Modulation der Acidität - hoch acide Brønsted-Säuren in der asymmetrischen Katalyse

Critical trends in synthetic organic chemistry in terms of organocatalysis

The utilization of small organic compounds as catalysts has advanced rapidly, and organocatalysis has emerged as a dominant technique complementary to metal-catalyzed organic conversions. The organocatalysis field has enhanced the progression of innovative approaches to make varied chiral molecules. Researchers have placed enormous effort towards designing and blending simpler organocatalysts to synthesize enantioselective molecules in good yields. This work emphasized the impact of enamine, iminium, hydrogen bonding, and phase transfer organocatalysts in organic synthesis. The monograph focused on the crucial methods to construct valuable molecules with high enantiomeric purity.

Catalytic Hydroetherification of Unactivated Alkenes Enabled by Proton-Coupled Electron Transfer

We report a catalytic, light-driven method for the intramolecular hydroetherification of unactivated alkenols to furnish cyclic ether products. These reactions occur under visible-light irradiation in the presence of an IrIII -based photoredox catalyst, a Brønsted base catalyst, and a hydrogen-atom transfer (HAT) co-catalyst. Reactive alkoxy radicals are proposed as key intermediates, generated by direct homolytic activation of alcohol O-H bonds through a proton-coupled electron-transfer mechanism. This method exhibits a broad substrate scope and high functional-group tolerance, and it accommodates a diverse range of alkene substitution patterns. Results demonstrating the extension of this catalytic system to carboetherification reactions are also presented.

Enantioselective Brønsted Acid Catalysis as a Tool for the Synthesis of Natural Products and Pharmaceuticals

Synthesis of biologically active molecules (whether at laboratory or industrial scale) remains a highly appealing area of modern organic chemistry. Nowadays, the need to access original bioactive scaffolds goes together with the desire to improve synthetic efficiency, while reducing the environmental footprint of chemical activities. Long neglected in the field of total synthesis, enantioselective organocatalysis has recently emerged as an environmentally friendly and indispensable tool for the construction of relevant bioactive molecules. Notably, enantioselective Brønsted acid catalysis has offered new opportunities in terms of both retrosynthetic disconnections and controlling stereoselectivity. The present report attempts to provide an overview of enantioselective total or formal syntheses designed around Brønsted acid-catalyzed transformations. To demonstrate the versatility of the reactions promoted and the diversity of the accessible motifs, this Minireview draws a systematic parallel between methods and retrosynthetic analysis. The manuscript is organized according to the main reaction types and the nature of newly-formed bonds.

Chiral Brønsted Acid-Catalyzed Enantioselective α-Amidoalkylation Reactions: A Joint Experimental and Predictive Study

Enamides with a free NH group have been evaluated as nucleophiles in chiral Brønsted acid-catalyzed enantioselective α-amidoalkylation reactions of bicyclic hydroxylactams for the generation of quaternary stereocenters. A quantitative structure-reactivity relationship (QSRR) method has been developed to find a useful tool to rationalize the enantioselectivity in this and related processes and to orient the catalyst choice. This correlative perturbation theory (PT)-QSRR approach has been used to predict the effect of the structure of the substrate, nucleophile, and catalyst, as well as the experimental conditions, on the enantioselectivity. In this way, trends to improve the experimental results could be found without engaging in a long-term empirical investigation.

The Brønsted Acid-Catalyzed, Enantioselective Aza-Diels-Alder Reaction for the Direct Synthesis of Chiral Piperidones

We disclose herein the first enantioselective aza-Diels-Alder reaction of β-alkyl-substituted vinylketene silyl-O,O-acetals and imines furnishing a broad range of optically highly enriched 4-alkyl-substituted 2-piperidones. As a catalyst for this one-pot reaction we employed a chiral phosphoric acid which effects a vinylogous Mannich reaction directly followed by ring-closure to the lactam. Subsequent fully diastereoselective transformations including hydrogenation, enolate alkylation, and lactam alkylation/reduction processes converted the cycloadducts into various highly substituted piperidines of great utility for the synthesis of natural products and medicinally active compounds.

Experimental and computational study of the catalytic asymmetric 4π-electrocyclization of N-heterocycles

The first asymmetric metal-catalyzed Nazarov cyclization of N-heterocycles has been developed. The use of a chiral catalyst allows the enantioselective electrocyclization of N-heterocycles under mild conditions and the corresponding products are obtained in good yields with excellent enantio- and diastereoselectivity. The reaction mechanism and the absolute configuration of the obtained products are explained by means of computational studies.

Metal-free enantioselective electrophilic activation of allenamides: stereoselective dearomatization of indoles

The effective and unprecedented chiral BINOL phosphoric acid catalyzed (1-10 mol%) dearomatization of indoles through electrophilic activation of allenamides (ee up to 94%), is documented. Besides the synthesis of 3,3-disubstituted indolenine cores, a dearomatization/hydrogen transfer cascade sequence is also presented as a new synthetic shortcut toward highly enantiomerically enriched indolines.