Ring-enlargement of dimethylaminopropenoyl cyclopropanes: an efficient route to substituted 2,3-dihydrofurans

Organocatalytic Cloke-Wilson Rearrangement: Carbocation-Initiated Tandem Ring Opening/Cyclization of Cyclopropanes under Neutral Conditions

We report a metal-, acid-, and base-free 2-(bromomethyl)naphthalene (2-BMN)-promoted organocatalytic Cloke-Wilson rearrangement of chain doubly activated cyclopropanes for the construction of 2,3-dihydrofurans via a carbocation-initiated tandem intramolecular ring-opening/recyclization process. The strategy is especially suitable for the construction of furan units in complex molecules, providing a solution to the problem of heavy-metal residues in dihydrofuran-containing drugs synthesized by traditional metal-based protocols. Thus, it is of potential interest in synthetic and medicinal chemistry.

Asymmetric Sequential Corey-Chaykovsky Cyclopropanation/Cloke-Wilson Rearrangement for the Synthesis of 2,3-Dihydrofurans

The first sequential Corey-Chaykovsky cyclopropanation/Cloke-Wilson rearrangement between propargyl sulfonium salts and acrylonitrile derivatives has been developed, affording the tetra-substituted 2,3-dihydrofurans in generally excellent yields (57-98%) with good diastereoselectivities (7:1-18:1). In addition, chiral propargyl sulfonium salt is also suitable for this strategy, giving the optically active 2,3-dihydrofurans with good enantioselectivities. This reaction sequence was designed upon in situ generated 10π-conjugated structures from the dearomatization of indole fragments and subsequent intramolecular 1,6-addition.

Substrate-oriented selectivity in the Mg-mediated conjugate addition of bromoform to electron-deficient alkenes

The Mg-mediated conjugate addition of bromoform to a variety of electron-deficient alkenes has been investigated. In the case of nitrodienes and dibenzylideneacetones, tribromomethylated products were isolated, whereas spiro-cyclopropanated products were obtained with cyclic dibenzylideneketones and 3-olefinic oxindoles. The spiro-cyclopropyl ketones derived from cyclic dibenzylideneketones were successfully transformed into fused furans via the Cloke-Wilson rearrangement.

Cu-catalyzed C–N bond cleavage of 3-aminoindazoles for the C–H arylation of enamines

We have presented a novel Cu-catalyzed stereoselective C–H arylation of enamines by using 3-aminoindazoles arylating agents via oxidative cleavage of two C–N bonds.

An approach to 7-aza-1-phosphanorbornane complexes: strain promoted rearrangement of 1-iminylphosphirane complexes and cycloaddition with olefins

1-Iminylphosphirane W(CO)5 complexes react with alkenes under an atmosphere of CO at 130 °C to form the original 7-aza-1-phosphanorbornane complexes. The reaction works well with both electron-rich and electron-poor alkenes. On the basis of DFT calculations, we propose a mechanism including the rearrangement of phosphirane into a dipolar five-membered ring and the following cycloaddition of this dipolar intermediate with alkenes.

Ammonium Acetate-Promoted One-Pot Tandem Aldol Condensation/Aza-Addition Reactions: Synthesis of 2,3,6,7-Tetrahydro-1H-pyrrolo[3,2-c]pyridin-4(5H)-ones

An efficient tandem intermolecular one-pot aldol condensation/aza-addition reaction of 2-methyl-3-carbamoylpyrroles and aldehydes was developed for the synthesis of 2,3,6,7-tetrahydro-1H-pyrrolo[3,2-c]pyridin-4(5H)-ones. The reaction proceeded using only 3.0 equiv of ammonium acetate promoter in green solvent poly(ethylene glycol)-400 at 100 °C to afford a series of pyrrolo[3,2-c]pyridinone derivatives in good to excellent yields.

Diastereoselective Synthesis of Spirocyclopropanes under Mild Conditions via Formal [2 + 1] Cycloadditions Using 2,3-Dioxo-4-benzylidene-pyrrolidines

A highly diastereoselective cyclopropanation of cyclic enones with sulfur ylides was developed under catalyst-free conditions, producing multifunctional spirocyclopropanes in generally excellent yields (up to 99% yield and >99:1 d.r.). The asymmetric version of this method was realized by using an easily available chiral sulfur ylide, affording products with moderate to good stereoselectivity.

Non-decarbonylative photochemical versus thermal activation of Bu4N[Fe(CO)3(NO)] - the Fe-catalyzed Cloke-Wilson rearrangement of vinyl and arylcyclopropanes

The base metal complex Bu₄N[Fe(CO)₃(NO)] (TBA[Fe]) catalyzes the rearrangement of vinyl and arylcyclopropanes both under thermal or photochemical conditions to give the corresponding vinyl or aryldihydrofurans in good to excellent yields. Under photochemical conditions the reaction is performed at room temperature. Spectroscopic investigations show that the metal carbonyl catalyst is not decarbonylated. The best performance was observed at a wavelength of 415 nm. icMRCI+Q analysis of the excited singlet and triplet states of the [Fe(CO)₃(NO)] anion was performed and used to calculate the vertical excitation energies which are in good agreement with the experimental data. CASSCF analysis indicates that the Fe center in all excited states of the ferrate becomes more electrophilic while adopting a distorted tetrahedral configuration. Both aspects have a positive synergistic effect on the formation of the initial π-complex with the incoming organic substrate.

PBr3-mediated [5 + 1] annulation of α-alkenoyl-α-carbamoyl ketene-S,S-acetals: access to substituted pyridine-2,6(1H,3H)-diones

Facile and efficient synthesis of substituted pyridine-2,6(1H,3H)-diones via an intramolecular [5 + 1] annulation of readily available α-alkenoyl-α-carbamoyl ketene-S,S-acetals mediated by phosphorus bromide under very mild is developed.

Catalytic asymmetric transannulation of NH-1,2,3-triazoles with olefins

A convenient one-pot asymmetric synthesis of 2,3-dihydropyrroles from in situ generated triflated triazoles and olefins is described that further expands the utility of azavinyl carbene chemistry and provides access to an important class of cyclic enamides. Mechanistic investigations support the involvement of triflated cyclopropylaldimine intermediates in the formation of 2,3-dihydropyrrole. To the best of our knowledge, this is the first example of a chiral Brønsted acid catalyzed rearrangement of cyclopropylimines into enantioenriched 2,3-dihydropyrroles.

Formal [4+2] annulation of enaminones and cyanomethyl sulfur ylide: one-pot access to polysubstituted pyridin-2(1H)-ones

One-pot synthesis of polysubstituted pyridin-2(1H)-ones by formal [4+2] annulation of enaminones and cyanomethyl sulfur ylide is described involving sequential nucleophilic vinylic substitution, intramolecular nucleophilic cyclization and dealkylation reactions.

[5C + 1N] annulations: two novel routes to substituted dihydrofuro[3,2-c]pyridines

Two novel routes based on [5C + 1N] annulations for the synthesis of 2,3-dihydrofuro[3,2-c]pyridines are described. Ammonium acetate (NH(4)OAc) is used as an ammonia source in both routes. The first route utilizes 1-acyl-1-[(dimethylamino)alkenoyl]cyclopropanes as a five-carbon 1,5-bielectrophilic species and combines the [5C + 1N] annulation and regioselective ring-enlargement of cyclopropyl ketone into one pot, whereas the second route utilizes 3-acyl-2-[(dimethylamino)alkenyl]-4,5-dihydrofurans as the five-carbon synthons, which involves a sequential intermolecular aza-addition, intramolecular aza-nucleophilic addition/elimination, and dehydration reaction.

One-pot tandem double-aldol reaction/aza-addition of acetylacetamides and o-phthalaldehyde leading to spiroindan-2,2'-pyrrolidines

A novel domino reaction based on o-phthalaldehyde and 3-oxo-N-arylbutanamide 1 has been developed which allows one-pot and efficient synthesis of structurally complex spiroindan-2,2'-pyrrolidines 2 and 3 with complete regioselectivity and high stereoselectivity from acyclic precursors.