Aldehyde-catalyzed epoxidation of unactivated alkenes with aqueous hydrogen peroxide.

The organocatalytic epoxidation of unactivated alkenes using aqueous hydrogen peroxide provides various indispensable products and intermediates in a sustainable manner. While formyl functionalities typically undergo irreversible oxidations when activating an oxidant, an atropisomeric two-axis aldehyde capable of catalytic turnover was identified for high-yielding epoxidations of cyclic and acyclic alkenes. The relative configuration of the stereogenic axes of the catalyst and the resulting proximity of the aldehyde and backbone residues resulted in high catalytic efficiencies. Mechanistic studies support a non-radical alkene oxidation by an aldehyde-derived dioxirane intermediate generated from hydrogen peroxide through the Payne and Criegee intermediates.

Catalytic Cascade Reactions Inspired by Polyketide Biosynthesis.

Aldol reactions belong to the most important methods for carbon-carbon bond formation and are also involved in one of the most astonishing biosynthetic processes: the biosynthesis of polyketides governed by an extraordinarily sophisticated enzymatic machinery. In contrast to the typical linear or convergent strategies followed in chemical synthesis, this late-stage catalysis concept allows Nature to assemble intermediates that are diversified into a broad range of scaffolds, which assume various crucial biological functions. To transfer this concept to small-molecule catalysis to access products beyond the natural systems, a stepwise approach to differentiate increasingly complex substrates was followed by investigating arene-forming polyketide cyclizations. An outline of our efforts to develop and apply these concepts are presented herein.

JoyaPhos: An Atropisomeric Teraryl Monophosphine Ligand.

A topologically well-defined atropisomeric teraryl monophosphine ligand system, prepared by a highly stereoselective arene-forming aldol condensation combined with a direct ester-to-anthracene transformation, is described herein. The ligands were evaluated for gold(I)-catalyzed [2+2] cycloaddition and cycloisomerization reactions as well as a unique intramolecular Pd-catalyzed C-N cross-coupling for the atroposelective synthesis of a N-aryl-indoline bearing a C-N stereogenic axis. The ligand structure induced up to 95:5 stereoselectivity in the asymmetric allylic alkylation reaction and features an interesting dynamic behavior as observed by X-ray crystallographic studies.

Catalyst-Controlled Stereodivergent Synthesis of Atropisomeric Multiaxis Systems.

Molecular scaffolds with multiple rotationally restricted bonds allow a precise spatial positioning of functional groups. However, their synthesis requires methods addressing the configuration of each stereogenic axis. We report here a catalyst-stereocontrolled synthesis of atropisomeric multiaxis systems enabling divergence from the prevailing stereochemical reaction path. By using ion-pairing catalysts in arene-forming aldol condensations, a strong substrate-induced stereopreference can be overcome to provide structurally well-defined helical oligo-1,2-naphthylenes. The configuration of up to four stereogenic axes was individually catalyst-controlled, affording quinquenaphthalenes with a unique topology.

Stereoselective Arene-Forming Aldol Condensation: Catalyst-Controlled Synthesis of Axially Chiral Compounds.

The fundamental role that aldol chemistry adopts in various disciplines, such as stereoselective catalysis or the biosynthesis of aromatic polyketides, illustrates its exceptional versatility. On the one hand, numerous aldol addition reactions reliably transfer the stereochemical information from catalysts into various valuable products. On the other hand, countless aromatic polyketide natural products are produced by an ingenious biosynthetic machinery based on arene-forming aldol condensations. With the aim of complementing aldol methodology that controls stereocenter configuration, we recently combined these two tenets by investigating small-molecule-catalyzed aldol condensation reactions that stereoselectively form diverse axially chiral compounds through the construction of a new aromatic ring.

Catalytic Arene-forming Aldol Condensation: Stereoselective Synthesis of Rotationally Restricted Aromatic Compounds.

By taking inspiration from the fascinating biosynthetic machinery that creates aromatic polyketides, our group investigates analogous reactions catalyzed by small molecules. We are particularly captivated by the prospects of intramolecular aldol condensation reactions to generate different rotationally restricted aromatic compounds. In a first project of our independent research group, a highly stereoselective amine catalyzed synthesis of axially chiral biaryls, tertiary aromatic amides and oligo-1,2-naphthylenes has been developed. In this article, we outline the twists and turns for our escape from the aromatic flatland to structurally intriguing chiral arene scaffolds relevant for various fields of application.

Stereoselective Arene-Forming Aldol Condensation: Synthesis of Configurationally Stable Oligo-1,2-naphthylenes.

Structurally well-defined oligomers are fundamental for the functionality of natural molecular systems and key for the design of synthetic counterparts. Herein, we describe a strategy for the efficient synthesis of individual stereoisomers of 1,2-naphthylene oligomers by iterative building block additions and consecutive stereoselective arene-forming aldol condensation reactions. The catalyst-controlled atropoenantioselective and the substrate-controlled atropodiastereoselective aldol condensation reaction provide structurally distinct ter- and quaternaphthalene stereoisomers, which represent configurationally stable analogues of otherwise stereodynamic, helically shaped ortho-phenylenes.