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

Natural sesquiterpene quinone/quinols: chemistry, biological activity, and synthesis

Covering: 2010 to 2021Sesquiterpene quinone/quinols (SQs) are characterized by a C15-sesquiterpenoid unit incorporating a C6-benzoquinone/quinol moiety. Numerous unprecedented carbon skeletons have been constructed with various connection patterns between the two parts. The potent anti-cancer, anti-inflammatory, anti-microbial, anti-viral, and fibrinolytic activities of SQs are associated with their diverse structures. The representative avarol has even entered the stage of clinical phase II research as an anti-HIV agent, and was developed as paramedic medicine against psoriasis. This review provides an overall summary of 558 new natural SQs discovered between 2010 and 2021, including seven groups and sixteen structure-type subgroups, which comprehensively recapitulates their chemical structures, spectral characteristics, source organisms, biological activities, synthesis, and biosynthesis, aiming to expand the application scope of this unique natural product resource.

Stereochemical editing logic powered by the epimerization of unactivated tertiary stereocenters

The stereoselective synthesis of complex targets requires the precise orchestration of chemical transformations that simultaneously establish the connectivity and spatial orientation of desired bonds. In this work, we describe a complementary paradigm for the synthesis of chiral molecules and their isomers, which tunes the three-dimensional structure of a molecule at a late stage. Key to the success of this strategy is the development of a mild and highly general photocatalytic method composed of decatungstate polyanion and disulfide cocatalysts, which enable the interconversion of unactivated tertiary stereogenic centers that were previously configurationally fixed. We showcase the versatility of this method-and the implementation of stereoediting logic-by the rapid construction of chiral scaffolds that would be challenging to access using existing tools and by the late-stage stereoediting of complex targets.

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.

Iminyl radical-triggered relay annulation for the construction of bridged aza-tetracycles bearing four contiguous stereogenic centers

Bridged tetracyclic nitrogen scaffolds are found in numerous biologically active molecules and medicinally relevant structures. Traditional methods usually require tedious reaction steps, and/or the use of structurally specific starting materials. We report an unprecedented, iminyl radical-triggered relay annulation from oxime-derived peresters and azadienes, which shows good substrate scope and functional group compatibility, and can deliver various bridged aza-tetracyclic compounds with complex molecular topology and four contiguous stereogenic centers (dr > 19 : 1) in a single operation. This transformation represents the first example of trifunctionalization of iminyl radicals through simultaneous formation of one C–N and two C–C bonds. DFT calculation studies were conducted to obtain an in-depth insight into the reaction pathways, which revealed that the reactions involved an interesting 1,6-hydrogen atom transfer process.

A novel radical relay annulation is realized for the construction of various bridged aza-tetracyclic compounds with complex molecular topology and four contiguous stereogenic centers (dr > 19 : 1) in a single operation.

Bioinspired Total Synthesis of Marine Anticancer Meroterpenoids Dysideanone B and Dysiherbol A and Structure Revision of Dysiherbol A

Our recent progress on the total synthesis of marine anticancer sesquiterpene quinone/hydroquinone dysideanone B and dysiherbol A is briefly highlighted. This success relied on some key transformations. The union of the terpene and quinone/hydroquinone moieties was realized through a site and stereoselective α-position alkylation of Wieland–Miescher ketone derivative with a bulky benzyl bromide. The 6/6/6/6-tetracycle of dysideanone B was constructed using an intramolecular radical cyclization and the 6/6/5/6-fused core structure of dysiherbol A was forged by an intramolecular Heck reaction, respectively. The possible origin of ethoxy group in dysideanone B was revealed by mimicking the isolation conditions at a late stage. The structure of dysiherbol A was revised through the total synthesis of this natural product. Schmalz’s synthesis of dysiherbol A was also included.

Enantioselective Total Synthesis and Structural Revision of Dysiherbol A

A 12-step total synthesis of the natural product dysiherbol A, a strongly anti-inflammatory and anti-tumor avarane meroterpene isolated from the marine sponge Dysidea sp., was elaborated. As key steps, the synthesis features an enantioselective Cu-catalyzed 1,4-addition/enolate-trapping opening move, an Au-catalyzed double cyclization to build up the tetracyclic core-carbon skeleton, and a late installation of the C5-bridgehead methyl group via proton-induced cyclopropane opening associated with spontaneous cyclic ether formation. The obtained pentacyclic compound (corresponding to an anhydride of the originally suggested structure for dysiherbol A) showed identical spectroscopic data as the natural product, but an opposite molecular rotation. CD-spectroscopic measurements finally confirmed that both the constitution and the absolute configuration of the originally proposed structure of (+)-dysiherbol A need to be revised.

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.

Unexpected Racemization in the Course of the Acetalization of (+)-(S)-5-Methyl-Wieland–Miescher Ketone with 1,2-Ethanediol and TsOH under Classical Experimental Conditions

(+)-(S) and (−)-(R)-5-methyl-Wieland-Miescher ketone (+)-1 and (−)-1, are important synthons in the diastereo and enantioselective syntheses of biological and/or pharmacological interesting compounds. A key step in these syntheses is the chemoselective C(1)O acetalization to (+)-5 and (−)-5, respectively. Various procedures for this transformation have been described in the literature. Among them, the classical procedure based on the use of 1,2-ethanediol and TsOH in refluxing benzene in the presence of a Dean-Stark apparatus. Within our work on bioactive natural products, it occurred to us to observe the partial racemization of (+)-5 in the course of the acetalization of (+)-1 by means of the latter methodology. Aiming to investigate this drawback, which, to our best knowledge, has no precedents in the literature, we acetalized with 1,2-ethanediol and TsOH in refluxing benzene and in the presence of a Dean–Stark apparatus under various experimental conditions, enantiomerically pure (+)-1. It was found that the extent of racemization depends on the TsOH/(+)-1 and 1,2-ethanediol/(+)-1 ratios. Mechanism hypotheses for this partial and unexpected racemization are provided.

Nature-inspired development of unnatural meroterpenoids as the non-toxic anti-colon cancer agents

Structural analogues of anti-cancer natural product, dysideanone, were synthesized starting from Wieland-Miescher ketone derivative. In vitro studies have been conducted to evaluate the anti-cancer potential of these unnatural meroterpenoids against colon cancer. Synthesized carbotetracycles were found to be more active as compared to their acyclic carbinol-derivatives. Unnatural carbotetracycles 4b-e, 4h, 4i and 12 were found to be highly effective against the human colon adenocarcinoma cells with IC₅₀ concentrations of 7.5-20 μM. In this series, the carbotetracyclic catechol 4e (IC₅₀ = 7.5 μM) and quinone 12 (IC₅₀ = 8 μM) were found to be the most potent compounds having the IC₅₀ of less than 10 μM with no cytotoxic effect on the normal cells. Downregulation of Cox-2 and survivin and cell cycle arrest eventually leading to apoptosis were found to be the underlying mechanism of the anti-cancer effect of these unnatural meroterpenoids.