Lewis acid-catalyzed novel [3+2] cycloaddition of methylenecyclopropanes with activated aldehydes or ketones

Methylenecyclopropanes react with activated aldehydes or ketones in the presence of various Lewis acids to give the corresponding [3+2] cycloaddition products having tetrahydrofuran (THF) skeleton in good yields and in some cases with high stereoselectivity.     

Lewis acid-catalyzed intermolecular [4 + 2] cycloaddition of 3-alkoxycyclobutanones to aldehydes and ketones

Intermolecular [4 + 2] cycloaddition between 3-alkoxycyclobutanones and aldehydes or ketones by the activation with boron trifluoride etherate is reported. The carbonyl compounds are inserted into the less substituted C2-C3 bond of the cyclobutanone ring of 6-alkyl-2-oxabicyclo[4.2.0]octan-7-ones to afford 1-alkyl-5,7-dioxabicyclo[4.4.0]decan-2-one derivatives regioselectively (>99:1) and diastereoselectively. On the other hand, [4 + 2] cycloaddition of 3-ethoxy-2,2-dialkylcyclobutanones at low temperature proceeds at the more substituted C2-C3 bond to yield 3,3-dialkyl-6-ethoxy-2,3,5,6-tetrahydro-4H-pyran-4-one derivatives with high regioselectivities. This [4 + 2] cycloaddition is developed into a one-pot synthesis of tri- or tetrasubstituted dihydro-gamma-pyrones from 3-ethoxycyclobutanones which are readily prepared from acid chloride and ethyl vinyl ether. The two regioselectivities observed in ring-opening of cyclobutanones can ascribe to thermodynamic stabilities of zwitterionic intermediates generated from tetrahydropyran-fused cyclobutanones and 3-ethoxycyclobutanones.     

Formal [4+2] cycloaddition of 3-phenylcyclobutanones with nitriles

Various 3-phenylcyclobutanones reacted with aliphatic and aromatic nitriles in the presence of Me₃SiOTf to afford dihydropyridones by formal [4+2] cycloaddition.     

Formal [4 + 2] cycloaddition of alkoxy-substituted donor-acceptor cyclobutanes and aldehydes catalyzed by Yb(OTf)3

The cycloaddition between 2-alkoxy-1,1-cyclobutane diesters and aromatic, heteroaromatic, or aliphatic aldehydes under Yb(OTf)(3) catalysis generates tetrahydropyrans in high yields with exclusive cis-stereochemistry.     

Formal [4+2] cycloaddition between 3-ethoxycyclobutanones and silyl enol ethers

3-Ethoxycyclobutanones reacted with silyl enol ethers to give formal [4+2] cycloadducts, 3-ethoxy-5-trimethylsiloxycyclohexanone derivatives, by using ethylaluminum dichloride as a Lewis acid. Highly oxygenated cyclohexanone derivatives were stereoselectively prepared by this method.     

Synthesis of aromatic aldehydes by organocatalytic [4+2] and [3+3] cycloaddition of α,β-unsaturated aldehydes

Organocatalytic inter- and intramolecular [4+2] and [3+3] cycloadditions of α,β-unsaturated aldehydes to give polysubstituted aromatic aldehydes are described. High periselectivity for the cycloadditions, with catalyst effects exerted by l-proline and pyrrolidine-HOAc, as well as cocatalyst, additive effects, has been observed.     

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.     

Palladium-catalyzed [3 + 2] cycloaddition reaction of vinylcyclopropanes with α,β-unsaturated esters or ketones

Reaction of vinylcyclopropanes having two electron-withdrawing groups with α,β-unsaturated esters or ketones in the presence of Pd₂(dba)₃.CHC1₃-1,2- bis(diphenylphosphino)ethane (dppe) or tributylphosphine catalyst gave vinyl- cyclopentanes in good yields.     

Formal intermolecular [2 + 2] cycloaddition reaction of allenamide [corrected] with alkenes via gold catalysis

An efficient method was developed to construct the densely functionalized cyclobutane[corrected] adducts through formal intermolecular cycloaddition of allenamides [corrected] with electron-rich olefins via gold catalysis, in which vinyl ethers/amides and electron-rich styrenes worked very well. In addition, a series of allenamide [corrected] dimerization products were prepared from the same allenamide [corrected] substrates.     

Formal [6+4] cycloaddition of a dicobalt acetylene complex with furan derivatives

An efficient method for constructing a 10-membered carbocycle with an oxygen bridge has been developed on the basis of a formal [6+4] cycloaddition reaction. Under the influence of EtAlCl₂, a dicobalt hexacarbonyl acetylene complex possessing a benzoyloxy group and an allylsilane moiety reacted with furan to give a 11-oxabicyclo[6.2.1]undec-9-ene derivative. On treatment with iodine, the cycloadduct underwent decomplexation followed by rearrangement of the oxygen bridge to afford a 11-oxabicyclo[5.3.1]undeca-1,5-diene derivative.     

Cationic iron(III) porphyrin-catalyzed [4 + 2] cycloaddition of unactivated aldehydes with simple dienes

Cationic iron(III) porphyrin was found to be an efficient catalyst for the highly chemoselective hetero-Diels-Alder-type reaction of aldehydes with 1,3-dienes. The catalyzed process did not require the use of electron-deficient aldehydes such as glyoxylic acid derivatives or activated electron-rich 1,3-dienes such as Danishefsky's diene and Rawal's diene. The high functional group tolerance and robustness of the catalyst were demonstrated. Further, the potential utility of the catalyst was demonstrated by performing the cycloaddition in the presence of water and by carrying out cycloaddition of an unactivated ketone such as cyclohexanone with a diene.     

[4+2]cycloaddition reactions of triarylphosphaalkene

Cycloaddition reactions of mesityl(diphenyliiifithylene)phosphine ($\text{1}$) were Investigated. With several dienes, no Diels-Alder reactions were oEserved. With azides, diphenyidiazomethane and 2,4,6-trimethylbenzonitrile oxide, the corresponding cycloadducts ($\text{4}$$\text{12}$$\text{17}$$\text{12}$were obtained. In the case of phenyl azide, a competing Staudinger reaction occurred leading to $\text{3}$.     

Rhodium N-heterocyclic carbene-catalyzed [4 + 2] and [5 + 2] cycloaddition reactions

[reactions: see text] A rhodium complex of N-heterocyclic carbene (NHC) has been developed for intra- and intermolecular [4 + 2] and intramolecular [5 + 2] cycloaddition reactions. This is the first use of a transition-metal NHC complex in a Diels-Alder-type reaction. For the intramolecular [4 + 2] cycloaddition reactions, all the dienynes studied were converted to their corresponding cycloadducts in 91-99% yields within 10 min. Moreover, up to 1900 turnovers have been obtained for the intramolecular [4 + 2] cycloaddition at 15-20 degrees C. For the intermolecular [4 + 2] cycloadditions, high yields (71-99%) of the corresponding cycloaddition products were obtained. The reaction time and yield were highly dependent upon the diene and the dienophile. For the intramolecular [5 + 2] cycloaddition reactions, all the alkyne vinylcyclopropanes studied were converted to their corresponding cycloadducts in 91-98% yields within 10 min. However, the catalytic system was not effective for an intermolecular [5 + 2] cycloaddition reaction.     

[2+2]- AND [4+2] cycloaddition reactions of 1,3-diazabutadienes with diphenylketene

1,3-Diazabutadienes react with diphenylketene in [2+2] - and [4+2] cycloaddition reactions, depending on the substitution pattern. Spectroscopic data (IR, ¹H- and ¹³C-NMR) and results of quantummechanical model calculations (ab initio 3-21G) are presented.     

Phosphine-promoted [3 + 2] cycloaddition between nonsubstituted MBH carbonates and trifluoromethyl ketones

A phosphine-promoted [3 + 2] cycloaddition from readily accessible MBH(Morita–Baylis–Hillman)carbonate and aryl trifluoromethyl ketone is described. The use of methyl vinyl ketone-derived allylic carbonate rather than common acrylate-derived counterpart renders the reaction pathway exclusive for 5-endo process, which enables the expeditious preparation of a range of trifluoromethylated 2,3-dihydrofuran in a chemospecific manner.     

Intramolecular [4+2] cycloaddition of an indolenine

Attempted transformation of readily accessible 18-methylenevincadifformine (I) to indolenine VI in acidic medium afforded unexpectedly the intramolecular [4+2] cycloadduct VIII in almost quantitative yield     

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.     

Synthesis of cyclopentenyl and cyclohexenyl ketones via [3 + 2] and [4 + 2] annulations of 1,2-allenic ketones简

In this paper, an efficient synthesis of cyclopentenyl ketones and cyclohexenyl ketones through tertiary phosphine catalyzed [3 ＋ 2] and [4 ＋ 2] annulations of 1,2-allenic ketones with activated alkenes has been developed. Compared with published synthetic methods on cyclopentenyl ketones and cyclohexenyl ketones, the protocols developed in this paper have the advantages such as readily available starting materials, mild reaction conditions and high efficiency.