Die asymmetrische Hetero-Diels-Alder-Reaktion in Synthesen biologisch relevanter Verbindungen

Diastereo- and Enantioselective Inverse-Electron-Demand Diels-Alder Cycloaddition between 2-Pyrones and Acyclic Enol Ethers

A broadly applicable diastereo- and enantioselective inverse-electron-demand Diels-Alder reaction of 2-pyrones and acyclic enol ethers is reported herein. Using a copper(II)-BOX catalytic system, bridged bicyclic lactones are obtained in very high yields (up to 99 % yield) and enantioselectivities (up to 99 % ee) from diversely substituted 2-pyrones and acyclic enol ethers. Mechanistic experiments as well as DFT calculations indicate the occurrence of a stepwise mechanism. The synthetic potential of the bridged bicyclic lactones is showcased by the enantioselective synthesis of polyfunctional cyclohexenes and cyclohexadienes, as well as a carbasugar unit.

Catalytic Asymmetric Aza-Diels-Alder Reaction of Ketimines and Unactivated Dienes

The enantioselective aza-Diels-Alder reaction is efficient for constructing chiral tetrahydropyridines, but the catalytic asymmetric aza-Diels-Alder reaction of ketimines with unactivated dienes is still a challenging topic. Herein, guided by computational screening, a highly enantioselective aza-Diels-Alder reaction of 2-aryl-3H-indol-3-ones with unactivated dienes was realized by using a B(C₆ F₅ )₃ /chiral phosphoric acid catalyst system under mild conditions. The reaction has a broad scope with respect to both aza-Diels-Alder reaction partners and hence offers rapid access to an array of tetrahydropyridine derivatives with pretty outcomes (up to 99 % yield, >20:1 dr and 98:2 er). The reaction is very efficient: lowering catalyst loadings for the model reaction to 0.1 mol %, enantioselectivity is still maintained. The synthetic utility was confirmed by transformations of the products. DFT calculations provide convincing evidence for the interpretation of stereoselection.

Switch From Pauli-Lowering to LUMO-Lowering Catalysis in Brønsted Acid-Catalyzed Aza-Diels-Alder Reactions

Brønsted acid-catalyzed inverse-electron demand (IED) aza-Diels-Alder reactions between 2-aza-dienes and ethylene were studied using quantum chemical calculations. The computed activation energy systematically decreases as the basic sites of the diene progressively become protonated. Our activation strain and Kohn-Sham molecular orbital analyses traced the origin of this enhanced reactivity to i) "Pauli-lowering catalysis" for mono-protonated 2-aza-dienes due to the induction of an asynchronous, but still concerted, reaction pathway that reduces the Pauli repulsion between the reactants; and ii) "LUMO-lowering catalysis" for multi-protonated 2-aza-dienes due to their highly stabilized LUMO(s) and more concerted synchronous reaction path that facilitates more efficient orbital overlaps in IED interactions. In all, we illustrate how the novel concept of "Pauli-lowering catalysis" can be overruled by the traditional concept of "LUMO-lowering catalysis" when the degree of LUMO stabilization is extreme as in the case of multi-protonated 2-aza-dienes.

A Unified Catalytic Asymmetric (4+1) and (5+1) Annulation Strategy to Access Chiral Spirooxindole-Fused Oxacycles

A unified catalytic asymmetric (N+1) (N=4, 5) annulation reaction of oxindoles with bifunctional peroxides has been achieved in the presence of a chiral phase-transfer catalyst (PTC). This general strategy utilizes peroxides as unique bielectrophilic four- or five-atom synthons to participate in the C-C and the subsequent umpolung C-O bond-forming reactions with one-carbon unit nucleophiles, thus providing a distinct method to access the valuable chiral spirooxindole-tetrahydrofurans and -tetrahydropyrans with good yields and high enantioselectivities under mild conditions. DFT calculations were performed to rationalize the origin of high enantioselectivity. The gram-scale syntheses and synthetic utility of the resultant products were also demonstrated.

Electrochemically Tuned Oxidative [4+2] Annulation and Dioxygenation of Olefins with Hydroxamic Acids

This work represents the first [4+2] annulation of hydroxamic acids with olefins for the synthesis of benzo[c][1,2]oxazines scaffold via anode-selective electrochemical oxidation. This protocol features mild conditions, is oxidant free, shows high regioselectivity and stereoselectivity, broad substrate scope of both alkenes and hydroxamic acids, and is compatible with terpenes, peptides, and steroids. Significantly, the dioxygenation of olefins employing hydroxamic acid is also successfully achieved by switching the anode material under the same reaction conditions. The study not only reveals a new reactivity of hydroxamic acids and its first application in electrosynthesis but also provides a successful example of anode material-tuned product selectivity.

Enantioselective Redox-Divergent Chiral Phosphoric Acid Catalyzed Quinone Diels-Alder Reactions

An efficient enantioselective construction of tetrahydronaphthalene-1,4-diones as well as dihydronaphthalene-1,4-diols by a chiral phosphoric acid catalyzed quinone Diels-Alder reaction with dienecarbamates is reported. The nature of the protecting group on the diene is key to the success of achieving high enantioselectivity. The divergent "redox" selectivity is controlled by using an adequate amount of quinones. Reversible redox switching without erosion of enantioselectivity was possible from individual redox isomers.

Regioselective Arylboration of 1,3-Butadiene

A method for the regioselective 1,2-arylboration of 1,3-butadiene, a feedstock chemical, is reported. The reactions result in the formation of products that can be easily elaborated to other structures. The mechanistic details of this process are also discussed.

A Scaffold-Diversity Synthesis of Biologically Intriguing Cyclic Sulfonamides

A "branching-folding" synthetic strategy that affords a range of diverse cyclic benzo-sulfonamide scaffolds is presented. Whereas different annulation reactions on common ketimine substrates build the branching phase of the scaffold synthesis, a common hydrogenative ring-expansion method, facilitated by an increase of the ring-strain during the branching phase, led to sulfonamides bearing medium-sized rings in a folding pathway. Cell painting assay was successfully employed to identify tubulin targeting sulfonamides as novel mitotic inhibitors.

Asymmetric Photocatalysis by Intramolecular Hydrogen-Atom Transfer in Photoexcited Catalyst-Substrate Complex

3-(2-Formylphenyl)-1-pyrazol-1-yl-propenones undergo an asymmetric photorearrangement to benzo[d]cyclopropa[b]pyranones with up to >99 % ee, which is catalyzed by a bis-cyclometalated rhodium catalyst in the presence of visible light. Mechanistic experiments and DFT calculations support a mechanism in which a photoexcited catalyst/substrate complex triggers an intramolecular hydrogen-atom transfer followed by a highly stereocontrolled hetero-Diels-Alder reaction. In this reaction scheme, the rhodium catalyst fulfills multiple functions by 1) enabling visible-light π→π* excitation of the catalyst-bound enone substrate, 2) facilitating the hydrogen-atom transfer, and 3) providing the asymmetric induction for the hetero-Diels-Alder reaction.

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.

Catalytic Asymmetric Inverse-Electron-Demand Hetero-Diels-Alder Reactions

In this review, the recent developments in catalytic asymmetric inverse-electron-demand hetero-Diels-Alder reaction, which is recognized as one of the most powerful routes to construct highly functionalized and enantioenriched six-membered heterocycles, are described. The article is organized on the basis of different kinds of electron-deficient heterodienes, including α,β-unsaturated ketones/aldehydes, o-benzoquinones, α,β-unsaturated imines, N-aryl imines, o-benzoqinone imides, and other aza-olefins.

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.