Photooxygenation of tetramethylallene competing (2+2) cycloaddition and ene-reactions with singlet oxygen

TPP-sensitized photooxygenation of tetramethylallene (4) in carbon tetrachloride yields acetone (5), 2,4-dimethyl-4-hydroxy-1-penten-3-one (8) and 2,4-dimethyl-1,4-pentadien-3-one (9) in a ratio of 35:20:45, besides minor amounts of resinous products and carbon dioxide. Isomerization of 4 to 2,4-dimethyl-1,3-pentadiene (6) does not occur under the reaction conditions. DABCO quenches the photooxygenation, whereas 2,4,6-tri-t-butylphenol (10) enhances the oxygen consumption rate but leaves the ratio of 5:8:9 unchanged. These results indicate that 4 is oxygenated by singlet oxygen. A mechanism is proposed according to which acetone is generated via a (2+2) cycloaddition whereas 8 and 9 are formed via an ene-reaction between 4 and singlet oxygen.     

Reaction of ene-bis(phosphinylallenes): [2+2] versus [4+2] cycloaddition

Reaction of ene-bis(phosphinylallenes), derived from ene-bis(propargyl alcohols) and chlorodiphenylphosphine, was investigated. Benzene-bridged bis(phosphinylallenes) exclusively gave intramolecular [2+2] cycloadducts in the presence of dimethyl fumarate in sharp contrast to the reaction of benzene-bridged bis(sulfinylallenes), which gave the corresponding [4+2] cycloadducts. On the other hand, substituted ethylene- or five-membered heterocycle-bridged bis(phosphinylallenes) provided [4+2] cycloadducts. Reaction of benzene-bridged diallene bearing both a sulfinyl group and a phosphinyl group on the two allenyl groups was also described.     

[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.     

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.     

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.     

Catalytic [2+2+2] and thermal [4+2] cycloaddition of 1,2-bis(arylpropiolyl)benzenes

We have determined that a cationic rhodium(I)/Segphos complex catalyzes an enantio- and diastereoselective intermolecular [2+2+2] cycloaddition of 1,2-bis(arylpropiolyl)benzenes with various monoalkynes at room temperature to give axially chiral 1,4-teraryls possessing an anthraquinone structure in good yields with good enantio- and diastereoselectivities. We have also determined that a thermal intramolecular [4+2] cycloaddition of 1,2-bis(arylpropiolyl)benzenes proceeds at 60 degrees C to give aryl-substituted naphthacenediones in moderate to good yields.     

Thermal intramolecular cycloaddition of 4-alkenylfulvene; highly regio- and diastereoselective formation of [4+2] adduct

4-Alkenylfulvenes were prepared by the annulation of 1,4-ynediones and allylidenetriphenylphosphorane and subjected to a thermal reaction. Highly regio- and stereoselective [4+2] cycloaddition is accomplished with 4-((R)-3-benzyloxypent-4-en-1-yl)fulvene and the resulting adduct is transformed into bicyclo[3.3.0]octene derivative.     

Chemistry of trans-resveratrol with singlet oxygen: [2+2] addition, [4+2] addition, and formation of the phytoalexin moracin M

Resveratrol (1) reacts with singlet oxygen by two major pathways: A [2+2] cycloaddition forming a transient dioxetane that cleaves into the corresponding aldehydes and a [4+2] cycloaddition forming an endoperoxide that, upon heating, undergoes a rearrangement to moracin M. The rate constant by which singlet oxygen is removed by 1 (k(T)) was determined by time-resolved infrared luminescence spectroscopy to be 1.5 × 10(6) M(-1) sec(-1) in CD(3)OD, smaller than previously reported values. Chemical reaction accounts for ca. 25% of k(T).     

Highly regio- and chemoselective [2 + 2 + 2] cycloaddition of 1,6-heptadiynes with allenes catalyzed by cobalt complexes

The CoI2(PPh3)2/Zn system effectively catalyzes the [2 + 2 + 2] ene-diyne cycloaddition of 1,6-heptadiynes with allenes in a highly regio- and chemoselective fashion to yield substituted benzene derivatives in good to excellent yields.     

Examination of homo-[3 + 2]-dipolar cycloaddition: mechanistic insight into regio- and diastereoselectivity

The reaction of 2,3-disubstituted-1,1-cyclopropanediesters with nitrones under Lewis acid conditions produces tetrahydro-1,2-oxazines in which the cis/trans relationship of the cyclopropanes is not conserved. Reacting nitrones with 2,3-cis-disubstituted cyclopropanes lead to 5,6-trans-oxazines, and 2,3-trans-disubstituted cyclopropanes lead to 5,6-cis-oxazines. This observed stereochemical inversion provides evidence for a stepwise annulation mechanism in the preparation of tetrahydro-1,2-oxazines.     

[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.     

Asymmetric [2 + 2] cycloaddition: total synthesis of (-)-swainsonine and (+)-6-epicastanospermine

[reaction: see text] An asymmetric total synthesis of (-)-swainsonine and (+)-6-epicastanospermine is described from a common intermediate, which is obtained through diastereoselective [2 + 2] cycloaddition of dichloroketene to a chiral enol ether.     

烯烃光敏氧化反应机理 II: 丙烯与单线态氧[2+2]环加成反应途径的解析

本文进一步报道丙烯与单线态氧[2+2]环加成反应途径的计算结果, 沿反应途径反应物间相互作用的详细情况和甲基在反应进程中的动态电子效应, 从而揭示烷基取代烯烃与单线态氧不易发生[2+2]环加成反应的原因. 为进一步探求单线态氧与烯烃[2+2]环加成反应的必要条件提供依据. 本文所用计算方法与前文相同.     

Synthesis of azapolycyclic systems via the intramolecular [4 + 2] cycloaddition chemistry of 2-(alkylthio)-5-amidofurans

A series of 2-(methylthio)-5-amidofurans containing tethered unsaturation were prepared via the reaction of dimethyl(methylthio)sulfonium tetrafluoroborate (DMTSF) with beta-alkoxy-gamma-dithiane amides 13-16 and 27-32 in 40-70% yield. Thermolysis of these furans resulted in an intramolecular Diels-Alder reaction (IMDAF). With the exception of 45 and 46, the oxa-bridged cycloadducts could not be isolated but immediately underwent a 1,2-methylthio shift to form bicyclic lactams in 60-100% yield. It was determined that incorporation of the amido carbonyl in the tether allowed the IMDAF reaction to proceed at a lower temperature. A dramatic example of this is seen in the IMDAF reaction of furan 35, in which the presence of an amido carbonyl as part of the dienophile tether, as opposed to the annealed ring (66) or the lack of a carbonyl (67), was necessary for the cycloaddition to occur. Furan 37, annealed to an azepine ring, underwent the IMDAF reaction at or below room temperature. To identify structural features that promote the IMDAF reaction, a computational study was undertaken. Ground-state and transition-state energies were calculated for furans 17, 33, 37, and 64 using the Becke 3LYP/6-31G model. The theoretical results were in good agreement with those observed. The results indicate that the incorporation of an amide carbonyl as part of the tether system works to place the dienophile in closer proximity to the furan ring, thereby lowering the activation energy needed to reach the transition state.     

Highly regio- and diastereoselective anionic [3 + 2] cycloaddition under phase transfer catalytic conditions

A highly trans and regioselective anionic formal [3 + 2] cycloaddition was achieved with allylic sulfones having an MBH acrylamide backbone under phase transfer organocatalytic conditions giving rise to the formation of unprecedented densely substituted cyclopentene derivatives.     

Synthesis of isoindolinones: Intramolecular [4+2] cycloaddition/retro [4+2] of pyridone propiolamides

A new synthesis of isoindolinones was discovered during a screening campaign aimed at the development of novel methods for the synthesis of pyridone-EZH2 inhibitor analogues. The reaction proceeds via an intramolecular [4+2] cycloaddition of a pyridone with a tethered propiolamide moiety followed by extrusion of isocyanic acid. The discovery, optimization, and scope of the methodology are described.