine natural products

This review covers the literature published in 2006 for marine natural products, with 758 citations (534 for the period January to December 2006) referring to compounds isolated from marine microorganisms and phytoplankton, green algae, brown algae, red algae, sponges, cnidaria, bryozoans, molluscs, tunicates and echinoderms. The emphasis is on new compounds (779 for 2006), together with their relevant biological activities, source organisms and country of origin. Biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.

Total Synthesis of Potent Antifungal Marine Bisoxazole Natural Products Bengazoles A and B

The bengazoles are a family of marine natural products that display potent antifungal activity and a unique structure, containing two oxazole rings flanking a single carbon atom. Total syntheses of bengazole A and B are described, which contain a sensitive stereogenic centre at this position between the two oxazoles. Additionally, the synthesis of 10-epi-bengazole A is reported. Two parallel synthetic routes were investigated, relying on construction of the 2,4-disubstituted oxazole under mild conditions and a diastereoselective 1,3-dipolar cycloaddition. Our successful route is high yielding, provides rapid access to single stereoisomers of the complex natural products and allows the synthesis of analogues for biological evaluation.

Some recent advances in the synthesis of polycyclic imidazole-containing marine natural products

This Highlight describes some of the recent synthetic work on imidazole-containing alkaloids isolated from marine sponges. Target molecules have been chosen which demonstrate synthetic strategies leading towards metabolites containing intact imidazole moieties as well as systems bearing modified imidazole rings.

Preparation and Diels−Alder Chemistry of 4-Vinylimidazoles

Various 4-vinylimidazole derivatives have been prepared from the corresponding 4-iodoimidazoles or from urocanic acid. Several methods for the elaboration of these vinylimidazoles and their Diels-Alder reactions are reported. All of the vinylimidazoles prepared in the course of this study react with N-phenylmaleimide quite readily with mild thermal activation providing a single cycloadduct, in most cases the initial, nonaromatic adduct. With more electron rich substrates, there is a tendency for these initial cycloadducts to undergo aromatization, ene reaction, and oxidation although this can be circumvented to a large extent by the choice of reaction conditions. Limited reactions were observed with other dienophiles, providing the expected cycloadducts in most cases, although an abnormal adduct was obtained in one case with dimethyl acetylene dicarboxylate. These substrates also participate in regioselective Diels-Alder reactions with monoactivated dienophiles, but require fairly forcing conditions, thus only providing the aromatized cycloadducts in modest yields. An investigation of substituent effects at the 2-position of the imidazole moiety was undertaken, in which electron-donating and weakly electron-withdrawing substituents are tolerated. In addition, several substrates with terminally substituted vinyl moieties have been investigated.

The total synthesis of siphonazole, a structurally unusual bis-oxazole natural product

The first synthesis of the unusual bis-oxazole natural product siphonazole is reported, both oxazole rings being constructed using rhodium carbene chemistry.

Antifouling potential of cyanobacteria: a mini-review

Cyanobacteria produce a variety of bioactive metabolites that may have allelochemical functions in the natural environment, such as in the prevention of fouling by colonising organisms. Chemical compounds from cyanobacteria are also of biotechnological interest, especially for clinical applications, because of their antibiotic, algicidal, cytotoxic, immunosupressive and enzyme inhibiting activities. Cyanobacterial metabolites have the potential for use in antifouling technology, since they show antibacterial, antialgal, antifungal and antimacrofouling properties which could be expoited in the prevention of biofouling on man-made substrata in the aquatic environment. Molecules with antifouling activity represent a number of types including fatty acids, lipopeptides, amides, alkaloids, terpenoids, lactones, pyrroles and steroids. The isolation of biogenic compounds and the determination of their structure may provide leads for future development of, for example, environmentally friendly antifouling paints. An advantage of exploring the efficacy of cyanobacterial products is that the organisms can be grown in mass culture, which can be manipulated to achieve optimal production of bioactive substances. Phycotoxins and related products from cyanobacteria may serve as materials for antimicro- and antimacrofouling applications. A survey of antibiotic compounds with antifouling potential revealed more than 21 different antifouling substances from 27 strains of cyanobacteria.