Synthesis and structural revision of marine eicosanoid agardhilactone

Iridium-Catalyzed Asymmetric Cascade Allylation/Retro-Claisen Reaction

An enantiomerically enriched 3-hydroxymethyl pentenal unit is one of the key structural cores in plenty of natural products and drug candidates with significant biological activities. However, very few synthetic methodologies for the facile construction of the related skeletons have been reported to date. Herein, an elegant iridium-catalyzed asymmetric cascade allylation/retro-Claisen reaction of readily available β-diketones with VEC was successfully developed, and a wide range of functionalized chiral 3-hydroxymethyl pentenal derivatives could be prepared in good yields with excellent enantioselectivities. Various 1,3-diketones and functionalized ketones containing different electron-withdrawing groups on the β-position were well tolerated as outstanding partners with high reactivity and excellent regio-/chemo-/enantioselectivity. The synthetic utility of product chiral 3-hydroxymethyl pentenal derivatives was well shown through gram-scale transformation, hydrogenation, cyclopropanation, hydroboration, and olefin metathesis. Moreover, this elegant protocol demonstrated synthetic applications in the concise synthesis of synthetically useful chiral building block (S)-Taniguchi lactone and the formal synthesis of natural product cytisine. A rational reaction pathway was proposed based on the experimental results and control experiments.

Origin of High Diastereoselectivity in Reactions of Seven-Membered-Ring Enolates

Unlike many reactions of their six-membered-ring counterparts, the reactions of chiral seven-membered-ring enolates are highly diastereoselective. Diastereoselectivity was observed for a range of substrates, including lactam, lactone, and cyclic ketone derivatives. The stereoselectivity arises from torsional and steric interactions that develop when electrophiles approach the diastereotopic π-faces of the enolates, which are distinguished by subtle differences in the orientation of nearby atoms of the ring.

Total Synthesis of Chlorinated Oxylipin Eiseniachloride B

Eiseniachloride B is a marine chlorinated oxylipin isolated from the brown alga Eisenia bicyclis. This natural product contains cyclopentane, chlorohydrin, and 14-membered lactone systems that incorporate five stereogenic centers. In this paper, we report on the total synthesis of structurally unique oxylipin eiseniachloride B from optically active lactol via ecklonialactone B in a linear sequence comprising 11 steps with a 12.1% overall yield.

Asymmetric total synthesis of (+)-ovafolinins A and B

(+)-Ovafolinins A and B are two homologous lignans containing unique polycyclic skeletons. Benefiting from a highly diastereoselective alkylation of (S)-Taniguchi lactone, a double Friedel-Crafts reaction, a global debenzylation and a Cu(OAc)₂-enabled benzylic oxidative cyclization, we present herein an efficient synthetic approach to (+)-ovafolinins A and B.

Total synthesis of marine eicosanoid (-)-hybridalactone

(-)-Hybridalactone (1) is a marine eicosanoid isolated from the red alga Laurencia hybrida. This natural product contains cyclopropane, cyclopentane, 13-membered macrolactone and epoxide ring systems incorporating seven stereogenic centers. Moreover, this compound has an acid-labile skipped Z,Z-diene motif. In this paper, we report on the total synthesis of (-)-hybridalactone (1). The unique eicosanoid (-)-hybridalactone (1) was synthesized starting from optically active γ-butyrolactone 2 in a linear sequence comprising 21 steps with an overall yield of 21.9%. A key step in the synthesis of (-)-hybridalactone (1) is the methyl phenylsulfonylacetate-mediated one-pot synthesis of the cis-cyclopropane-γ-lactone derivative. This reaction provided an efficient and stereoselective access to cis-cyclopropane-γ-lactone 12. Further elaboration of the latter compounds through desulfonylation, epoxidation, oxidation, Wittig olefination and Shiina macrolactonization afforded (-)-hybridalactone.

Survey of marine natural product structure revisions: a synergy of spectroscopy and chemical synthesis

The structural assignment of new natural product molecules supports research in a multitude of disciplines that may lead to new therapeutic agents and or new understanding of disease biology. However, reports of numerous structural revisions, even of recently elucidated natural products, inspired the present survey of techniques used in structural misassignments and subsequent revisions in the context of constitutional or configurational errors. Given the comparatively recent development of marine natural products chemistry, coincident with modern spectroscopy, it is of interest to consider the relative roles of spectroscopy and chemical synthesis in the structure elucidation and revision of those marine natural products that were initially misassigned. Thus, a tabulated review of all marine natural product structural revisions from 2005 to 2010 is organized according to structural motif revised. Misassignments of constitution are more frequent than perhaps anticipated by reliance on HMBC and other advanced NMR experiments, especially when considering the full complement of all natural products. However, these techniques also feature prominently in structural revisions, specifically of marine natural products. Nevertheless, as is the case for revision of relative and absolute configuration, total synthesis is a proven partner for marine, as well as terrestrial, natural products structure elucidation. It also becomes apparent that considerable 'detective work' remains in structure elucidation, in spite of the spectacular advances in spectroscopic techniques.

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.

Marine natural products: synthetic aspects

An overview of marine natural products synthesis during 2005 is provided. In a similar vein to earlier installments in this series, the emphasis is on total syntheses of molecules of contemporary interest, new total syntheses, and syntheses that have resulted in structure confirmation or stereochemical assignments.