Showing papers on "Enone published in 2015"

Hydrogen‐Borrowing and Interrupted‐Hydrogen‐Borrowing Reactions of Ketones and Methanol Catalyzed by Iridium

Abstract: Reported herein is the use of catalytic [{Ir(cod)Cl}2] to facilitate hydrogen-borrowing reactions of ketone enolates with methanol at 65 °C. An oxygen atmosphere accelerates the process, and when combined with the use of a bulky monodentate phosphine ligand, interrupts the catalytic cycle by preventing enone reduction. Subsequent addition of pro-nucleophiles to the reaction mixture allowed a one-pot methylenation/conjugate addition protocol to be developed, which greatly expands the range of products that can be made by this methodology.

Efficient Access to Bicyclo[4.3.0]nonanes: Copper‐Catalyzed Asymmetric Silylative Cyclization of Cyclohexadienone‐Tethered Allenes

Abstract: The creation of three consecutive chiral carbon centers in one step is achieved using Cu-catalyzed asymmetric silylative cyclization of cyclohexadienone-tethered allenes. Through regioselective β-silylation of the allene and subsequent enantioselective 1,4-addition to cyclohexadienone, this tandem reaction could afford cis-hydrobenzofuran, cis-hydroindole, and cis-hydroindene frameworks with excellent yields (80-98%) and enantioselectivities (94-98% ee) bearing vinylsilane and enone substructures. Meanwhile, this mild transformation is generally compatible with a wide range of functional groups, which allows further conversion of the bicyclic products to bridged and tricyclic ring structures.

Enantioselective Lewis Acid Catalysis in Intramolecular [2 + 2] Photocycloaddition Reactions: A Mechanistic Comparison between Representative Coumarin and Enone Substrates

Abstract: The intramolecular [2 + 2] photocycloaddition of three 4-(alk-4-enyl)coumarins and three 1-(alk-4-enoyl)-2,3-dihydropyridones was studied in the absence and in the presence of Lewis acids (irradiation wavelength λ = 366 nm). Spectral and kinetic data were collected for the respective parent compounds with a pent-4-enyl and a pent-4-enoyl chain. For the substrates with a methyl group in cis- or trans-position of the terminal alkene carbon atom (hex-4-enyl and hex-4-enoyl substitution), the stereochemical outcome of the [2 + 2] photocycloaddition was investigated. The mechanistic course of the uncatalyzed coumarin reactions was found to be a singlet pathway, whereas Lewis acid-catalyzed reactions proceeded with higher reaction rates in the triplet manifold. Contrary to that, the dihydropyridones underwent a fast triplet reaction in the absence of the Lewis acid. In the presence of a chiral Lewis acid the reactions slowed down but, due to the high extinction coefficient of the Lewis acid/dihydropyridone complexes at λ = 366 nm, still resulted in high enantioselectivity.

Deoxygenative C-C Bond-Forming Processes via a Net Four-Electron Reductive Coupling.

Abstract: The nickel-catalyzed coupling of enones or enals with alkynes in the presence of silane and titanium alkoxide reductants provides direct access to skipped diene products. The process involves a net four-electron reductive coupling and proceeds with deoxygenation of the starting enone or enal. A new class of well-defined nickel(0) precatalysts bearing an unhindered N-heterocyclic carbene ligand, which was developed in optimization of the process, is essential for the efficiency of the transformation. The strategy allows the high reactivity of α,β-unsaturated carbonyl substrates to be utilized in couplings with simultaneous extrusion of the oxygen atom, thus enabling a traceless strategy for alkene installation.

Chiral Cationic CpxRu(II) Complexes for Enantioselective Yne-Enone Cyclizations

Abstract: The cyclopentadienyl (Cp) group is a ligand of great importance for many transition-metal complexes used in catalysis. Cationic CpRuII complexes with three free coordination sites are highly versatile catalysts for many atom-economic transformations. We report the synthesis of a family of CpxRuII complexes with chiral Cp ligands keeping the maximum number of available coordination sites. The cationic members are efficient and selective catalysts for yne-enone cyclizations via formal hetero-Diels–Alder reactions. The transformation proceeds in <1 h at −20 °C and provides pyrans in up to 99:1 er. Unsaturated ester or Weinreb-amide substrates directly yield the iridoid skeleton.

Stereospecific Rhodium-Catalyzed Allylic Substitution with Alkenyl Cyanohydrin Pronucleophiles: Construction of Acyclic Quaternary Substituted α,β-Unsaturated Ketones.

Abstract: A highly regio- and stereospecific rhodium-catalyzed allylic alkylation of tertiary allylic carbonates with alkenyl cyanohydrin pronucleophiles is described. This protocol offers a fundamentally novel approach toward the synthesis of acyclic quaternary-substituted α,β-unsaturated ketones and thereby provides a new cross-coupling strategy for target directed synthesis. A particularly attractive feature with this process is the ability to directly couple di-, tri- and tetrasubstituted alkenyl cyanohydrin pronucleophiles to prepare the corresponding α,β-unsaturated ketone derivatives in a highly selective manner. Additionally, the chemoselective 1,4-reduction of the enone products provides rapid access to acyclic enantiomerically enriched α,α′-dialkyl-substituted ketones, which are challenging motifs to prepare using conventional enolate alkylation.

Regulatory Mechanism of the Enantioselective Intramolecular Enone [2+2] Photocycloaddition Reaction Mediated by a Chiral Lewis Acid Catalyst Containing Heavy Atoms

Abstract: The asymmetric catalysis of the intramolecular enone [2+2] photocycloaddition has been subject of extensive experimental studies, however theoretical insight to its regulatory mechanism is still sparse. Accurate quantum chemical calculations at the CASPT2//CASSCF level of theory associated with energy-consistent relativistic pseudopotentials provide a basis for the first regulation theory that the enantioselective reaction is predominantly controlled by the presence of relativistic effects, that is, spin-orbit coupling resulting from heavy atoms in the chiral Lewis acid catalyst.

Mechanistic analysis of an asymmetric palladium-catalyzed conjugate addition of arylboronic acids to β-substituted cyclic enones

Abstract: An asymmetric palladium-catalyzed conjugate addition reaction of arylboronic acids to enone substrates was investigated mechanistically. Desorption electrospray ionization coupled to mass spectrometry was used to identify intermediates of the catalytic cycle and delineate differences in substrate reactivity. Our findings provide evidence for the catalytic cycle proceeding through formation of an arylpalladium(II) cation, subsequent formation of an arylpalladium–enone complex, and, ultimately, formation of the new C–C bond. Reaction monitoring in both positive and negative ion modes revealed that 4-iodophenylboronic acid formed a relatively stable trimeric species under the reaction conditions.

Gold(I)‐Assisted α‐Allylation of Enals and Enones with Alcohols

Abstract: The intermolecular α-allylation of enals and enones occurs by the condensation of variously substituted allenamides with allylic alcohols. Cooperative catalysis by [Au(ItBu)NTf2] and AgNTf2 enables the synthesis of a range of densely functionalized α-allylated enals, enones, and acyl silanes in good yield under mild reaction conditions. DFT calculations support the role of an α-gold(I) enal/enone as the active nucleophilic species.

Recyclable Hypervalent-Iodine-Mediated Dehydrogenative α,β'-Bifunctionalization of β-Keto Esters Under Metal-Free Conditions.

Abstract: We have developed a method for recyclable hypervalent-iodine-mediated direct dehydrogenative α,β'- bifunctionalization of β-ketoesters and β-diketones under metal-free conditions, which affords a straightforward way to synthesize benzo-fused 2,3-dihydrofurans. This efficient, mild method, which has a wide substrate scope and good functional-group tolerance, was used for the multistep synthesis of the protected aglycone of a naturally occurring phenolic glycoside. A mechanism involving Michael addition to an enone intermediate and subsequent oxidative cyclization is proposed.

Palladium(II)-catalyzed intramolecular carboxypalladation–olefin insertion cascade: direct access to indeno[1,2-b]furan-2-ones

Abstract: A catalytic, atom-economical, domino 5-endo-dig cyclization–intramolecular olefin insertion sequence was developed under mild conditions. Aryl alkynoic acids bearing a tethered enone partner afforded the indeno[1,2-b]furan-2-ones, the core skeleton present in a number of biologically significant molecules including the natural product solanacol, under ligand-free, palladium-catalyzed reaction conditions in high yields. The competitive β-hydride elimination in the final step leading to the conjugated analogs was avoided by the addition of lithium bromide. A plausible mechanism for this domino sequence is proposed involving intramolecular carboxypalladation and olefin insertion steps.

Formation of an Endoperoxide upon Chromium-Catalyzed Allylic Oxidation of a Triterpene by Oxygen.

Abstract: The chromium-catalyzed allylic oxidation of triterpene 1 with O2 and N-hydroxyphthalimide (NHPI, 5 equiv) formed endoperoxide 2 in 76% yield at ambient temperature. Unlike standard allylic oxidations, this oxidation is catalytic in chromium because oxygen, not the chromium reagent, is the oxidant. This oxidation is sensitive to the precise structure of the substrate. The endoperoxide is only formed if ring A is unsaturated and ring C contains an enone. A mechanism is proposed that involves the coupling of two stabilized radicals on rings A and C to form endoperoxide 2.

Iron(iii) halide or iodine-promoted synthesis of 3-haloindene derivatives from o-alkynylarene chalcones

Abstract: o-Alkynylarene chalcones, when treated with ferric halides/iodine, undergo cyclization to give synthetically important 3-haloindene derivatives in good yields. The reactions proceed through Lewis acid-promoted intramolecular nucleophilic addition of the alkyne unit to the enone moiety followed by halide capture of the resulting vinyl carbocation intermediate. The reactions are operationally simple, require only inexpensive and environmentally friendly reagents and are applicable to a range of substrates.

Highly efficient synthesis of novel methyl 132-methylene mesopyropheophorbide a and its stereoselective Michael addition reaction

Abstract: Treatment of methyl mesopyropheophorbide a with formaldehyde under basic conditions gave a novel 132-methylene derivative in 85% yield; under acidic conditions, the corresponding 20-hydroxymethyl derivative was obtained in 65% yield. The high reactivity of the enone structural motif existed in the former product provides a unique way to construct some novel chlorophyll a derivatives for various applications. Stereoselective Michael reaction of this compound is studied and discussed.

New Preparation of Benzylic Manganese Chlorides by the Direct Insertion of Magnesium into Benzylic Chlorides in the Presence of MnCl2·2LiCl

Abstract: Functionalized benzylic manganese chlorides were smoothly prepared by the direct insertion of magnesium into benzylic chlorides in the presence of MnCl2·2LiCl. Reactions with acid chlorides, aldehydes, an allyl bromide, and an enone proceed without any additional transition metal.

Synthetic and Mechanistic Study of the Catalytic Enantioselective Preparation of Primary β‑Amino Ketones from Enones and a Fluorinated Gabriel Reagent

Abstract: The salen μ-oxo complex of aluminum 1 catalyzes the asymmetric 1,4-addition of the novel ammonia equivalent 3,4,5,6-tetrafluorophthalimide to unsaturated ketones. All of the reagents are inexpensive and are readily available. The products are formed in up to 89% yield and up to 96% ee. The tetrafluorophthalimide group is removed under mild chemoselective conditions and in high yields to afford the free primary amines. Mechanistic studies suggest that the reaction occurs through a dual activation mechanism. A pre-equilibrium formation of a 1:1 complex between tetrafluorophthalimide and the catalyst is observed. The rate-determining step is the addition of tetrafluorophthalimide catalyst complex to the catalyst activated enone. These mechanistic studies provide important clues for the further development of catalytic asymmetric reactions.

A practical deca-gram scale ring expansion of (R)-(-)-carvone to (R)-(+)-3-methyl-6-isopropenyl-cyclohept-3-enone-1

Abstract: A route to enantiopure (R)-(+)-3-methyl-6-isopropenyl-cyclohept-3-enone-1, an intermediate for terpenoids, has been developed and includes a highly chemo- and regioselective Tiffeneau-Demjanov reaction. Starting from readily available (R)-(-)-carvone, this robust sequence is available on a deca-gram scale and uses flow chemistry for the initial epoxidation reaction. The stereochemistry of the addition of two nucleophiles to the carbonyl group of (R)-(-)-carvone has been determined by X-ray diffraction studies and chemical correlation.

Competitive Gold-Promoted Meyer-Schuster and oxy-Cope Rearrangements of 3-Acyloxy-1,5-enynes: Selective Catalysis for the Synthesis of (+)-(S)-γ-Ionone and (-)-(2S,6 R)-cis-γ-Irone.

Abstract: We report a simple, highly stereoselective synthesis of (+)-(S)-γ-ionone and (-)-(2S,6R)-cis-γ-irone, two characteristic and precious odorants; the latter compound is a constituent of the essential oil obtained from iris rhizomes. Of general interest in this approach are the photoisomerization of an endo trisubstituted cyclohexene double bond to an exo vinyl group and the installation of the enone side chain through a [(NHC)Au(I)]-catalyzed Meyer-Schuster-like rearrangement. This required a careful investigation of the mechanism of the gold-catalyzed reaction and a judicious selection of reaction conditions. In fact, it was found that the Meyer-Schuster reaction may compete with the oxy-Cope rearrangement. Gold-based catalytic systems can promote either reaction selectively. In the present system, the mononuclear gold complex [Au(IPr)Cl], in combination with the silver salt AgSbF6 in 100:1 butan-2-one/H2O, proved to efficiently promote the Meyer-Schuster rearrangement of propargylic benzoates, whereas the digold catalyst [{Au(IPr)}2(μ-OH)][BF4] in anhydrous dichloromethane selectively promoted the oxy-Cope rearrangement of propargylic alcohols.