Experimental and theoretical studies on the bismuth-triflate-catalysed cycloisomerisation of 1,6,10-trienes and aryl polyenes

Modular terpene synthesis enabled by mild electrochemical couplings

The synthesis of terpenes is a large field of research that is woven deeply into the history of chemistry. Terpene biosynthesis is a case study of how the logic of a modular design can lead to diverse structures with unparalleled efficiency. This work leverages modern nickel-catalyzed electrochemical sp²-sp³ decarboxylative coupling reactions, enabled by silver nanoparticle-modified electrodes, to intuitively assemble terpene natural products and complex polyenes by using simple modular building blocks. The step change in efficiency of this approach is exemplified through the scalable preparation of 13 complex terpenes, which minimized protecting group manipulations, functional group interconversions, and redox fluctuations. The mechanistic aspects of the essential functionalized electrodes are studied in depth through a variety of spectroscopic and analytical techniques.

Novel Tdp1 Inhibitors Based on Adamantane Connected with Monoterpene Moieties via Heterocyclic Fragments

Tyrosyl-DNA phosphodiesterase 1 (Tdp1) is a promising target for anticancer therapy due to its ability to counter the effects topoisomerase 1 (Top1) poison, such as topotecan, thus, decreasing their efficacy. Compounds containing adamantane and monoterpenoid residues connected via 1,2,4-triazole or 1,3,4-thiadiazole linkers were synthesized and tested against Tdp1. All the derivatives exhibited inhibition at low micromolar or nanomolar concentrations with the most potent inhibitors having IC₅₀ values in the 0.35–0.57 µM range. The cytotoxicity was determined in the HeLa, HCT-116 and SW837 cancer cell lines; moderate CC₅₀ (µM) values were seen from the mid-teens to no effect at 100 µM. Furthermore, citral derivative 20c, α-pinene-derived compounds 20f, 20g and 25c, and the citronellic acid derivative 25b were found to have a sensitizing effect in conjunction with topotecan in the HeLa cervical cancer and colon adenocarcinoma HCT-116 cell lines. The ligands are predicted to bind in the catalytic pocket of Tdp1 and have favorable physicochemical properties for further development as a potential adjunct therapy with Top1 poisons.

Molecular bismuth(iii) monocations: structure, bonding, reactivity, and catalysis

Structurally defined, molecular bismuth(iii) cations show remarkable properties in coordination chemistry, Lewis acidity, and redox chemistry, allowing for unique applications in synthetic chemistry.

Total Synthesis of Cryptotrione

The total synthesis of cryptotrione (1) was enabled by substrate-controlled diastereoselective construction of the bicyclo[3.1.0]hexene framework through platinum-catalyzed enyne cycloisomerization and Lewis acid induced polyene cyclization to construct the abietane-type tricyclic diterpene skeleton. The stereogenic tertiary carbon center in the side chain was installed in a diastereodivergent manner by conjugate addition reactions.

Hyperconjomer stereocontrol of cationic polyene cyclisations

Polyene cyclisations are a powerful method for the direct generation of molecular complexity. This paper describes the use of computational methods to investigate the stereoselectivity of cationic polyene cyclisations of geranylbenzene derivatives. The outcomes highlight the different reactivity of hyperconjomers during the key Friedel-Crafts alkylation step, and informed a successful strategy for the synthesis of (±)-taiwaniaquinone G with improved levels of stereoselectivity.

Are Brønsted Acids the True Promoter of Metal-Triflate-Catalyzed Glycosylations? A Mechanistic Probe into 1,2-cis-Aminoglycoside Formation by Nickel Triflate

Metal triflates have been utilized to catalytically facilitate numerous glycosylation reactions under mild conditions. In some methods, the metal triflate system provides stereocontrol during the glycosylation, rather than the nature of protecting groups on the substrate. Despite these advances, the true activating nature of metal triflates remains unclear. Our findings indicated that the in situ generation of trace amounts of triflic acid from metal triflates can be the active catalyst species in the glycosylation. This fact has been mentioned previously in metal triflate-catalyzed glycosylation reactions; however, a thorough study on the subject and its implications on stereoselectivity has yet to be performed. Experimental evidence from control reactions and 19F NMR spectroscopy have been obtained to confirm and quantify the triflic acid released from nickel triflate, for which it is of paramount importance in achieving a stereoselective 1,2-cis-2-amino glycosidic bond formation via a transient anomeric triflate. A putative intermediate resembling that of a glycosyl triflate has been detected using variable temperature NMR (1H and 13C) experiments. These observations, together with density functional theory calculations and a kinetic study, corroborate a mechanism involving triflic acid-catalyzed stereoselective glycosylation with N-substituted trifluoromethylbenzylideneamino protected electrophiles. Specifically, triflic acid facilitates formation of a glycosyl triflate intermediate which then undergoes isomerization from the stable α-anomer to the more reactive β-anomer. Subsequent SN2-like displacement of the reactive anomer by a nucleophile is highly favorable for the production of 1,2-cis-2-aminoglycosides. Although there is a previously reported work regarding glycosyl triflates, none of these reports have been confirmed to come from the counter ion of the metal center. Our work provides supporting evidence for the induction of a glycosyl triflate through the role of triflic acid in metal triflate-catalyzed glycosylation reactions.

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.

Biomimetic Platinum-Promoted Polyene Polycyclizations: Influence of Alkene Substitution and Pre-cyclization Conformations

Results of kinetic experiments and quantum chemical computations on a series of platinum-promoted polycyclization reactions are described. Analyses of these results reveal a reactivity model that reaches beyond the energetics of the cascade itself, incorporating an ensemble of pre-cyclization conformations of the platinum-alkene reactant complex, only a subset of which are productive for bi- (or larger) cyclization and lead to products. Similarities and differences between this scenario, including reaction coordinates for polycyclization, for platinum- and enzyme-promoted polycyclization reactions are highlighted.

Studies in Cyclization of Aromatic Epoxy-acyclicpolyprenes: Lewis Superacids and Titanocene Chloride

Two catalytic cascade cyclization methods (radical and cationic) to obtain aromatic polycyclic diterpenes hydroxylated at C3 starting from aromatic epoxypolyprenes were developed. The catalytic use of the Lewis superacid bismuth triflate produces a good yield of cyclized 2 from epoxypolyprene 3. This research may well direct future efforts to the synthesis of bioactive natural products.

Enolizable Carbonyls and N,O-Acetals: A Rational Approach for Room-Temperature Lewis Superacid-Catalyzed Direct α-Amidoalkylation of Ketones and Aldehydes

An efficient catalytic room-temperature direct α-amidoalkylation of carbonyl donors, that is, ketones and aldehydes with unbiased N,O-acetals, is described. Sn(NTf2 )4 is an optimal catalyst to promote this challenging transformation at low loading and the reaction shows promising scope. A comprehensive and rational evaluation of this reaction has led to the establishment of an empirical scale of nucleophilic reactivity for a broad set of ketones that should be helpful in the synthetic design and development of carbonyl α-functionalization methods.

Simple salts of abundant metals (Fe, Bi, and Ti) supported on montmorillonite as efficient and recyclable catalysts for regioselective intramolecular and intermolecular hydroalkoxylation reactions of double bonds and tandem processes

Montmorillonites doped with metal cations are efficient and recyclable catalysts for hydroalkoxylation reactions and amenable to flow chemistry.

Bismuth(iii) triflate-catalysed tandem cyclisations towards complex polycyclic ethers

A series of complex polycyclic ethers have been synthesised under very mild conditions using a low catalyst loading of bismuth(iii) triflate.

Carbon-CArbon bond formation by Lewis superacid catalysis

Diverse intramolecular cyclizations involving the formation of CC bonds are described using catalytic methodologies based on Lewis superacids. Examples are presented on 1,6-diene cyclizations to gem-dimethylcyclohexane structures. Tandem cyclization of trienes are described to afford bicyclic structures in reactions involving rearrangements. Hydroarylation of olefins and of allenes is developed in catalytic FriedelCrafts-type coupling processes, which can give rise to tandem reactions. The olfactory evaluation of the series of prepared compounds is also presented.

Divergent total synthesis of triptolide, triptonide, tripdiolide, 16-hydroxytriptolide, and their analogues

A divergent route was developed for the formal total synthesis of triptolide, triptonide, and tripdiolide, as well as a total synthesis of 16-hydroxytriptolide and their analogues in an enantioselective form. Common advanced intermediate 5 was concisely assembled by employing an indium(III)-catalyzed cationic polycyclization reaction and a palladium-catalyzed carbonylation-lactone formation reaction as key steps. This advanced intermediate was readily converted to the above natural products by using palladium-catalyzed cross-coupling or the Claisen rearrangement reaction as key steps. Additionally, preliminary structure-cytotoxic activity relationship studies of C13 suggested that it might be a new modification site that could still retain the cytotoxicity.

Design of stereoelectronically promoted super lewis acids and unprecedented chemistry of their complexes

A new family of stereoelectronically promoted aluminum and scandium super Lewis acids is introduced on the basis of state-of-the-art computations. Structures of these molecules are designed to minimize resonance electron donation to central metal atoms in the Lewis acids. Acidity of these species is evaluated on the basis of their fluoride-ion affinities relative to the antimony pentafluoride reference system. It is demonstrated that introduced changes in the stereochemistry of the designed ligands increase acidity considerably relative to Al and Sc complexes with analogous monodentate ligands. The high stability of fluoride complexes of these species makes them ideal candidates to be used as weakly coordinating anions in combination with highly reactive cations instead of conventional Lewis acid-fluoride complexes. Further, the interaction of all designed molecules with methane is investigated. All studied acids form stable pentavalent-carbon complexes with methane. In addition, interactions of the strongest acid of this family with very weak bases, namely, H2, N2, carbon oxides, and noble gases were investigated; it is demonstrated that this compound can form considerably stable complexes with the aforementioned molecules. To the best of our knowledge, carbonyl and nitrogen complexes of this species are the first hypothetical four-coordinated carbonyl and nitrogen complexes of aluminum. The nature of bonding in these systems is studied in detail by various bonding analysis approaches.

Diastereoselective Pt catalyzed cycloisomerization of polyenes to polycycles

Application of a tridentate NHC containing pincer ligand to Pt catalyzed cascade cyclization reactions has allowed for the catalytic, diastereoselective cycloisomerization of biogenic alkene terminated substrates to the their polycyclic counterparts.

Bi(OTf)3-catalysed synthesis of substituted indanes by a double hydroarylation of unactivated 1,3-dienes

A straightforward synthesis of indane derivatives by sequential hydroarylation of 1,3-dienes catalysed by bismuth(iii) triflate is presented.

Catalytic platinum-initiated cation-olefin reactions with alkene terminating groups

A series of phosphine-Pt(2+)-catalysts is reported, which enable the oxidative cascade cyclization of poly-alkene substrates. When the terminus is appropriately arranged and a catalyst reoxidation mediator is included, several polycyclic all carbon skeletons can be obtained. In one example, a chiral P2Pt(+2) catalyst provides up to 79% ee.

New trends in bismuth-catalyzed synthetic transformations

This review covers uses of bismuth catalysts since 2005 with a special emphasis on the emerging applications of such catalysts. Low toxicity, low catalytic loading, synergistic effects with other catalysts, and some hydrocompatibility properties confer to bismuth salts major advantages. The expanding activity in the field clearly highlights the growing potential of bismuth catalysts. The article is not a comprehensive review on bismuth catalysis but a selection of its most promising uses in challenging synthetic transformations.