Tetraphenylphosphonium Tetrafluoroborate/1,1,1,3,3,3-Hexafluoroisopropanol (Ph4PBF4/HFIP) Effecting Epoxide-Initiated Cation–Olefin Polycyclizations

Stereoselective Approach to the Core Structure of (+)-Phainanoid A via Strategically Engineered Cascade Polyene Cyclization

Stereoselective synthesis of 3b and its cascade polyene cyclization to 18b have been described. Acyclic polyene 3b was prepared from allyl bromide 4 and 1,3-dithiane 5, and intermediates 4 and 5 were synthesized from the commercially available geraniol (6) and cyclopenten-2-one (8), respectively, using enantioselective reduction of ketone, Johnson-Claisen rearrangement, and the Suzuki reaction as key steps. Au(I)-mediated diastereoselective polyene cyclization of 3b efficiently afforded tetracyclic compound 18b.

The catalytic asymmetric polyene cyclization of homofarnesol to ambrox

Polyene cyclizations are among the most complex and challenging transformations in biology. In a single reaction step, multiple carbon-carbon bonds, ring systems and stereogenic centres are constituted from simple, acyclic precursors1-3. Simultaneously achieving this kind of precise control over product distribution and stereochemistry poses a formidable task for chemists. In particular, the polyene cyclization of (3E,7E)-homofarnesol to the valuable naturally occurring ambergris odorant (-)-ambrox is recognized as a longstanding challenge in chemical synthesis1,4-7. Here we report a diastereoselective and enantioselective synthesis of (-)-ambrox and the sesquiterpene lactone natural product (+)-sclareolide by a catalytic asymmetric polyene cyclization by using a highly Brønsted-acidic and confined imidodiphosphorimidate catalyst in the presence of fluorinated alcohols. Several experiments, including deuterium-labelling studies, suggest that the reaction predominantly proceeds through a concerted pathway in line with the Stork-Eschenmoser hypothesis8-10. Mechanistic studies show the importance of the enzyme-like microenvironment of the imidodiphosphorimidate catalyst for attaining exceptionally high selectivities, previously thought to be achievable only in enzyme-catalysed polyene cyclizations.

Catalyst- and Additive-free, Regioselective 1,6-Hydroarylation of para-Quinone Methides with Anilines in HFIP

A simple and efficient method for the synthesis of diarylmethyl-functionalized anilines through the hexafluoroisopropanol (HFIP)-mediated regioselective 1,6-hydroarylation reaction of para-quinone methides (p-QMs) with anilines under catalyst- and additive-free conditions is reported. Various kinds of p-QMs and amines (e. g. primary, secondary and tertiary amines) are well tolerated in this transformation without the pre-protection of amino group, and the corresponding products could be generated with good to excellent yields and satisfactory regioselectivity under the optimized reaction conditions. In addition to adaptable amine compounds, indoles and their derivatives are also compatible with this reaction system. This transformation can be easily extended to a gram scale-synthesis level to synthesize the target product. Furthermore, it is worth noting that some complex small aniline molecules with biological activity can be selectively modified using this method. The possible reaction mechanism is proposed through the step-by-step control experiments and DFT calculations, showing that the key process for achieving the regioselective 1,6-hydroarylation of p-QMs is the hydrogen bonding effect of HFIP to substrates.

A biomimetic approach for the concise total synthesis of greenwaylactams A-C

A concise, racemic total synthesis of three sesquiterpenoid alkaloids (greenwaylactams A-C) exhibiting an unprecedented 8-membered benzolactam is disclosed. Key transformations of this work include the ring expansion through cleavage of an indole via Witkop oxidation, as well as an HFIP mediated cationic cyclisation to build up the pentacyclic carbon skeleton.

Biomimetic chlorine-induced polyene cyclizations harnessing hypervalent chloroiodane-HFIP assemblies

While bromo- and iodocyclizations have recently been successfully implemented, the challenging chlorocyclizations have been scantly investigated. We present a selective and generally applicable concept of chlorination-induced polyene cyclization by utilizing HFIP-chloroiodane networks mimicking terpene cyclases. A manifold of different alkenes was converted with excellent selectivities (up to d.r. >95 : 5). The cyclization platform was even extended to several structurally challenging terpenes and terpenoid carbon frameworks.

Ab Initio Metadynamics Simulations of Hexafluoroisopropanol Solvent Effects: Synergistic Role of Solvent H-Bonding Networks and Solvent-Solute C-H/π Interactions

The solvent effects in Friedel-Crafts cycloalkylation of epoxides and Cope rearrangement of aldimines were investigated by using ab initio molecular dynamics simulations. Explicit molecular treatments were applied for both reactants and solvents. The reaction mechanisms were elucidated via free energy calculations based on metadynamics simulations. The results reveal that both reactions proceed in a concerted fashion. Key solvent-substrate interactions are identified from the structures of transition states with explicit solvent molecules. The remarkable promotion effect of hexafluoroisopropanol solvent is ascribed to the synergistic effect of H-bonding networks and C-H/π interactions with substrates.

Alcohols as Efficient Intermolecular Initiators for a Highly Stereoselective Polyene Cyclisation Cascade

The use of benzylic and allylic alcohols in HFIP solvent together with Ti(Oi Pr)4 has been shown to trigger a highly stereoselective polyene cyclisation cascade. Three new carbon-carbon bonds are made during the process and complete stereocontrol of up to five new stereogenic centers is observed. The reaction is efficient, has high functional group tolerance and is atom-economic generating water as a stoichiometric by-product. A new polyene substrate-class is employed, and subsequent mechanistic studies indicate a stereoconvergent mechanism. The products of this reaction can be used to synthesize steroid-analogues in a single step.

Enzyme-like polyene cyclizations catalyzed by dynamic, self-assembled, supramolecular fluoro alcohol-amine clusters

Terpene cyclases catalyze one of the most powerful transformations with respect to efficiency and selectivity in natural product (bio)synthesis. In such polyene cyclizations, structurally highly complex carbon scaffolds are built by the controlled ring closure of linear polyenes. Thereby, multiple C,C bonds and stereocenters are simultaneously created with high precision. Structural pre-organization of the substrate carbon chain inside the active center of the enzyme is responsible for the product- and stereoselectivity of this cyclization. Here, we show that in-situ formed fluorinated-alcohol-amine supramolecular clusters serve as artificial cyclases by triggering enzyme-like reactivity and selectivity by controlling substrate conformation in solution. Because of the dynamic nature of these supramolecular assemblies, a broad range of terpenes can be produced diastereoselectively. Mechanistic studies reveal a finely balanced interplay of fluorinated solvent, catalyst, and substrate as key to establishing nature's concept of a shape-selective polyene cyclization in organic synthesis.

InI3-catalyzed polyene cyclization of allenes and its application in the total synthesis of seven abietane-type diterpenoids

A novel polyene cyclization using the allene group as the initiator has been successfully developed. This methodology provides an efficient strategy for the construction of an abietane-type tricyclic skeleton with a functionalizable C2-C3 double bond and features a wide substrate scope and excellent stereoselectivities. Potential utility of this approach has been well demonstrated by the collective total synthesis of seven abietane-type diterpenoids. Specifically, (±)-2,3-dihydroxyferruginol and (±)-2,3-dihydroxy-15,16-dinor-ent-pimar-8,11,13-triene were synthesized for the first time.

Short, Divergent, and Enantioselective Total Synthesis of Bioactive ent-Pimaranes

We present the first total synthesis of eight ent-pimaranes via a short and enantioselective route (11-16 steps). Key features of the divergent synthesis are a Sharpless asymmetric dihydroxylation, a Brønsted acid catalyzed cationic bicyclization, and a mild Rh-catalyzed arene hydrogenation for rapid access to a late synthetic branching point. From there on, selective functional group manipulations enable the synthesis of ent-pimaranes bearing different modifications in the A- and C-rings.

HFIP in Organic Synthesis

1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) is a polar, strongly hydrogen bond-donating solvent that has found numerous uses in organic synthesis due to its ability to stabilize ionic species, transfer protons, and engage in a range of other intermolecular interactions. The use of this solvent has exponentially increased in the past decade and has become a solvent of choice in some areas, such as C-H functionalization chemistry. In this review, following a brief history of HFIP in organic synthesis and an overview of its physical properties, literature examples of organic reactions using HFIP as a solvent or an additive are presented, emphasizing the effect of solvent of each reaction.

Access to the Syn diastereomers of cryptophane cages using HFIP

Cryptophane cages can adopt either an anti or syn configuration that present different recognition properties. While the synthesis of anti-cryptophanes is well reported, the synthesis of syn-cryptophanes remains a challenge. Herein, we demonstrate that the use of HFIP as a co-solvent during the second ring closure reaction significantly affects the regioselectivity, providing easier access to the syn-cryptophane stereomers.

Studies on the Origin of the Stabilizing Effects of Fluorinated Alcohols and Weakly Coordinated Fluorine-Containing Anions on Cationic Reaction Intermediates

Many synthetic methods that use fluorinated alcohols as solvents have been reported, and the fluorinated alcohols have been found to be crucial to the success of these methods. In addition, there have been reports indicating that adding a weakly coordinated fluorine-containing anion, such as BF₄^-, PF₆^-, or SbF₆^-, to fluorinated alcohols can improve yields. The boosting effect of fluorinated alcohols is attributed mainly to hydrogen bond activation. A few studies have suggested that the very polar fluorinated alcohols can stabilize cationic reaction intermediates. However, how they do so and why weakly coordinated fluorine-containing anions improve yields have not been studied in depth. Here, we used quaternary ammonium cations, a quaternary phosphonium cation, and a triaryl-substituted carbocation as models for short-lived cationic intermediates and studied the possible interactions of these cations with fluorinated alcohols and BF₄^-, PF₆^-, or SbF₆^-. On the basis of the results, we propose that the C-F dipoles of fluorinated alcohols and the E-F dipoles (where E is B, P, or Sb) of weakly coordinated fluorine-containing anions stabilized these cations by intermolecular charge-dipole interactions. We deduced that in the same fashion the C-F and E-F dipoles can thermodynamically stabilize cationic reaction intermediates.

HFIP as Protonation Reagent and Solvent for Regioselective Alkylation of Indoles with All-Carbon Centers

The regio- and chemoselective construction of indole bearing an all-carbon center at the C3-position, a versatile bioactive building block, by C(sp²)-C(sp³) formation with olefins has been achieved through utilization of hexafluoroisopropanol (HFIP) as the protonation reagent and solvent. The catalytic reactions are operationally simple and green compared with previous reports utilizing elaborated olefins and catalysts. This protocol allows for alkylation of a variety of substituted indoles with diverse of styrene type alkenes in excellent yields and with high selectivity. Application of this protocol to the synthesis of drug was pursued and with an improved yield in contrast to previous art. Catalytic kinetics and deuterium-labeling experiments suggest that the rate-determining step involves the protonation of olefin by HFIP to generate carbocation, followed by electrophilic addition to indole derivative.

HFIP-promoted halo-carbocyclizations of N- and O-tethered arene–alkene substrates to access all halo (X = Br, I, Cl)-functionalized tetrahydroquinoline and chroman cores

Herein, a HFIP-promoted mild and efficient method for the synthesis of all halo (X = Br, I, Cl)-functionalized tetrahydroquinoline and chroman building blocks is disclosed.

HFIP-mediated three-component imidization of electron-rich arenes with in situ formed spiroindolenines for facile construction of 2-arylspiroindolenines

The three-component reaction of o-aminobenzaldehydes with 5-hydroxyindole and electron-rich arenes has been achieved through HFIP-mediated cascade hydride transfer/dearomative cyclization/CDC-type imidization at room temperature under air.

Direct activation of alcohols via perrhenate ester formation for an intramolecular dehydrative Friedel–Crafts reaction

A general and highly efficient intramolecular dehydrative Friedel–Crafts reactions via Re2O7 mediated hydroxyl group activation is described for the syntheses of tetrahydronaphthalene, tetrahydroquinoline, tetrahydroisoquinoline, chromane, and isochromane derivatives.

HFIP-mediated 2-aza-Cope rearrangement: metal-free synthesis of α-substituted homoallylamines at ambient temperature

An efficient metal-free strategy for the synthesis of α-substituted homoallylamine derivatives has been developed via a 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP)-promoted 2-aza-Cope rearrangement of aldimines, generated in situ by condensation of aldehydes with easily accessible 1,1-diphenylhomoallylamines. This reaction provides rapid access to α-substituted homoallylamines with excellent functional group tolerance and yields. The reaction takes place at room temperature and no chromatographic purification is required for product isolation. The synthetic utility of the current method is further demonstrated by the transformation of the obtained benzophenone ketimines into N-unprotected homoallylamines, an α-amino alcohol and an α-amino amide.

An Endo-Selective Epoxide-Opening Cascade for the Fast Assembly of the Polycyclic Core Structure of Marine Ladder Polyethers

The rapid synthesis of marine ladder polyethers from polyepoxide precursors (in analogy with the biosynthetic pathway hypothesized by Nakanishi) is hampered by the fact that the exo-selective epoxide-opening cyclization cascade that gives THF-type polyethers is preferred over the endo-selective cascade that gives the desired products. We found that perfluoro-tert-butanol (PFTB) cooperating with 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM]BF₄ ) can promote endo-selective epoxide-opening cyclization reactions of trisubstituted epoxy alcohols. Starting from readily accessible homochiral polyepoxy alcohols with a methyl group at all the endo-cyclization sites, we were able to construct polyethers up to five consecutive fused 6-, 7-, and/or 8-membered rings in one step. Notably, molecules with the 7/7/6/6 and 7/7/6/7/6 polyether frameworks of hemibrevetoxin B and brevenal, respectively, could be synthesized in 40 % and 17 % chemical yields.

Polyether Natural Product Inspired Cascade Cyclizations: Autocatalysis on π-Acidic Aromatic Surfaces

Anion-π catalysis functions by stabilizing anionic transition states on aromatic π surfaces, thus providing a new approach to molecular transformation. The delocalized nature of anion-π interactions suggests that they serve best in stabilizing long-distance charge displacements. Aiming therefore for an anionic cascade reaction that is as charismatic as the steroid cyclization is for conventional cation-π biocatalysis, reported here is the anion-π-catalyzed epoxide-opening ether cyclizations of oligomers. Only on π-acidic aromatic surfaces having a positive quadrupole moment, such as hexafluorobenzene to naphthalenediimides, do these polyether cascade cyclizations proceed with exceptionally high autocatalysis (rate enhancements k_auto /k_cat >10⁴  m^-1 ). This distinctive characteristic adds complexity to reaction mechanisms (Goldilocks-type substrate concentration dependence, entropy-centered substrate destabilization) and opens intriguing perspectives for future developments.

HFIP-Promoted Synthesis of Substituted Tetrahydrofurans by Reaction of Epoxides with Electron-Rich Alkenes

In the present work, the employment of fluorinated alcohols, specifically 1,1,1,3,3,3-hexafluoroisopropanol (HFIP), as solvent and promoter of the catalyst-free synthesis of substituted tetrahydrofuranes through the addition of electron-rich alkenes to epoxydes is described. The unique properties of this fluorinated alcohol, which is very different from their non-fluorinated analogs, allows carrying out this new straightforward protocol under smooth reaction conditions affording the corresponding adducts in moderate yields in the majority of cases. Remarkably, this methodology has allowed the synthesis of new tetrahydrofuran-based spiro compounds as well as tetrahydrofurobenzofuran derivatives. The scope and limitations of the process are also discussed. Mechanistic studies were also performed pointing towards a purely ionic or a SN2-type process depending on the nucleophilicity of the alkene employed.

Cobalt-Catalyzed Hydrogen-Atom Transfer Induces Bicyclizations that Tolerate Electron-Rich and Electron-Deficient Intermediate Alkenes

A novel CoII -catalyzed polyene cyclization was developed that is uniquely effective when performed in hexafluoroisopropanol as the solvent. The process is presumably initiated by metal-catalyzed hydrogen-atom transfer (MHAT) to 1,1-disubstituted or monosubstituted alkenes, and the reaction is remarkable for its tolerance of internal alkenes bearing either electron-rich methyl or electron-deficient nitrile substituents. Electron-rich aromatic terminators are required in both cases. Terpenoid scaffolds with different substitution patterns are obtained with excellent diastereoselectivities, and the bioactive C20-oxidized abietane diterpenoid carnosaldehyde was made to showcase the utility of the nitrile-bearing products. Also provided are the results of several mechanistic experiments that suggest the process features an MHAT-induced radical bicyclization with late-stage oxidation to regenerate the aromatic terminator.

A Relay Strategy Actuates Pre-Existing Trisubstituted Olefins in Monoterpenoids for Cross-Metathesis with Trisubstituted Alkenes

A retrosynthetic disconnection-reconnection analysis of epoxypolyenes-substrates that can undergo cyclization to podocarpane-type tricycles-reveals relay-actuated Δ^6,7-functionalized monoterpenoid alcohols for ruthenium benzylidene catalyzed olefin cross-metathesis with homoprenyl benzenes. Successful implementation of this approach provided several epoxypolyenes as expected (E/Z, ca. 2-3:1). The method is further generalized for the cross-metathesis of pre-existing trisubstituted olefins in other relay-actuated Δ^6,7-functionalized monoterpenoid alcohols with various other trisubstituted alkenes to form new trisubstituted olefins. Epoxypolyene cyclization of an enantiomerically pure, but geometrically impure, epoxypolyene substrate provides an enantiomerically pure, trans-fused, podocarpane-type tricycle (from the E-geometrical isomer).

Hexafluoroisopropanol-Mediated Redox-Neutral α-C(sp3)-H Functionalization of Cyclic Amines via Hydride Transfer

Hexafluoroisopropanol has been demonstrated as the versatile promoter for redox-neutral α-C(sp³)-H functionalization of cyclic amines via the cascade [1,5]-hydride transfer/cyclization strategy. A wide range of cyclic amines are functionalized into bioactive tetrahydroquinolines, quinazolines, benzoxazines, and benzotriazepines in moderate to excellent yields. This protocol features additive-free conditions, operational simplicity, and wide substrate scope.

Exploiting hexafluoroisopropanol (HFIP) in Lewis and Brønsted acid-catalyzed reactions

Hexafluoroisopropanol (HFIP) is a solvent with unique properties that has recently gained attention for promoting a wide range of challenging chemical reactions. It was initially believed that HFIP was almost exclusively involved in the stabilization of cationic intermediates, owing to its high polarity and low nucleophilicity. However, in many cases, the mechanism of action of HFIP appears to be more complex. Recent findings reveal that many Lewis and Brønsted acid-catalyzed transformations conducted in HFIP additionally involve cooperation between the catalyst and HFIP hydrogen-bond clusters, akin to Lewis- or Brønsted acid-assisted-Brønsted acid catalysis. This feature article showcases the remarkable versatility of HFIP in Lewis and Brønsted acid-catalyzed reactions, with an emphasis on examples yielding mechanistic insight.

Mimicking Halimane Synthases: Monitoring a Cascade of Cyclizations and Rearrangements from Epoxypolyprenes

We have developed and rationalized a biomimetic transformation mimicking halimane synthases based on a Lewis acid-catalyzed cascade of cyclizations and rearrangements of epoxypolyprenes. Two rings, three stereogenic centers, and a new double bond were generated in a single chemical operation. Based on this cascade transformation, we achieved a unified strategy toward the stereoselective total syntheses of halimene-type terpenoids and analogues as a proof-of-concept study. This method has been applied to the rapid synthesis of diterpene isotuberculosinol, a virulence factor of Mycobacterium tuberculosis as a representative example.

Mutated variants of squalene-hopene cyclase: enzymatic syntheses of triterpenes bearing oxygen-bridged monocycles and a new 6,6,6,6,6-fusded pentacyclic scaffold, named neogammacerane, from 2,3-oxidosqualene

Squalene-hopene cyclase (SHC) catalyzes the conversion of acyclic squalene molecule into a 6,6,6,6,5-fused pentacyclic hopene and hopanol. SHC is also able to convert (3S)-2,3-oxidosqualene into 3β-hydroxyhopene and 3β-hydroxyhopanol and can generate 3α-hydroxyhopene and 3α-hydroxyhopanol from (3R)-2,3-oxidosqualene. Functional analyses of active site residues toward the squalene cyclization reaction have been extensively reported, but investigations of the cyclization reactions of (3R,S)-oxidosqualene by SHC have rarely been reported. The cyclization reactions of oxidosqualene with W169X, G600F/F601G and F601G/P602F were examined. The variants of the W169L generated new triterpene skeletons possessing a 7-oxabicyclo[2.2.1]heptane moiety (oxygen-bridged monocycle) with (1S,2S,4R)- and (1R,2S,4S)-stereochemistry, which were produced from (3R)- and (3S)-oxidosqualenes, respectively. The F601G/P602F double mutant also furnished a novel triterpene, named neogammacer-21(22)-en-3β-ol, consisting of a 6,6,6,6,6-fused pentacyclic system, in which Me-29 at C-22 of the gammacerane skeleton migrated to C-21. We propose to name this novel scaffold neogammacerane. The formation mechanisms of the enzymatic products from 2,3-oxidosqualene are discussed.

Chemoselective Borane-Catalyzed Hydroarylation of 1,3-Dienes with Phenols

A B(C₆ F₅ )₃ -catalyzed hydroarylation of a series of 1,3-dienes with various phenols has been established through a combination of theoretical and experimental investigations, affording structurally diverse ortho-allyl phenols. DFT calculations show that the reaction proceeds through a borane-promoted protonation/Friedel-Crafts pathway involving a π-complex of a carbocation-anion contact ion pair. This protocol features simple and mild reaction conditions, broad functional-group tolerance, and low catalyst loading. The obtained ortho-allyl phenols could be further converted into flavan derivatives using B(C₆ F₅ )₃ with good cis diastereoselectivity. Furthermore, this transformation was applied in the late-stage modification of pharmaceutical compounds.

Fluorinated Alcohol-Promoted Reaction of Chlorohydrocarbons with Diverse Nucleophiles for the Synthesis of Triarylmethanes and Tetraarylmethanes

This article reports an efficient synthesis of triarylmethanes and tetraarylmethanes from chlorohydrocarbons with miscellaneous nucleophiles in fluorinated alcohols, featuring metal-free, wide substrate scope, excellent functional group tolerance, and mild reaction conditions.