Lewis acid-mediated [3+2] cycloaddition between hydrazones and olefins

[3+2] Cycloaddition between hydrazones and olefins was accelerated in the presence of a stoichiometric amount of BF₃·OEt₂ or a catalytic amount of Zr(OTf)₄, Hf(OTf)₄, or Sc(OTf)₃ under mild conditions. The corresponding pyrazolidine derivatives were obtained in moderate to high yields using this novel [3+2] Lewis acid catalysis.     

Photochemical [3 + 2] cycloaddition of 2-cyanochromone to olefins

Irradiation of 2-cyanochromone in the presence of olefins gave rise to [3 + 2] cycloadducts together with the formation of normal [2 + 2] cycloadducts with the product ratio being temperature-dependent. The mechanism involving a vinyl nitrene intermediate was proposed.     

Rh-catalyzed enantioselective [2 + 2] cycloaddition of alkynyl esters and norbornene derivatives

[reaction: see text] The enantioselective [2 + 2] cycloaddition of alkynes possessing an ester functionality and norbornene derivatives proceeded efficiently using a chiral rhodium catalyst. The chiral tri- and tetracyclic cyclobutenes were obtained in moderate to high ee.     

Concise [4+3] cycloaddition reaction of pyrroles leading to tropinone derivatives

A concise [4+3] cycloaddition reaction of pyrroles with 2-(silyloxy)allyl cations has been developed. The oxyallyl cations stabilized with a methylthio group or geminal methyl groups were generated from the corresponding allylic alcohols under the influence of a Brönsted acid (Tf₂NH), respectively. The use of N-nosyl-protected pyrroles as the four-carbon unit was found to give tropinone derivatives in high yield.     

Stereoselective [3+2+2] cycloaddition utilizing optically active binuclear Fischer carbene complexes with alkynes

The reaction of optically active α,β-unsaturated binuclear Fischer carbene complexes with alkynes gave planar chiral cycloheptatriene chromium complexes via [3+2+2] cycloaddition with high diastereoselectivity.     

Recent advances in enantioselective [2 + 2 + 2] cycloaddition

Enantioselective cycloaddition using chiral transition metal catalysts is an atom-economical and efficient synthetic tool for the construction of chiral carbo- and heterocyclic skeletons. This short account discloses our recent results of inter- and intramolecular enantioselective [2 + 2 + 2] cycloadditions of alkyne and/or alkene moiety(ies). Chiral iridium complexes catalyzed the alkyne trimerization for the generation of axial chirality(ies), and chiral rhodium ones catalyzed alkyne-alkyne-alkene cyclization for the generation of a quaternary carbon including spirocyclic system.     

Rh-catalyzed intermolecular and enantioselective [4 + 2] cycloaddition of 1,3-dienes with dimethyl acetylenedicarboxylate

A Rh-BINAP complex catalyzed an intermolecular and enantioselective [4 + 2] cycloaddition of 1-monosubstituted, 1,1- or 1,2-disubstituted buta-1,3-dienes with dimethyl acetylenedicarboxylate to give chiral cyclohexa-1,4-dienes.     

Catalytic enantioselective [2 + 4] and [2 + 2] cycloaddition reactions with propiolamides

We report here the catalytic and highly enantioselective [2 + 4] and [2 + 2] cycloaddition reactions of electron-rich dienes or silyl enol ethers with electron-deficient propiolamide derivatives induced by copper(II).3-(2-naphthyl)-L-alanine amide complex.     

Electrochemical reduction of dibromodiketones facile [3+2] cycloaddition with olefins

Electrochemical reduction of 2,2-dibromo-1,3-diketones in the presence of olefins afforded the [3+2] cycloadducts, 2,3-dihydrofuran derivatives, regioselectively.     

Diastereo- and enantioselective palladium-catalyzed dearomative [4+2] cycloaddition of 3-nitroindoles

A Pd-catalyzed asymmetric decarboxylative [4+2] cycloaddition of 3-nitroindoles and vinyl benzoxazinanones is developed through a dearomatization approach. The reaction provides an efficient protocol for constructing a series of chiral tetrahydro-5H-indolo[2,3-b]quinolines in high yields and with excellent diastereo- and enantioselectivities.     

Ni-catalyzed [3+2+2] cycloaddition of ethyl cyclopropylideneacetate and 1,3-diynes. Application to the three-component cycloaddition

The Ni-catalyzed [3+2+2] cocyclization between ethyl cyclopropylideneacetate (1) and 1,3-diynes afforded cycloheptadiene derivatives. The three-component reaction of 1, 1,3-diynes, and alkynes proceeded with good yield and high selectivity. Scope of the substrates was studied, and the origin of chemo- and regioselectivity of the reaction is discussed.     

Photochemical [3 + 2] cycloaddition of α,β-acetylenic ketones with simple olefins

Irradiation of 3-pentyn-2-one (1) in the presence of tetramethylethylene, isobutylene, and cis- and trans -2-butene leads to vinyldihydrofurans 3,5,6, 8 and 9 (Table 1) in a novel [3 + 2] photochemical cycloaddition reaction. A related adduct 13 is formed between tetramethylethylene and 5,5-dimethyl-3-hexyn-2-one (2). A mechanism incorporating an initial biradical that closes to a carbene (eqn 3) is proposed to account for these reactions.     

Rhodium-catalyzed chemo-, regio-, and enantioselective [2 + 2 + 2] cycloaddition of alkynes with isocyanates

[reaction: see text] We have developed a cationic rhodium(I)/modified-BINAP complex-catalyzed chemoselective [2 + 2 + 2] cycloaddition of alkynes with isocyanates leading to a wide range of 2-pyridones. This method was successfully applied to the chemo-, regio-, and enantioselective synthesis of axially chiral 2-pyridones from unsymmetrical alpha,omega-diynes, bearing an ortho-substituted phenyl at one terminal position, and alkyl isocyanates.     

Rhodium-catalyzed enantioselective [2+2+2] cycloaddition of diynes with unfunctionalized alkenes

A chiral rhodium complex catalyzed an enantioselective [2+2+2] cycloaddition of unsymmetrical diynes with norbornene, and tetracyclic products were obtained in good to excellent ee. The cycloaddition of a symmetrical diyne with styrene derivatives as coupling partners gave bicyclic products in good ee.     

Au-containing all-carbon 1,3-dipoles: generation and [3+2] cycloaddition reactions

A novel approach to generate Au-containing all-carbon 1,3-dipoles is developed via an unprecedented migration-fragmentation of ketals/acetals. These in situ generated dipoles undergo rapid [3+2] cycloaddition under mild conditions with various enones/enals, electron-rich aromatic aldehydes, and N-benzylindole, leading to flexible formation of highly functionalized dihydrofurans and cyclopentenes.     

Highly enantioselective synthesis of silahelicenes using Ir-catalyzed [2+2+2] cycloaddition

Silahelicenes, which contain two silole moieties in a helically chiral structure, were synthesized by a chiral Ir-catalyzed intermolecular [2+2+2] cycloaddition of tetraynes with diynes along with a Ni-mediated intramolecular [2+2+2] cycloaddition. The photophysical properties of the obtained highly enantiomerically enriched silahelicenes (up to 93% ee) were also measured.     

[3,3] Valenceisomeric azo bridged carbocycles and cyclic hydrazones: intramolecular [3+2] cycloaddition and [4+2] cycloreversions through n-methylation1

Unsaturated azo bridged carbocycles 1, 2, 5, 8, 12, 14 and 16 can easily be methylated with Me₃OBF₄ or MeI. Depending on structural and steric requirement and the anion, the quaternary salts obtained are stable (1-Me⁺, 2-Me⁺, 5a-Me⁺, 14a/b-Me⁺, 16a/b-Me⁺ with BF_4-), undergo [4+2] cycloreversion (8a-Ne⁺, 12-Me⁺) or intramolecular [3+2] cycloaddition after intermediate deprotonation, whereby the unusual hydrazine derivatives, the cage compounds 3-H⁺ , 4-H⁺, 6-H⁺ and 11-H⁺ are formed. Systems which contain the N=N and C=C function in 1,5-positions are isomeric with their [3,3] rearrangement products, the hydrazones endo-7, endo-10, endo-15 and endo-17. Methylation of the latter provokes the same consecutive reactions as for their azo isomers. These have been demonstrated to be the crucial intermediates for the formation of cage Compound (e.g. endo-7b-Me⁺ → 5b-Me⁺ → 6-H⁺ ). Intermolecular methyl migration of quaternized azo compounds has been established, explaining the high yields of cage compounds which can be produced by the “b-series” only.