Total Synthesis of the Cytotoxic Marine Triterpenoid Isodehydrothyrsiferol Reveals Partial Enantiodivergency in the Thyrsiferol Family of Natural Products

Establishment of "Ring-Size-Divergent" Synthetic Strategy: Divergent Synthesis, Stereochemical Assignments, and Biological Activity Studies of Nerolidol-Type Sesquiterpenoids and Feroniellins

Biomimetic epoxide-opening cascade cyclizations of polyepoxides enable the efficient and rapid construction of polyether skeletons. In this study, we discovered a method for switching the cyclization mode from tetrahydrofuran to tetrahydropyran (THP) formation in epoxide-opening cascades of polyepoxides. The THP formation proceeded via an epoxonium-ion intermediate by simple heating in neutral water. Next, by expanding the switching reaction, we successfully established a "ring-size-divergent" synthetic strategy that enabled the synthesis of the five-, six-, and seven-membered ether rings from identical diepoxide cyclization precursors under simple acidic or neutral conditions. The "ring-size-divergent" synthetic strategy was applied to the short divergent synthesis of nerolidol-type sesquiterpenoids and feroniellins, resulting in the revision of the proposed stereochemistry of certain natural products and the determination of all of the absolute configurations. Additionally, the anti-inflammatory activities of the synthetic samples were evaluated.

Naturally Occurring Organohalogen Compounds-A Comprehensive Review

The present volume is the third in a trilogy that documents naturally occurring organohalogen compounds, bringing the total number-from fewer than 25 in 1968-to approximately 8000 compounds to date. Nearly all of these natural products contain chlorine or bromine, with a few containing iodine and, fewer still, fluorine. Produced by ubiquitous marine (algae, sponges, corals, bryozoa, nudibranchs, fungi, bacteria) and terrestrial organisms (plants, fungi, bacteria, insects, higher animals) and universal abiotic processes (volcanos, forest fires, geothermal events), organohalogens pervade the global ecosystem. Newly identified extraterrestrial sources are also documented. In addition to chemical structures, biological activity, biohalogenation, biodegradation, natural function, and future outlook are presented.

Asymmetric Total Syntheses, Stereostructures, and Cytotoxicities of Marine Bromotriterpenoids Aplysiol B (Laurenmariannol) and Saiyacenol A

There are marine cytotoxic bromotriterpenoids, named the thyrsiferol family that are structurally characterized by some tetrahydropyran (THP) and tetrahydrofuran (THF) rings. The thyrsiferol family belongs to natural products that are often difficult to determine their stereostructures even by the current, highly advanced spectroscopic methods, especially in acyclic systems including stereogenic tetrasubstituted carbon centers. In such cases, it is effective to predict and synthesize the possible stereostructures. Herein, to elucidate ambiguous stereostructures and unassigned absolute configurations of aplysiol B, laurenmariannol, and saiyacenol A, members of the thyrsiferol family, we carried out their asymmetric chemical syntheses featuring 6‐exo and 5‐exo oxacyclizations of epoxy alcohol precursors and 6‐endo bromoetherification of a bishomoallylic alcohol. In this paper, we report total assignments of their stereostructures through their asymmetric chemical syntheses and also their preliminary cytotoxic activities against some tumor cells. These results could not have been achieved without depending on asymmetric total synthesis.

Critical Switching of Cyclization Modes of Polyepoxides in Acidic Aqueous Media and Neutral Water: Synthesis and Revised Structure of a Nerolidol-Type Sesquiterpenoid

Biomimetic epoxide-opening cascades of polyepoxides enable the efficient and rapid construction of polyether frameworks. Herein, we show that the epoxide-opening cascade cyclization that affords tetrahydrofuran products in acidic aqueous media produces tetrahydropyran (THP) in neutral water. THP formation proceeded by simply heating polyepoxides in neutral water and followed a different cyclization mode from those observed so far. The novel cascade cyclization in H₂ O was applied to the synthesis of a new nerolidol-type sesquiterpenoid, resulting in revision of the proposed structure and determination of the absolute configuration.

Marine natural products

Covering: January to December 2017This review covers the literature published in 2017 for marine natural products (MNPs), with 740 citations (723 for the period January to December 2017) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1490 in 477 papers for 2017), together with the relevant biological activities, source organisms and country of origin. Reviews, biosynthetic studies, first syntheses, and syntheses that led to the revision of structures or stereochemistries, have been included. Geographic distributions of MNPs at a phylogenetic level are reported.