Cylindrospermopsins

Aside from the pyrimidine preparations which follow well-established strategies, several new pathways have been developed. In model studies for the synthesis of cylindrospermopsin, Weinreb and Keen reported a novel route to N-hydroxydihydrouracil 33 from a,P-unsaturated ester 30 followed by aromatization to give pyrimidone 34 <00TL4307>. 

Recent developments in palladium-catalyzed coupling reactions have arisen in the pyrimidine field as well. The Sonogashira coupling of bromopyrimidine 85 with alkynes 86 produced pyrimidines 87, important intermediates reported by Hart and co-workers in their approach to the cylindrospermopsin substructure . 

Hepatotoxins include microcystins, which are cyclic heptapeptides (Fig. 5.1a) and cylindrospermopsin, a sulfated guanidinium alkaloid (Fig. 5. lb). Microcystins bind to certain protein phosphatases responsible for regulating the distribution of cytoskeletal proteins (Zurawell et al. 2005 Leflaive and Ten-Hage 2007). Hepatocytes exposed to microcystins eventually undergo cellular deformation, resulting in intra-hepatic bleeding and, ultimately, death (Carmichael 2001 Batista et al. 2003). In contrast, cylindrospermopsin appears to have a different mode of activity, possibly involving inhibition of protein or nucleotide synthesis (Codd et al. 1999 Froscio et al. 2003 Reisner et al. 2004). Nevertheless, microcystins are the most common cyanotoxins isolated from cyanobacterial blooms (Sivonen and Jones 1999). 

Fig. 5.1 Common cyanobacterial hepatotoxins. (a) Generalized structure of microcystin, a cyclic heptapeptide. Note that X and Z are L-amino acids. For example, microcystin-LR possesses lysine and arginine residues at X and Z, respectively, (b) Cylindrospermopsin, a hepatotoxic alkaloid from Cylindrospermopsis raceborskii...

Froscio SM, Humpage AR, Burcham PC, Falconer IR (2003) Cylindrospermopsin-induced protein synthesis inhibition and its dissociation from acute toxicity in mouse hepatocytes. Environ Toxicol 18 243-251... 

Preufiel K, Stiiken A, Wiedner C, Chorus I, Fastner J (2006) First report of cylindrospermopsin producing Aphanizomenon flos-aquae (Cyanobacteria) from two German lakes. Toxicon... 

Reisner M, Carmeli S, Werman M, Sukenik A (2004) The cyanobacterial toxin cylindrospermopsin inhibits pyrimidine nucleotide synthesis and alters cholesterol distribution in mice. 

Seifert M, McGregor G, Eaglesham G, Wickramasinghe W, Shaw G (2007) First evidence for the production of cylindrospermopsin and deoxy-cylindrospermopsin by the benthic freshwater cyanobacterium Lyngbya wollei (Farlow ex Gomont) Speziale and Dyck. Harmful Algae 6 73-80... 

Cyanobacteria toxins are toxins produced by certain species of blue-green algae that have become a major environmental and public health concern. The behavior of cyanotoxins during chlorination treatment has been recently reviewed by Merel et al. [129]. Chlorination DBFs have been reported only for the hepatotoxins microcystin-LR and cylindrospermopsin. Other cyanotoxins, such as nodularins, saxitoxins, and anatoxins, have yet to be investigated. Different isomers of six chlorination products of microcystin-LR have been characterized dihydroxy-microcystin, monochloro-microcystin, monochloro-hydroxy-microcystin, monochloro-dihydroxy-microcystin, dichloro-dihydroxy-microcystin, and trichloro-hydroxy-microcystin. Only two chlorination DBFs have been reported so far for cylindrospermopsin 5-chloro-cylindros-permopsin and cylindrospermopsic acid [129]. Chlorination of microcystin, cylindrospermopsin, and nodularins seems to reduce the mixture toxicity however, this aspect has not been extensively studied [129]. 

An international intercomparison exercise in the determination of microcystin, carried out by using the most common methods (LC/DAD, ELISA and LC/MS) indicated that LC/DAD is affected by lower precision [234], while the coupling of the LC technique with ELISA permit the achievement of high sensitivity and specificity in the determination of microcystins and nodularin [235] without the need of pre-concentration the method meets the World Health Organization guidelines (1 pg L ). The combination of ELISA characterization and LC analysis with fluorescence, UV, and tandem MS detections, allowed the first identification of cylindrospermopsin, an algal toxin that caused the poisoning of up to 148 persons in Australia [236],... 

This procedure was successfully used as part of the total synthesis of the freshwater cyanobacterial hepatotoxins cylindrospermopsin 741 and 7-epicylindrospermopsin 742 <2001JA8851, 2002JA3939>. 

Cylindrospermopsin has also been isolated from Umezakia natans and Aphanizomenon ovalisporum. Algal blooms that produce cylindrospermopsin are widespread in tropical waters where occurrences of gastro-... 

The novel structure of cylindrospermopsin, with a guanidine embedded in a tricyclic system, six chiral centers, and polar sulfate, uracil and guanidine functional groups, makes its synthesis challenging. Its potent toxicity makes the synthesis of cylindrospermopsin an important problem that has been the subject of intense interest.1,7 8... 

The polar functionality presents a major problem in the synthesis of cylindrospermopsin because the natural product cannot be extracted into organic solvents, making its separation from water-soluble reagents difficult. Clearly, the sulfate ester should be introduced in the last step. The guanidine should also be introduced late in the synthesis and should be protected as a less basic acyl guanidine until as late as possible. Finally, the uracil is also polar and so must be either introduced late in the synthesis or protected as a dialkoxypyrimidine. 

We envisioned that the B ring of cylindrospermopsin (1) could be prepared by an intramolecular SN2 reaction of the guanidine of 4 on the bro-moketone. Equilibration would provide the desired equatorial side chain. Reduction of the ketone, deprotection and sulfation would complete the synthesis of cylindrospermopsin. Addition of acetylene 5 to pyrimidine... 

The spectral data of 24 are very similar to those of the right half of cylindrospermopsin (1), while those of 25 are quite different. For instance, the absorption of the side chain methine hydrogen H7 of 24 at 8 4.70 (d, 1, / = 4. 0 Hz) is identical to that of cylindrospermopsin and very different from that of 25 at 8 4.44 (d, 1, J = 6.8 Hz). Therefore, we concluded that 24 has the same stereochemistry as cylindrospermopsin. The spectral data for 25 was later used to support the structural assignment of 7-epicylindrospermopsin (2) in which the side chain hydrogen absorbs at 8 4.50 (d, 1, J = 6.6 Hz).5... 

Having established that the end game of the proposed synthesis of cylindrospermopsin (1) from bromoketone 4 was viable, we turned our attention to the preparation of acetylene 5 from 4-methoxy-3-methylpyridine (7). 4-Methoxy-3-methylpyridine (7) was prepared by modifications of the literature procedure.17,18 3-Methyl-4-nitropyridine A-oxide (27),19 was treated with K2CO3 in methanol at 70 °C to displace the nitro group to... 

Hydrolysis of 63 in concentrated hydrochloric acid at 100 °C for 6 h afforded 95% of uracil diol 65 with H and 13C NMR spectral data virtually identical to those of cylindrospermopsin except for the protons and carbons close to C12. A similar hydrolysis of 64 provided 95% of 66. This hydrolysis is perhaps the most remarkable step in the synthesis. Very harsh conditions are needed to hydrolyze the dimethoxypyrimidine to the uracil. However, the reaction is remarkably clean, accompanied only by the desired hydrolysis of the acetate ester, but no decomposition or epimerization at any of the stereocenters. Presumably, the protonated guanidine and uracil make it hard to protonate either alcohol and solvolyze to form a trication. 

The key step in the synthesis was bromination of ketone 59, followed by hydrogenation to liberate the free guanidine, which underwent an intramolecular Sn2 reaction to form the tetrahydropyrimidine ring B. Further hydrogenation reduced the ketone to yield 42% of 63 containing the fully functionalized tricyclic system and protected hydroxymethyl-uracil side chain of cylindrospermopsin. Hydrolysis of the pyrimidine of 63 in concentrated hydrochloric acid at reflux and selective monosulfation completed the synthesis of cylindrospermopsin. 

We are grateful to the National Institutes of Health (GM-46470) for financial support. We thank Dr. Christine Hofstetter for assistance in obtaining NMR data and Dr. Thomas C. Harvey for carrying out preliminary experiments on another route to cylindrospermopsin. 

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