Stereoselective synthesis of a tricyclic guanidinium model of cylindrospermopsin

Formal total synthesis of (±)-7-deoxycylindrospermopsin and its 8-epi isomer

A concise formal synthesis of (±)-7-deoxycylindrospermopsin in 14 steps was reported.

Preparation of an ABC tricyclic model of the cylindrospermopsin alkaloids via a biomimetically inspired pathway

The protected guanidine18was convertedviaa 4-step sequence (Boc-deprotection, tethered Biginelli condensation, deallylation/decarboxylation and reduction) to give the tricyclic guanidines26and27, models of the tricyclic core of cylindrospermopsin1.

A synthetic method to access symmetric and non-symmetric 2-(N,N'-disubstituted)guanidinebenzothiazoles

Symmetric and non-symmetric 2-(N-H, N-methyl, N-ethylenyl and N-aryl)guanidinebenzothiazoles were synthesized from the reaction of ammonia, methylamine, pyrrolidine and aniline with dimethyl benzo[d]thiazol-2-yl-carbonodithioimidate as intermediate. The products were characterized by ¹H-, ¹³C-NMR spectroscopy and three of them by X-ray diffraction analysis. HN-phenyl protons formed intramolecular hydrogen bonds that assist the stereochemistry of the second substituent, whereas the HN-alkyl protons were involved in intermolecular hydrogen bonding.

Synthesis of skeletal analogues of saxitoxin derivatives and evaluation of their inhibitory activity on sodium ion channels Na(V)1.4 and Na(V)1.5

Skeletal analogues of saxitoxin (STX) that possess a fused-type tricyclic ring system, designated FD-STX, were synthesized as candidate sodium ion channel modulators. Three kinds of FD-STX derivatives 4a-c with different substitution at C13 were synthesized, and their inhibitory activity on sodium ion channels was examined by means of cell-based assay. (-)-FD-STX (4a) and (-)-FD-dcSTX (4b), which showed moderate inhibitory activity, were further evaluated by the use of the patch-clamp method in cells that expressed Na(V)1.4 (a tetrodotoxin-sensitive sodium channel subtype) and Na(V)1.5 (a tetrodotoxin-resistant sodium channel subtype). These compounds showed moderate inhibitory activity towards Na(V)1.4, and weaker inhibitory activity towards Na(V)1.5. Uniquely, however, the inhibition of Na(V)1.5 by (-)-FD-dcSTX (4b) was "irreversible".

Total synthesis of (+)-batzelladine A and (-)-batzelladine D, and identification of their target protein

Asymmetric total synthesis of batzelladine A (1) and batzelladine D (2) has been achieved. Our synthesis of batzelladines features 1) stereoselective construction of the cyclic guanidine system by means of successive 1,3-dipolar cycloaddition reaction and subsequent cyclization, 2) direct esterification of the bicyclic carboxylic acid 35 with the guanidine alcohol 8 or 59 to construct the whole carbon skeleton of batzelladines, and 3) one-step formation of the alpha,beta-unsaturated aldehyde 53 from the primary alcohol 47 with tetra-n-propylammoniumperruthenate (TPAP), providing an efficient route to the left-hand bicyclic guanidine alcohol of batzelladine A (1). With the synthetic compounds 1 and 2 in hand, their target protein was examined by using immobilized CD4 and gp120 affinity gels. The results indicated that batzelladines A (1) and D (2) bind specifically to CD4.

A general, Brønsted acid-catalyzed hetero-Michael addition of nitrogen, oxygen, and sulfur nucleophiles

[reaction: see text] Strong Brønsted acids such as bis(trifluoromethanesulfon)imide catalyze the hetero-Michael addition of carbamates, alcohols, and thiols to alpha,beta-unsaturated ketones, alkylidene malonates, and acrylimides. Scope, reaction rates, and yields are superior to comparable Lewis acid-catalyzed processes.

[Total synthesis of marine cyclic guanidine compounds and development of novel guanidine type asymmetric organocatalysts]

Crambescidins and batzelladines, novel marine guanidine alkaloids, have unique pentacyclic and tricyclic guanidine core structures, respectively. They display a considerable array of biological activity and not surprisingly have attracted considerable synthetic interest. The first total synthesis of crambescidin 359 (7) and stereoselective total synthesis of batzelladine D (11) were accomplished based on a successive 1,3-dipolar cycloaddition reaction strategy. During synthetic studies of 7, the absolute stereochemistry was revealed. Based on the structure of 7, the novel C2-symmetric pentacyclic guanidine compounds 69a-d were designed and synthesized as guanidine organocatalysts. The catalyst 69b works efficiently as an asymmetric catalyst of the alkylation reaction of the glycynate-benzophenone Schiff base 73, which gives 74 with 80-90% ee.

Oral toxicity of the cyanobacterial toxin cylindrospermopsin in male Swiss albino mice: determination of no observed adverse effect level for deriving a drinking water guideline value

The cyanobacterial toxin cylindrospermopsin (CYN) is a frequent contaminant of freshwaters throughout the world, including those that are sources of drinking water. The first cases of human poisoning attributed to this toxin occurred from a treated drinking water supply in Queensland, Australia, in 1979. The toxin causes extensive damage to the liver, kidneys, spleen, heart, and other organs. It is known to be a potent protein synthesis inhibitor, but there is mounting evidence for genotoxicity and that it metabolizes to even more toxic forms. As part of a risk assessment process leading to a guideline for a safe drinking water level for this toxin, we performed a series of experiments to determine a no-observed-adverse-effect level (NOAEL) for this toxin. In the first trial male mice were exposed to CYN-containing cyanobacterial extract in their drinking water (0-657 microg CYN kg(-1) day(-1)) for 10 weeks. In the second trial mice received purified CYN by daily gavage (0-240 microg CYN kg(-1) day(-1)) for 11 weeks. Body and organ weights were recorded; urine, serum, and hematology analyses were performed; and histopathological examination of tissues was carried out. Body weights were significantly increased at low doses (30 and 60 microg kg(-1) day(-1)) and decreased at high doses (432 and 657 microg kg(-1) day(-1)). Liver and kidney weights were significantly increased at doses of 240 microg kg(-1) day(-1) and 60 microg kg(-1) day(-1), respectively. Serum bilirubin levels were significantly increased and bile acids significantly decreased at doses of 216 microg kg day(-1) and greater. Urine total protein was significantly decreased at doses above 60 microg kg(-1) day(-1). The kidney appeared to be the more sensitive organ to this toxin. If it is assumed that increased organ weights and changes in functional capacity are responses to an underlying toxic effect, then the NOAEL based on this data is 30 microg kg(-1) day(-1), which, with standard calculations and uncertainty factors, provides a proposed guideline safety value of 1 microg/L in drinking water.

The Palm Island mystery disease 20 years on: a review of research on the cyanotoxin cylindrospermopsin

Poisoning of humans resulting from consumption of water affected by the toxic cyanobacterium Cylindrospermopsis raciborskii was first reported almost 20 years ago from Palm Island, northern Queensland, Australia. Since that time a great deal has been learned about this organism and cylindrospermopsin (CYN), the toxin it produces. This article reviews the information now available to us. It summarizes aspects of the chemistry of the toxin-now known to be produced by some cyanobacterial species other than C. raciborskii-and its biosynthesis and chemical synthesis in vitro, as well as its detection and measurement by chemical and biological assay. Some of the factors affecting toxin production by cultured isolates of C. raciborskii are reviewed and the conditions that cause its release from the cells described. The occurrence of CYN in water bodies and the management strategies used to minimize the harmful effects of the toxin are outlined. These include a range of water-treatment practices now in place to remove CYN-producing organisms and/or to neutralize the toxin together with some management procedures that have been tried, with varying degrees of success, to prevent buildup of blooms of the offending organisms. Some of the public-health considerations arising from exposure to water supplies affected by CYN are summarized along with the risk factors and guidance values as they are currently applied. Among the more recent developments described are those that come from the application of molecular techniques for characterizing toxic and nontoxic strains and for exploring the genetic aspects of CYN production.

In vitro hepatotoxicity of the cyanobacterial alkaloid cylindrospermopsin and related synthetic analogues

Cylindrospermopsin (CY), a sulfate ester of a tricyclic guanidine substituted with a hydroxymethyluracil, is a cyanobacterial toxin of increasing environmental import as it frequently occurs in drinking water reservoirs. As a toxin, CY mainly targets the liver but also involves other organs. In hepatocytes CY inhibits the synthesis of protein and of glutathione, leading to cell death. The total chemical synthesis of CY has recently been reported (Xie et al., 2000, J. Am. Chem. Soc. 22, 5017-5024). The synthesis has provided analogues of CY to study aspects of the relationship between chemical structure and activity that contribute to toxicity. Protein synthesis inhibition was measured in vitro using a rabbit reticulocyte system. Primary cultures of rat hepatocytes were used to determine the biological activity of CY and analogues in intact cells. Protein synthesis and cell glutathione levels were measured. We could distinguish between CY transport and biological activity by comparing the results in vitro to those in intact cells. The role of the sulfate group in CY toxicity was examined by comparing biological effects of CY with that of CY-DIOL (synthetic CY lacking the sulfate group). The sulfate group was found not to play a role in CY activity or in its uptake into cells, since there was no significant difference in biological activity in vitro or in cells between natural CY and CY-DIOL. The orientation of the hydroxyl group at C7 also had no impact on biological activity or transport of CY, since the C7 epimer of CY (EPI-CY) and the corresponding diol (EPI-DIOL) had activity similar to RAC-CY in vitro and in intact cells. AB-MODEL, the analogue lacking an intact C ring, and the methyl and hydroxyl groups of ring A could inhibit protein synthesis (but at concentrations 500-1000-fold higher than natural CY). Other structurally simpler synthetic analogues lacked biological activity.

Stereoselective total syntheses and reassignment of stereochemistry of the freshwater cyanobacterial hepatotoxins cylindrospermopsin and 7-epicylindrospermopsin

A stereoselective total synthesis of the structure 1 proposed for the freshwater cyanobacterial heptatotoxin cylindrospermopsin has been accomplished in approximately 30 operations starting from commercially available 4-methoxypyridine. Utilizing methodology developed by Comins, the tetrasubstituted piperidine A-ring unit of the hepatotoxin was efficiently constructed. The two remaining stereocenters in the natural product were then set by a stereospecific intramolecular N-sulfinylurea Diels-Alder cyclization/Grignard ring opening/allylic sulfoxide [2,3]-sigmatropic rearrangement sequence previously developed in these laboratories, leading to key intermediate 29. The stereochemical assignment of alcohol 29, which contains all six of the stereogenic centers of the natural product, was confirmed by an X-ray crystal structure determination of a derivative. Installation of the D-ring uracil moiety was effected by using our new methodology developed for this purpose, and construction of the C-ring guanidine completed the total synthesis of racemic structure 1. However, the (1)H NMR data for this compound do not match that of cylindrospermopsin, but instead agree with the data reported for 7-epicylindrospermopsin, a minor toxic metabolite that co-occurs with cylindrospermopsin. Therefore, we propose a revision of the stereochemical assignments of these natural products such that cylindrospermopsin is now represented as structure 2 and 7-epicylindrospermopsin is 1. This reassignment was further confirmed by Mitsunobu inversion of the C-7 alcohol 51 to epimer 52, and conversion of this compound to tetracyclic diol 57, which has previously been transformed to cylindrospermopsin (2).

Substituted guanidines: introducing diversity in combinatorial chemistry

The guanidine moiety is an important motif present in many biologically active compounds. Fully substituted guanidines are of key importance for the development of bioactive molecules. The present paper reports on an efficient procedure for the direct solid-phase conversion of amines to fully substituted guanidines under very mild conditions.