Marine natural products: synthetic aspects

Copper-Catalyzed Synthesis of N-Fused Quinolines via C(sp3)-H Activation-Radical Addition-Cyclization Cascade

A novel copper-catalyzed cyclization reaction for the synthesis of pyrazolo[1,5-a]quinoline, triazolo[1,5-a]quinoline, and pyrrolo[1,2-a]quinoline derivatives is described. The process is initiated by di-tert-butyl peroxide-mediated C(sp3)-H activation to generate the α-functionalized radical, which supervenes a cascade radical addition/cyclization sequence to access the N-fused quinolines in good yields with broad functional group tolerance.

Metal- and Base-Free Radical Cascade Cyclization/Hydrolysis of CN-Containing 1,6-Enynes with Ethers to Access Polyheterocycles

A convenient and straightforward approach for the radical cascade cyclization/hydrolysis of CN-containing 1,6-enynes with simple ethers under metal- and base-free conditions is described. This strategy provides a variety of valuable ethers-substituted polyheterocycles via the construction of three C-C bonds, one C=O bond, and two new six-membered rings within a single procedure. The resulting products can smoothly undergo follow-up conversions to various useful scaffolds. The methodology shows excellent functional group tolerance, high step- and atom- economy, and mild reaction conditions, which can be further scaled up to gram quantity in a satisfactory yield.

Recent progress in the total synthesis of marine brominated sesquiterpene aplydactone

Aplydactone is a brominated sesquiterpene isolated from the sea hare Aplysia dactylomela. Structurally, it features a complex cage-like skeleton containing a highly strained tricyclic-[4.2.0.0^3,8]-4-decanone system. Its unique structural features have fascinated many synthetic chemists. In this review, the synthetic efforts towards aplydactone in the last five years are summarized in two categories including nonbiomimetic synthesis and biomimetic synthesis based on the core synthetic strategy. These syntheses set a classical and instructive example for the syntheses of other marine natural products.

Base-promoted domino radical cyclization of 1,6-enynes

A good regioselective, high atom-economical and transition-metal-free method for the synthesis of α-functionalized ether derivatives via the domino radical cyclization of 1,6-enynes is described. A series of α-functionalized ether derivatives could be easily obtained in good yields with wide functional group tolerance by using less toxic and inexpensive Cs₂CO₃ as the base. The control experiment results show that the reaction involves a radical process. This strategy provides a regioselective way toward the formation of dual C-C bonds in one step.

tert-Butyl peroxybenzoate mediated formation of 3-alkylated quinolines from N-propargylamines via a cascade radical addition/cyclization reaction

A novel method for constructing 3-alkylated quinolines was developed via tert-butyl peroxybenzoate mediated cycloaddition between N-propargyl aromatic amines and ethers.

Visible-light-induced selective amination of enol ethers with N-alkoxyamides by using DDQ as a photoredox catalyst

Nitrogen-centered radical (NCR) mediated amination of enol ethers has been developed under visible-light irradiation and metal-free conditions.

Visible-light-mediated radical insertion/cyclization cascade reaction: synthesis of phenanthridines and isoquinolines from isocyanides

The development of a visible-light promoted radical insertion/cyclization cascade reaction to generate phenanthridines and isoquinolines from isocyanides under mild reaction conditions is reported.

Synthetic strategies to terpene quinones/hydroquinones

The cytotoxic and antiproliferative properties of many natural sesquiterpene-quinones and -hydroquinones from sponges offer promising opportunities for the development of new drugs. A review dealing with different strategies for obtaining bioactive terpenyl quinones/hydroquinones is presented. The different synthetic approches for the preparation of the most relevant quinones/hydroquinones are described.

Recent synthetic studies leading to structural revisions of marine natural products

Because of the highly unique structures of marine natural products, there are many examples of structures that were originally proposed based on spectral analyses but later proven incorrect. In many cases, the total syntheses of the originally proposed structures of marine natural products has confirmed their incorrectness and the subsequent total syntheses of the newly proposed structures proved the revised structures. This review will show such cases appearing after 2005 and demonstrate how the true structures were elucidated.

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.

Synthesis of macrocyclic shellfish toxins containing spiroimine moieties

An overview of the structure and biological activity of macrocyclic polyketides derived from dinoflagellates that contain unusual cyclic imine units is provided. The total and partial syntheses of these molecules are discussed with an emphasis on the construction of the spiroimine functionality thought to be the key pharmacophore of these fact-acting shellfish toxins.