Unifying the Synthesis of a Whole Family of Marine Meroterpenoids through a Biosynthetically Inspired Sequence of 1,2-Hydride and Methyl Shifts as Key Step

Marine meroterpenoids have attracted a great deal of attention from synthetic research groups due to their attractive and varied biological activities and their unique and diverse structures. In most cases, however, further biological studies have been severely limited mainly to the scarcity of natural supply and because almost none of the reported syntheses methods has enabled unified access for a large number of marine meroterpenoids with aureane and avarane skeletons. Based on our previous publications and the study of recent manuscripts on marine meroterpenoids, we have conceived a unified strategy for these fascinating marine compounds with aureane or avarane skeletons using available drimane compounds as starting materials. The key step is a biosynthetic sequence of 1,2-hydride and methyl shifts. This strategy is of great synthetic value to access marine meroterpenoids through easy chemical synthetic procedures. Finally, several retrosynthetic proposals are made for the future synthesis of several members of this class of meroterpenoids, focused on consolidating these 1,2-rearrangements as a versatile and unified strategy that could be widely used in the preparation of these marine meroterpenoids.

Total Synthesis of Marine Natural Products (+)-Strongylin A and Corallidictyal D by Regio- and Stereoselective Cyclization of Alkenyl Benzenes

An expeditious access to marine natural products (+)-strongylin A and corallidictyal D is described. A TFA/Et₃SiH-induced reductive isomerization of enols I to alkenyl benzenes II followed by a selectivity-controlled cyclization in the presence of HCl and BF₃·Et₂O affords benzofuran III and benzopyran IV, respectively. The applicability of this HCl-induced cyclization is showcased by a regio- and stereoselective synthesis of corallidictyal D, while BF₃·Et₂O-promoted cyclization posterior to rearrangement of an alkenyl benzene provides a regioselectively different benzopyran, (+)-strongylin A.

Bio-inspired construction of a tetracyclic ring system with an avarane skeleton: total synthesis of dactyloquinone A

We report the asymmetric construction of an avarane skeleton. The strategy involves a Lewis acid-catalyzed cyclization reaction, which drives the methyl groups of two different configurations at the C-4 site to migrate by 1, 2-rearrangement.

Total Synthesis of Anti-Cancer Meroterpenoids Dysideanone B and Dysiherbol A and Structural Reassignment of Dysiherbol A

The first total synthesis of marine anti-cancer meroterpenoids dysideanone B and dysiherbol A have been accomplished in a divergent way. The synthetic route features: 1) a site and stereoselective α-position alkylation of a Wieland-Miescher ketone derivative with a bulky benzyl bromide to join the terpene and aromatic moieties together and set the stage for subsequent cyclization reactions; 2) an intramolecular radical cyclization to construct the 6/6/6/6-tetracycle of dysideanone B and an intramolecular Heck reaction to forge the 6/6/5/6-fused core structure of dysiherbol A. A late-stage introduction of the ethoxy group in dysideanone B reveals that this group might come from the solvent ethanol. The structure of dysiherbol A has been revised based on our chemical total synthesis.

Computational Investigation of Multifaceted Cationic Rearrangement and Stereo- and Regioselectivity in the Formation of Dysideanone's Analogues

Mechanistic studies of regiodivergent arylations of cycloalkanols to furnish enantioenriched dysideanone's analogues are performed by employing density functional theory (DFT) calculations (B3LYP-D3(SMD)/6-311++G**//B3LYP-D3/6-31+G** level of theory). On the basis of our calculations, remote γ'-C-H arylation is preferred for unsubstituted carbinol 1, an outcome from combined factors like carbocationic stability, less steric hindrance during C-C coupling, and facile dearomatization. Meanwhile, in the presence of dimethyl substituent 1^Me, regioselective γ-arylation is favored by 3.4 kcal/mol, and both findings are in agreement with the reported experimental observations. Most importantly, we concur that the barrier associated with the formation of carbocation 6 and its substituted analogues correlates with the C-H arylation outcomes. Furthermore, the β-arylation route remains unlikely for all the reaction pathways explored in this study.

A Concise Route for the Synthesis of Tetracyclic Meroterpenoids: (±)-Aureol Preparation and Mechanistic Interpretation

A new concise general methodology for the synthesis of different tetracyclic meroterpenoids is reported: (±)-aureol (1), the key intermediate of this general route. The synthesis of (±)-aureol (1) was achieved in seven steps (28% overall yield) from (±)-albicanol. The key steps of this route include a C-C bond-forming reaction between (±)-albicanal and a lithiated arene unit and a rearrangement involving 1,2-hydride and 1,2-methyl shifts promoted by BF₃•Et₂O as activator and water as initiator.

Bioactive Prenyl- and Terpenyl-Quinones/Hydroquinones of Marine Origin †

The sea is a rich source of biological active compounds, among which terpenyl-quinones/hydroquinones constitute a family of secondary metabolites with diverse pharmacological properties. The chemical diversity and bioactivity of those isolated from marine organisms in the last 10 years are summarized in this review. Aspects related to synthetic approaches towards the preparation of improved bioactive analogues from inactive terpenoids are also outlined.

Access to Wieland-Miescher Diketone-Derived Building Blocks by Stereoselective Construction of the C-9 Quaternary Carbon Center Using the Mukaiyama Aldol Reaction

The Mukaiyama aldol reaction has been used to efficiently install a lateral chain at the C-9 position of the Wieland-Miescher ketone derivative 3 within two steps, representing a shortcut compared to that of the classical sequences. The treatment of the silylated enol ether 8 with a wide range of acetals in the presence of tin tetrachloride led to a the diastereoselective construction of the C-9 quaternary center of 33 new building blocks derived from the Wieland-Miescher ketone derivative 3.

A novel approach to sesquiterpenoid benzoxazole synthesis from marine sponges: nakijinols A, B and E–G

Nakijinols A, B, E, F and G were efficiently synthesized via the ring closure of the N-(2-hydroxyphenyl)-formamide or -acetamide moiety.

A modular synthesis of tetracyclic meroterpenoid antibiotics

Stachyflin, aureol, smenoqualone, strongylin A, and cyclosmenospongine belong to a family of tetracyclic meroterpenoids, which, by nature of their unique molecular structures and various biological properties, have attracted synthetic and medicinal chemists alike. Despite their obvious biosynthetic relationship, only scattered reports on the synthesis and biological investigation of individual meroterpenoids have appeared so far. Herein, we report a highly modular synthetic strategy that enabled the synthesis of each of these natural products and 15 non-natural derivatives. The route employs an auxiliary-controlled Diels-Alder reaction to enable the enantioselective construction of the decalin subunit, which is connected to variously substituted arenes by either carbonyl addition chemistry or sterically demanding sp²-sp³ cross-coupling reactions. The selective installation of either the cis- or trans-decalin stereochemistry is accomplished by an acid-mediated cyclization/isomerization reaction. Biological profiling reveals that strongylin A and a simplified derivative thereof have potent antibiotic activity against methicillin-resistant Staphylococcus aureus.

Regiodivergent Remote Arylation of Cycloalkanols to Dysideanone's Fused Carbotetracycles and Its Bridged Isomers

Regiodivergent γ and γ' arylations across an all-carbon quaternary center of cycloalkanols to access enantioenriched fused and bridged carbotetracycles are reported. The conformation of the carbocation guided either sequential stereospecific β-C-Me/γ-C-H-shifts or β-C-Me/γ'-C-H-shifts, providing fused carbotetracyclic analogs of dysideanone or bridged tetracycles, respectively. The reaction is highly stereoselective in building three contiguous stereocenters, where one, two, or three could be all-carbon quaternary centers. Interestingly, mechanistic studies revealed a crucial role of a methyl substituent in controlling regioselectivity.

Evolution of a Polyene Cyclization Cascade for the Total Synthesis of (-)-Cyclosmenospongine

We report a full account on the development of a unique cationic polyene cyclization for the total synthesis of the tetracyclic meroterpenoid (-)-cyclosmenospongine. A highly convergent three-component coupling strategy enabled rapid access to individual cyclization precursors that were tested for their reactivity. The successful transformation generates three rings and sets four consecutive stereocenters in a single operation proceeding in a highly efficient manner to give exclusively the trans-decalin framework. In addition, we found that the enol ether geometry and the relative configuration of C3 and C8 are crucial for the success of the polyene cyclization.

Marine natural products

This review covers the literature published in 2013 for marine natural products (MNPs), with 982 citations (644 for the period January to December 2013) 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 (1163 for 2013), together with the relevant biological activities, source organisms and country of origin. Reviews, biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.