Chemische Synthesen und chemische Biologie von Carboxylpolyether‐Ionophoren: Aktuelle Entwicklungen

Stereo- and Site-Selective Crotylation of Alcohol Proelectrophiles via Ruthenium-Catalyzed Hydrogen Auto-Transfer Mediated by Methylallene and Butadiene

Iodide-bound ruthenium-JOSIPHOS complexes catalyze the redox-neutral C-C coupling of primary alcohols with methylallene (1,2-butadiene) or 1,3-butadiene to form products of anti-crotylation with good to excellent levels of diastereo- and enantioselectivity. Distinct from other methods, direct crotylation of primary alcohols in the presence of unprotected secondary alcohols is possible, enabling generation of spirastrellolide B (C9-C15) and leucascandrolide A (C9-C15) substructures in significantly fewer steps than previously possible.

Rational Design of Silicon-Based Zinc Ionophores

Ionophores transport ions across biological membranes and have wide-ranging applications, but a platform for their rapid development does not exist. We report a platform for developing ionophores from metal-ion chelators, which are readily available with wide-ranging affinities and specificities, and structural data that can aid rational design. Specifically, we fine-tuned the binding affinity and lipophilicity of a Zn^II -chelating ligand by introducing silyl groups proximal to the Zn^II -binding pocket, which generated ionophores that performed better than most of the currently known Zn^II ionophores. Furthermore, these silicon-based ionophores were specific for Zn^II over other metals and exhibited better antibacterial activity and less toxicity to mammalian cells than several known Zn^II ionophores, including pyrithione. These studies establish rational design principles for the rapid development of potent and specific ionophores and a new class of antibacterial agents.

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.

Total Synthesis of the Spliceosome Modulator Pladienolide B via Asymmetric Alcohol-Mediated syn- and anti-Diastereoselective Carbonyl Crotylation

The potent spliceosome modulator pladienolide B, which bears 10 stereogenic centers, is prepared in 10 steps (LLS). Asymmetric alcohol-mediated carbonyl crotylations catalyzed by ruthenium and iridium that occur with syn- and anti-diastereoselectivity, respectively, were used to form the C20-C21 and C10-C11 C-C bonds.

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.