Critical Switching of Cyclization Modes of Polyepoxides in Acidic Aqueous Media and Neutral Water: Synthesis and Revised Structure of a Nerolidol-Type Sesquiterpenoid

Establishment of "Ring-Size-Divergent" Synthetic Strategy: Divergent Synthesis, Stereochemical Assignments, and Biological Activity Studies of Nerolidol-Type Sesquiterpenoids and Feroniellins

Biomimetic epoxide-opening cascade cyclizations of polyepoxides enable the efficient and rapid construction of polyether skeletons. In this study, we discovered a method for switching the cyclization mode from tetrahydrofuran to tetrahydropyran (THP) formation in epoxide-opening cascades of polyepoxides. The THP formation proceeded via an epoxonium-ion intermediate by simple heating in neutral water. Next, by expanding the switching reaction, we successfully established a "ring-size-divergent" synthetic strategy that enabled the synthesis of the five-, six-, and seven-membered ether rings from identical diepoxide cyclization precursors under simple acidic or neutral conditions. The "ring-size-divergent" synthetic strategy was applied to the short divergent synthesis of nerolidol-type sesquiterpenoids and feroniellins, resulting in the revision of the proposed stereochemistry of certain natural products and the determination of all of the absolute configurations. Additionally, the anti-inflammatory activities of the synthetic samples were evaluated.

Decoded fingerprints of hyperresponsive, expanding product space: polyether cascade cyclizations as tools to elucidate supramolecular catalysis

Simple enough to be understood and complex enough to be revealing, cascade cyclizations of diepoxides are introduced as new tools to characterize supramolecular catalysis. Decoded product fingerprints are provided for a consistent set of substrate stereoisomers, and shown to report on chemo-, diastereo- and enantioselectivity, mechanism and even autocatalysis. Application of the new tool to representative supramolecular systems reveals, for instance, that pnictogen-bonding catalysis is not only best in breaking the Baldwin rules but also converts substrate diastereomers into completely different products. Within supramolecular capsules, new cyclic hemiacetals from House–Meinwald rearrangements are identified, and autocatalysis on anion–π catalysts is found to be independent of substrate stereochemistry. Decoded product fingerprints further support that the involved epoxide-opening polyether cascade cyclizations are directional, racemization-free, and interconnected, at least partially. The discovery of unique characteristics for all catalysts tested would not have been possible without decoded cascade cyclization fingerprints, thus validating the existence and significance of privileged platforms to elucidate supramolecular catalysis. Once decoded, cascade cyclization fingerprints are easily and broadly applicable, ready for use in the community.

Hyperresponsive XL product space identifies polyether cascade fingerprinting as an attractive tool to elucidate supramolecular catalysis, including pnictogen-bonding, capsule and anion–π catalysts.

Bioinspired Ether Cyclizations within a π-Basic Capsule Compared to Autocatalysis on π-Acidic Surfaces and Pnictogen-Bonding Catalysts

While the integration of supramolecular principles in catalysis attracts increasing attention, a direct comparative assessment of the resulting systems catalysts to work out distinct characteristics is often difficult. Herein is reported how the broad responsiveness of ether cyclizations to diverse inputs promises to fill this gap. Cyclizations in the confined, π-basic and Brønsted acidic interior of supramolecular capsules, for instance, are found to excel with speed (exceeding general Brønsted acid and hydrogen-bonding catalysts by far) and selective violations of the Baldwin rules (as extreme as the so far unique pnictogen-bonding catalysts). The complementary cyclization on π-acidic aromatic surfaces remains unique with regard to autocatalysis, which is shown to be chemo- and diastereoselective with regard to product-like co-catalysts but, so far, not enantioselective.

Establishing a "Ring-Size-Divergent" Synthetic Strategy: Synthesis, Structural Revision, and Absolute Configuration of Feroniellins

Feroniellin analogs isolated from Feroniella lucida possess a furanocoumarin skeleton connected to monoterpenic five- to seven-membered ethereal rings by an ether linkage and exhibit a broad spectrum of biological activities. In this contribution, we intended to establish a "ring-size-divergent" synthetic strategy for the monoterpenic five- to seven-membered ethereal rings through the chemical sythesis of feroniellins. The short and comprehensive synthesis of feroniellins was achieved in only two steps from easily available bergamottin based on the "ring-size-divergent" strategy. In addition, these syntheses resulted in revision of the proposed structures for feroniellins A and B and the determination of all the absolute configurations of feroniellins; their preliminary anti-inflammatory activities were investigated as well.

Water clusters as bifunctional catalysts in organic chemistry: the hydrolysis of oxirane and its methyl derivatives

This contribution explores the bifunctional catalytic activity of water clusters ((H₂O)_n with n = 1-5) in organic chemistry similar to that observed in the formation of H₂SO₄ in acid rain (Chem. Commun., 53, 3516, (2017)). We considered for this purpose the Hydrolysis of Epoxides (HE), in particular, that of oxirane and its methyl derivatives. Surrounding water molecules with H-bond cooperative effects decrease the activation energy of the rate-limiting step of HE in condensed phase, especially when they lead to an anti-periplanar attack on the alkoxide leaving group. Furthermore, the water molecules have a bifunctional catalytic role in HE by (i) increasing the nucleophilic and electrophilic character of the attacking oxygen atom and the leaving group of the reaction, respectively, and (ii) placing the reactants in a suitable disposition for the substitution to occur. Overall, this investigation provides relevant insights into the collective action of water molecules on organic reactions in neutral, basic and acid media.

Total synthesis and complete configurational assignment of amphirionin-2

Amphirionin-2 is a linear polyketide metabolite that exhibits potent and selective cytotoxic activity against certain human cancer cell lines. We disclose herein the first total synthesis of amphirionin-2 and determination of its absolute configuration. Our synthesis featured an extensive use of cobalt-catalyzed Mukaiyama-type cyclization of γ-hydroxy olefins for stereoselective formation of all the tetrahydrofuran rings found in the natural product, and a late-stage Stille-type coupling for convergent assembly of the entire carbon backbone. Four candidate diastereomers of amphirionin-2 were synthesized in a unified, convergent manner, and their spectroscopic/chromatographic properties were compared with those of the authentic material. The present study culminated in the reassignment of the C5/C7 relative configuration, assignment of the C12/C18 relative configuration, and determination of the absolute configuration of amphirionin-2.

An extensive application of cobalt-catalyzed Mukaiyama-type cyclization of γ-hydroxy olefins and a late-stage Stille-type reaction enabled syntheses of four diastereomers of amphirionin-2 to establish its absolute configuration.

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.