Biomimetic synthesis of (±)-9,10-deoxytridachione

Synthetic study toward tridachiapyrone B

A convergent approach to the skeleton of tridachiapyrone B is described taking advantage of the desymmetrization of α,α'-dimethoxy-γ-pyrone leading to α-crotyl-α'-methoxy-γ-pyrone in one step. To construct the quaternary carbon of the 2,5-cyclohexadienone of the target, a strategy based on the Robinson-type annulation of an aldehyde derived from α-crotyl-α'-methoxy-γ-pyrone was applied. The grafting of the simplified target's side chain was demonstrated through an oxidative anionic oxy-Cope rearrangement of the tertiary alcohol arising from the 1,2-addition of a 1,3-dimethylallyl reagent to 2,5-cyclohexadienone connected to the α'-methoxy-γ-pyrone motif.

Bioinspired Asymmetric Total Synthesis of Emeriones A–C**

We report asymmetric bioinspired total syntheses of the fungal metabolites emeriones A–C via stereoselective oxidations of two bicyclo[4.2.0]octadiene diastereomers. The central bicyclic scaffolds are prepared in an 8π/6π electrocyclization cascade of a stereodefined pentaene, which contains the fully assembled side chains of the emeriones. The anti‐aldol side chain is made using a Paterson‐aldol addition, and the epoxide of the dioxabicyclo[3.1.0]hexane side chain via ring‐closure onto an oxidized acetal. Our work has enabled the structural revision of emerione C, and resulted in the synthesis of a “missing” family member, which we call emerione D. DFT calculations identified two methyl groups that govern torquoselectivity in the 8π/6π cascade.

Two Steps to Bicyclo[4.2.0]octadienes from Cyclooctatetraene: Total Synthesis of Kingianic Acid A

Synthetic approaches to bicyclo[4.2.0]octadiene natural products frequently employ the synthesis of linear tetraenes to initiate a biosynthetic 8π/6π-electrocyclization cascade. This work forges a functionalized bicyclo[4.2.0]octadiene in two steps from cyclooctatetraene. The versatility of this method is demonstrated through natural product synthesis, including the first total synthesis of kingianic acid A and formal syntheses of kingianins A, D, and F and cryptobeilic acid D ethyl ester. The unexpected formation of an E,E,Z,E-tetraene byproduct is rationalized through density functional theory modeling.

Animal biosynthesis of complex polyketides in a photosynthetic partnership

Complex polyketides are typically associated with microbial metabolism. Here, we report that animals also make complex, microbe-like polyketides. We show there is a widespread branch of fatty acid synthase- (FAS)-like polyketide synthase (PKS) proteins, which sacoglossan animals use to synthesize complex products. The purified sacogolassan protein EcPKS1 uses only methylmalonyl-CoA as a substrate, otherwise unknown in animal lipid metabolism. Sacoglossans are sea slugs, some of which eat algae, digesting the cells but maintaining functional chloroplasts. Here, we provide evidence that polyketides support this unusual photosynthetic partnership. The FAS-like PKS family represents an uncharacterized branch of polyketide and fatty acid metabolism, encoding a large diversity of biomedically relevant animal enzymes and chemicals awaiting discovery. The biochemical characterization of an intact animal polyketide biosynthetic enzyme opens the door to understanding the immense untapped metabolic potential of metazoans.

Complex polyketides are usually produced by microbes, whereas the origin of polyketides found in animals remained unknown. This study shows that sacoglossan animals, such as sea slugs, employ fatty acid synthase-like proteins to produce microbe-like polyketides.

Terminal substituent effects on the reactivity, thermodynamics, and stereoselectivity of the 8π-6π electrocyclization cascades of 1,3,5,7-tetraenes

M06-2X/6-31+G(d,p) computations are reported for the 8π-6π electrocyclization cascades of 1,3,5,7-tetraenes. The rate-determining step for these cascades is typically the second (6π) ring closure. According to experiment and theory, un- and monosubstituted tetraenes readily undergo 8π electrocyclic ring closure to form 1,3,5-cyclooctatrienes; however, the 6π electrocyclizations of these cyclooctatriene intermediates are slow and reversible, and mixtures of monocyclic and bicyclic products are formed. Computations indicate that di- and trisubstituted tetraenes undergo facile but less exergonic 8π electrocyclization due to a steric clash that destabilizes the 1,3,5-cyclooctatriene intermediates. Relief of this steric clash ensures the subsequent 6π ring closures of these intermediates are both kinetically facile and thermodynamically favorable, and only the bicyclic products are observed for the cascade reactions of naturally occurring tri- and tetrasubstituted tetraenes (in agreement with computations). The 6π electrocyclization step of these cascade electrocyclizations is also potentially diastereoselective, and di- and trisubstituted tetraenes often undergo cascade reactions with high diastereoselectivities. The exo mode of ring closure is favored for these 6π electrocyclizations due to a steric interaction that destabilizes the endo transition state. Thus, theory explains both the recalcitrance of the unsubstituted 1,3,5,7-octatetraene and 1-substituted tetraenes toward formation of the bicyclo[4.2.0]octa-2,4-diene products, as well as the ease and the stereoselectivity with which terminal di- and trisubstituted tetraenes are known to react biosynthetically.

Recent advances in stereoselective synthesis of 1,3-dienes

The aim of this review is to present the latest developments in the stereoselective synthesis of conjugated dienes, covering the period 2005-2010. Since the use of this class of compounds is linked to the nature of their appendages (aryls, alkyls, electron-withdrawing, and heterosubstituted groups), the review has been categorized accordingly and illustrates the most representative strategies and mechanisms to access these targets.

Biomimetic synthesis of pyrone-derived natural products: exploring chemical pathways from a unique polyketide precursor

Our biomimetic hypothesis proposes that families of diverse natural products with complex core structures such as 9,10-deoxytridachione, photodeoxytridachione and ocellapyrone A are derived in nature from a linear and conformationally strained all-( E) tetraene-pyrone precursor. We therefore synthesized such a precursor and investigated its biomimetic transformation under a variety of reaction conditions, both to the above natural products as well as to diverse isomers which we propose to be natural products "yet to be discovered". We also report herein the first synthesis of the natural product iso-9,10-deoxytridachione.

Marine natural products

This review covers the literature published in 2005 for marine natural products, with 704 citations (493 for the period January to December 2005) referring to compounds isolated from marine microorganisms and phytoplankton, green algae, brown algae, red algae, sponges, coelenterates, bryozoans, molluscs, tunicates and echinoderms. The emphasis is on new compounds (812 for 2005), 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.

Cascade Reactions in Total Synthesis

The design and implementation of cascade reactions is a challenging facet of organic chemistry, yet one that can impart striking novelty, elegance, and efficiency to synthetic strategies. The application of cascade reactions to natural products synthesis represents a particularly demanding task, but the results can be both stunning and instructive. This Review highlights selected examples of cascade reactions in total synthesis, with particular emphasis on recent applications therein. The examples discussed herein illustrate the power of these processes in the construction of complex molecules and underscore their future potential in chemical synthesis.

Triplet photosensitization in cyercene A and related pyrones

Mollusks of the Sacoglossa order contain a variety of polypropionate metabolites that are characterized by a pyrone chromophore, such as cyercene A and 9,10-deoxytridachione. Most often the pyrone is a 2-methoxy-gamma-pyrone but occasionally is a 4-methoxy-alpha-pyrone or hydropyrone. Members of this class of metabolites have been shown to undergo photochemical reactions of biosynthetic importance. An example is the photochemical conversion of 9,10-deoxytridachione to photodeoxytridachione, which has been observed in several mollusks. In this report, a series of gamma-pyrones and their alpha-pyrone analogs were synthesized and analyzed for photosensitizing activity. In all cases studied, the gamma-pyrone was a more efficient triplet sensitizer than the corresponding alpha-pyrone. Included in this set of molecules was the Sacoglossan metabolite cyercene A and its alpha-pyrone isomer. When irradiated in the presence of oxygen, cyercene A produced singlet oxygen at significantly higher rate than the corresponding alpha-pyrone isomer. Furthermore, the photoisomerization of cyercene A was quenched by piperylene with con-committant isomerization of the piperylene indicating that the isomerization proceeded through a triplet excited state. In contrast, the isomerization of the alpha-pyrone analog was not quenched. The implications of these photochemical results in terms of the biosynthesis and biological activity of Sacoglossan polypropionate metabolites are discussed.

Chemical diversity in opisthobranch molluscs from scarcely investigated Indo-Pacific areas

Opisthobranch molluscs with both a rich variety of secondary metabolites and great biomedical potential represent the most intensively studied group of molluscs in natural product chemistry. We review here the chemical investigations into secondary metabolites of "sea slugs" from less-studied Indian, Chinese and Egyptian coasts, giving an overview of their most relevant biological activities. In addition to the biomedical interest of the metabolites, in which both structures and organisms often lose their own importance, this chapter emphasizes the phyletic and geographic distribution of the compounds in order to provide a further informational base for chemotaxonomical generalizations.

A Novel Oxidative Rearrangement of 6-Methoxypyran-2-ones

As part of our continuing studies of pyrone-containing natural products, a series 6-methoxypyran-2-ones were synthesized. These were found to react with molecular oxygen at 20 degrees C, and this novel reaction yielded a series of highly functionalized alpha,beta-butenolides. [reaction: see text]

Stereoselective Syntheses of the Bioactive Polypropionates Aureothin, N-Acetylaureothamine, and Aureonitrile

[reaction: see text] Concise total syntheses of the bioactive polypropionates aureothin, N-acetylaureothamine, and aureonitrile are described.