Tandem Diels-Alder/fragmentation approach to the synthesis of eleutherobin

Rapid Access to the Structural Core of Aflavinines via Stereoselective Tandem Intramolecular Diels-Alder Cycloaddition Controlled by the Allylic 1,3-Strain

An expedient route to access the functionalized structural core of aflavinines has been developed starting from three readily available fragments over 12 steps in 29.1% overall yield without using any transition metal catalysis. The key feature of this approach is a tandem intramolecular Diels-Alder cycloaddition to complete the hexacyclic framework with the correct stereochemistry and all the requisite structural elements in place to achieve the total synthesis of aflavinine and its congeners.

C-C fragmentation: origins and recent applications

It has been 60 years since Eschenmoser and Frey disclosed the archetypal CC fragmentation reaction. New fragmentations and several variants of the original quickly followed. Many of these variations, which include the Beckmann, Grob, Wharton, Marshall, and Eschenmoser-Tanabe fragmentations, have been reviewed over the intervening years. A close examination of the origins of fragmentation has not been described. Recently, useful new methods have flourished, particularly fragmentations that give alkynes and allenes, and such reactions have been applied to a range of complex motifs and natural products. This Review traces the origins of fragmentation reactions and provides a summary of the methods, applications, and new insights of heterolytic CC fragmentation reactions advanced over the last 20 years.

Phylogeny drives large scale patterns in Australian marine bioactivity and provides a new chemical ecology rationale for future biodiscovery

Twenty-five years of Australian marine bioresources collecting and research by the Australian Institute of Marine Science (AIMS) has explored the breadth of latitudinally and longitudinally diverse marine habitats that comprise Australia's ocean territory. The resulting AIMS Bioresources Library and associated relational database integrate biodiversity with bioactivity data, and these resources were mined to retrospectively assess biogeographic, taxonomic and phylogenetic patterns in cytotoxic, antimicrobial, and central nervous system (CNS)-protective bioactivity. While the bioassays used were originally chosen to be indicative of pharmaceutically relevant bioactivity, the results have qualified ecological relevance regarding secondary metabolism. In general, metazoan phyla along the deuterostome phylogenetic pathway (eg to Chordata) and their ancestors (eg Porifera and Cnidaria) had higher percentages of bioactive samples in the assays examined. While taxonomy at the phylum level and higher-order phylogeny groupings helped account for observed trends, taxonomy to genus did not resolve the trends any further. In addition, the results did not identify any biogeographic bioactivity hotspots that correlated with biodiversity hotspots. We conclude with a hypothesis that high-level phylogeny, and therefore the metabolic machinery available to an organism, is a major determinant of bioactivity, while habitat diversity and ecological circumstance are possible drivers in the activation of this machinery and bioactive secondary metabolism. This study supports the strategy of targeting phyla from the deuterostome lineage (including ancestral phyla) from biodiverse marine habitats and ecological niches, in future biodiscovery, at least that which is focused on vertebrate (including human) health.

Design, synthesis and antiviral activity studies of schizonepetin derivatives

A series of schizonepetin derivatives have been designed and synthesized in order to obtain potent antivirus agents. The antiviral activity against HSV-1 and influenza virus H3N2 as well as the cytotoxicity of these derivatives was evaluated by using cytopathic effect (CPE) inhibition assay in vitro. Compounds M2, M4, M5 and M34 showed higher inhibitory activity against HSV-1 virus with the TC₅₀ values being in micromole. Compounds M28, M33, and M35 showed higher inhibitory activity against influenza virus H3N2 with their TC₅₀ values being 96.4, 71.0 and 75.4 μM, respectively. Preliminary biological activity evaluation indicated that the anti-H3N2 and anti-HSV-1 activities improved obviously through the introduction of halogen into the structure of schizonepetin.

Marine diterpene glycosides

Marine diterpene glycosides (MDGs) respresent a small but highly significant group of the much larger class of marine diterpenes. The three well-studied examples of MDGs are eleutherobins, pseudopterosins and fuscosides, all of which exhibit extremely promising biological activity. The eleutherobins are potent anti-mitotic agents, and the pseudopterosins and fuscosides are potent anti-inflammatory agents. This review discusses the structures and biological activities of these compounds, as well as their biosynthesis and synthesis.

Efficient highly selective synthesis of methyl 2-(ethynyl)alk-2(E)-enoates and 2-(1'-chlorovinyl)alk-2(Z)-enoates from 2-(methoxycarbonyl)-2,3-allenols

Highly regio- and stereoselective reactions of readily available 2-(methoxycarbonyl)-2,3-allenols 1 with oxalyl chloride in the presence of Et(3)N or DMSO afforded methyl 2-(ethynyl)alk-2(E)-enoates (E)-2 and 2-(1'-chlorovinyl)alk-2(Z)-enoates (Z)-3, respectively, in moderate to good yields.

Using singlet oxygen to synthesize polyoxygenated natural products from furans

[Reaction: see text]. Singlet oxygen is a powerful tool in the armament of the synthetic organic chemist and possibly in that of nature itself. In this Account, we illustrate a small selection of the many ways singlet oxygen can be harnessed in the laboratory to aid in the construction of the complex molecular motifs found in natural products. A more philosophical question is also addressed: namely, how much do singlet oxygen oxidations influence the biogenesis of these natural products? All the synthetic examples surveyed in this Account can be characterized as belonging to the same class because they all involve the oxidation of a substituted furan nucleus by singlet oxygen. Readily accessible and relatively simple furans can be transformed into a host of complex motifs present in a diverse range of natural products by the action of singlet-oxygen-mediated reaction sequences. These reactions are highly advantageous because they frequently deliver a rapid and dramatic increase in molecular complexity in high yield. Furthermore, an unusually wide structural diversity is exhibited by the molecular motifs obtained from these reaction sequences. For example, relatively minor modifications to the starting substrate and to the reaction conditions may lead to products as variable as spiroketal lactones, 3-keto-tetrahydrofurans, various types of bis-spiroketals, 4-hydroxy cyclopentenones, or spiroperoxylactones. In addition, two more specialized examples are discussed in this Account. The core of the prunolide molecules and the chinensine family of natural products were rapidly synthesized using effective and short singlet oxygen mediated strategies; this adds weight to the assertion that singlet oxygen is a very effective moderator of complex cascade reaction sequences. We also show how our synthetic investigations have provided evidence that these same strategies might be used in the biogenesis of these molecules. In the cases of the chinensines and the litseaverticillols, an entire and diverse family of natural products was synthesized beginning from known naturally occurring furan-bearing terpenes. Additionally, in several cases, intermediates in our syntheses have been isolated from natural sources, which suggests that we have followed the same synthetic paths as nature. Certainly, the limit of the synthetic potential of singlet oxygen has not yet been reached, and we can look forward to seeing the boundaries expand in the future in a slew of new and interesting ways.

trans-RhCl(CO)(PPh3)2-Catalyzed Monomeric and Dimeric Cycloisomerization of Propargylic 2,3-Dienoates. Establishment of α,β-Unsaturated δ-Lactone Rings by Cyclometallation

Cyclometallation of two unsaturated carbon-carbon bonds usually requires the application of low-valent metal catalysts, which could cleave the propargylic ester linkage. Thus, it is desirable to identify a catalyst which could undergo cyclometallation without cleaving the propargylic ester linkage. In this paper, we used trans-RhCl(CO)(PPh(3))(2) to realize the cyclometallation of propargylic 2,3-dienoates. The substituents at the 4-position of allenoate moiety nicely control the reaction pathway: when the 4-position of propargylic 2,3-dienoate 1 was monosubstituted with an aryl group, the bicyclic intermediate 7 formed by the cyclometallation could highly selectively undergo carbometalation with the alkyne moiety in the second molecule of propargylic 2,3-dienoate 1 to afford metallabicyclic intermediates 8a or 8b. Subsequent reductive elimination would afford 9, which could undergo an intramolecular Diels-Alder reaction resulting in the formation of polycyclic bis(delta-lactone)-containing structures 2. The intermediate could be trapped by adding 3-methoxyprop-1-yne affording cyclization-aromatization product 4p highly selectively. If the substituent at the 4-positon of the 2,3-allenoate moiety has a beta-H atom, sequential unimolecular cyclometallation/beta-H elimination/reductive elimination occurs to afford cross-conjugated 5(Z)-alkylidene-4-alkenyl-5,6-dihydropyran-2-ones. The Z-stereochemistry of the exo double bond was determined by the cyclometallation. Some of the alpha,beta-unsaturated delta-lactones could be easily converted to other synthetically useful compounds via reduction reaction, hydrogenation, and iodination/coupling protocol.

Studies on Highly Stereoselective Addition−Elimination Reactions of 3-(Methoxycarbonyl)-1,2-allen-4-ols with MX. An Efficient Synthesis of 3-(Methoxycarbonyl)-2-halo-1,3(Z)-dienes

3-(Methoxycarbonyl)-2-halo-1,3(Z)-dienes were prepared highly stereoselectively via SN2'-type addition-elimination reactions of 3-(methoxycarbonyl)-1,2-allen-4-ols with MX. These products may easily undergo Negishi or Sonogashira coupling reactions to yield a series of stereodefined polysubstituted (E)-1,3-dienes.

Synthesis of Substituted 1,3-Diene Synthetic Equivalents by a Ru-Catalyzed Diyne Hydrative Cyclization

Catalyzed by [CpRu(CH3CN)3]PF6, the hydrative cyclization of dipropargylic sulfone substrates provides an effective way to synthesize highly functionalized substituted 3-sulfolenes. The amount of water is crucial for the reactivity of this cycloisomerization reaction. The scope and limitations of the Ru-catalyzed cycloisomerization are discussed. A marked ketone-directing effect was observed for the first time in ruthenium-catalyzed cyclizations. A plausible mechanism for the ketone-directed cycloisomerization is also rationalized. The utility of this method was demonstrated by both sulfur dioxide extrusion of the 3-sulfolenes to afford 1,3-dienes and subsequent inter- or intramolecular Diels-Alder reactions.

A Formal Total Synthesis of Eleutherobin Using the Ring-Closing Metathesis (RCM) Reaction of a Densely Functionalized Diene as the Key Step: Investigation of the Unusual Kinetically Controlled RCM Stereochemistry

Asymmetric oxyallylation reactions and ring-closing metathesis have been used to synthesize compound 3, a key advanced intermediate used in the total synthesis of eleutherobin reported by Danishefsky and co-workers. The aldehyde 6, which is readily prepared from commercially available R-(-)-carvone in six steps in 30 % overall yield on multigram quantities, was converted into the diene 5 utilizing two stereoselective titanium-mediated Hafner-Duthaler oxyallylation reactions. The reactions gave the desired products (8 and 12) in high yields (73 and 83 %, respectively) as single diastereoisomers, with the allylic alcohol already protected as the p-methoxyphenyl (PMP) ether, which previous work has demonstrated actually aids ring-closing metathesis compared to other protective groups and the corresponding free alcohol. Cyclization under forcing conditions, using Grubbs' second-generation catalyst 13, gave the ten-membered carbocycle (E)-14 in 64 % yield. This result is in sharp contrast to similar, but less functionalized, dienes, which have all undergone cyclization to give the Z stereoisomers exclusively. A detailed investigation of this unusual cyclization stereochemistry by computational methods has shown that the E isomer of the ten-membered carbocycle is indeed less thermodynamically stable than the corresponding Z isomer. In fact, the selectivity is believed to be due to the dense functionality around the ruthenacyclobutane intermediate that favors the trans-ruthenacycle, which ultimately leads to the less stable E isomer of the ten-membered carbocycle under kinetic control. During the final synthetic manipulations the double bond of enedione (E)-16 isomerized to the more thermodynamically stable enedione (Z)-4, giving access to the advanced key-intermediate 3, which was spectroscopically and analytically identical to the data reported by Danishefsky and co-workers, and thereby completing the formal synthesis of eleutherobin.

Intramolecular Diels−Alder Reactions of Optically Active Allenic Ketones: Chirality Transfer in the Preparation of Substituted Oxa-Bridged Octalones

The intramolecular Diels-Alder reaction of allenic ketones containing a furyl unit (IMDAF) to generate oxatricyclic systems in good yields is described. The alkene dienophiles 1ab give poor yields of the cycloadducts 2ab, presumably due to the facile retro Diels-Alder reaction. However, the analogous allenic dienophile 7 afforded the desired cycloadduct 8 in 91% yield on treatment with dimethylaluminum chloride. When the allene bears an alkyl substituent on the terminal carbon, complete diastereoselectivity is seen in the IMDAF, e.g. cyclization of 14 gave only the cycloadduct 15 in 80% yield presumably due to greater steric hindrance in the transition state II as compared to that in I. Finally we report complete chirality transfer of the stereochemistry of an allene to the carbon framework of the oxatricyclic system. Thus, the optically active allenic ketone 20 afforded only the desired cycloadduct 21 with the correct absolute stereochemistry needed for the synthesis of the arisugacin class of natural products.

Ruthenium-Catalyzed Diyne Hydrative Cyclization: Synthesis of Substituted 1,3-Diene Synthons

[reaction: see text]. A novel and versatile strategy for the synthesis of highly functionalized substituted 3-sulfolenes based on [CpRu(CH3CN)3]PF6-catalyzed hydrative cyclization has been developed. A marked ketone directing effect in ruthenium-catalyzed cyclization was observed for the first time. This provides complementary chemoselectivity for the synthesis of 3-sulfolenes and other cyclic enones. The utility of this method has been demonstrated by SO2 extrusion of 3-sulfolenes to afford 1,3-dienes and the subsequent inter- and intramolecular Diels-Alder reaction.

Studies on the Base-Promoted Conversion of Conjugated Alkynyl Esters to α-Substituted α-Allenyl Esters

We report the development of an efficient method for the conversion of a variety of conjugated alkynyl esters to alpha-substituted conjugated allenyl esters (racemic) through the use of strong amide bases. Substantially improved yields over typical enolate formation conditions were observed with the use of 2 equiv of lithium diisopropylamide. Trapping studies indicate that the second equivalent of base likely leads to the dianion intermediate, which upon addition of methyl iodide, trimethylsilyl chloride, or tributyltin chloride gives mixtures of alpha-substituted conjugated allenyl and beta,gamma-alkynyl deconjugated esters. Further optimization revealed that additive salts such as LiCl lead primarily to the allenyl product while the use of HMPA as a cosolvent gives the beta,gamma-alkynyl deconjugated alkylation product. The role of base, base concentration, and electrophile on product yield and selectivity is also discussed.