Total Synthesis of Purported Cephalosporolides H and I, Penisporolide B, and Their Stereoisomers

Emergence of 2,3,5-trisubstituted tetrahydrofuran natural products and their synthesis

The emergence of various 2,3,5-trisubstituted tetrahydrofuran natural products in the recent literature and their synthesis is the focus of this review. These molecules exhibit varied bioactivities and have garnered the interest of several synthetic chemists owing to their efficient synthesis. A few of them have been synthesized and their absolute stereo structure has been confirmed for the first time. These will be appealing candidates in future synthetic investigations along with the untouched molecules. Thus, this compilation will reveal these molecules for expansion of their diversity within the realm of both synthesis and bioactivity studies.

Can an n(O) → π* Interaction Provide Thermodynamic Stability to Naturally Occurring Cephalosporolides?

The stereocontrolled synthesis of naturally occurring products containing a 5,5-spiroketal molecular structure represents a major synthetic problem. Moreover, in a previous work, the stereocontrolled synthesis of cephalosporolide E (ceph E), which presumably was obtained from its epimer congener (ceph F) through an acid-mediated equilibration process, was reported. Consequently, we performed a theoretical investigation to provide relevant information regarding the title question, and it was found that the higher thermodynamic stability of ceph E, relative to ceph F, is caused by an n → π* interaction between a lone electron pair of the oxygen atom of the spiroketal ring (n_O) and the antibonding orbital of the carbonyl group (π*_C=O). Although similar stereoelectronic interactions have been disclosed in other molecular structures, its presence in ceph E, and very likely in other related naturally occurring products, represents a novel nonanomeric stabilizing effect that should be introduced into the chemical literature.

Cobalt-Catalyzed Reductive Dimethylcyclopropanation of 1,3-Dienes

Dimethylcyclopropanes are valuable synthetic targets that are challenging to access in high yield using Zn carbenoid reagents. Herein, we describe a cobalt-catalyzed variant of the Simmons-Smith reaction that enables the efficient dimethylcyclopropanation of 1,3-dienes using a Me₂ CCl₂ /Zn reagent mixture. The reactions proceed with high regioselectivity based on the substitution pattern of the 1,3-diene. The products are vinylcyclopropanes, which serve as substrates for transition-metal-catalyzed ring-opening reactions, including 1,3-rearrangement and [5+2] cycloaddition. Preliminary studies indicate that moderately activated monoalkenes are also amenable to dimethylcyclopropanation under the conditions of cobalt catalysis.

Dual Lewis Acid/Photoredox-Catalyzed Addition of Ketyl Radicals to Vinylogous Carbonates in the Synthesis of 2,6-Dioxabicyclo[3.3.0]octan-3-ones

A combined Lewis acid/photoredox catalyst system enabled the intramolecular umpolung addition of ketyl radicals to vinylogous carbonates in the synthesis of 2,6-dioxabicyclo[3.3.0]octan-3-ones. This reaction proceeded on a variety of aromatic ketones to provide THF rings in good yield (up to 95%). Although diastereoselectivity was found to be modest (1.4-5:1) for the C-C bond forming reaction, the minor diastereomers were converted to 2,6-dioxabicyclo[3.3.0]octan-3-ones by an efficient Lewis acid-mediated epimerization cascade in up to 90% yield.

Marine natural products

Covering: 2016. Previous review: Nat. Prod. Rep., 2017, 34, 235-294This review covers the literature published in 2016 for marine natural products (MNPs), with 757 citations (643 for the period January to December 2016) 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 (1277 in 432 papers for 2016), 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.

A NMR method for relative stereochemical assignments of the tricyclic core of cephalosporolides, penisporolides and related synthetic analogues

A new NMR method is developed to discriminate the four possible diastereomeric SAFLs, leading to revisions of 11 synthetic compounds.