Solvent dependence of singlet oxygen / substrate interactions in ene-reactions, (4+2)- and (2+2)-cycloaddition reactions

Product formation of singlet oxygen reactions with simple olefins occurring as ene-reactions, (4+2)- and (2+2)-cycloaddition reactions is independent on solvent polarity. Thus, 2,3-dimethyl-2-butene (1) and 2-methy]-2-butene (3), 1,3-cyclohexadiene (6), and benzvalene (8) yield allylic hydroperoxides (2) and(4) (54%) + (5) (46%), endoperoxide (7), and dioxetane (9), respectively. The rates of the ene-reactions and (4+2)-cycloaddition reactions are only slightly dependent, those of the (2+2)-cycloaddition reaction, however,are clearly dependent on solvent polarity. “Physical” quenching of singlet oxygen by the olefins is negligible, but substantial by the sensitizer tetraphenylporphin (TPP) in chlorinated solvents.     

Photooxygenation of the helimers of (-)-isocolchicine: regio- and facial selectivity of the [4 + 2] cycloaddition with singlet oxygen and surprising endoperoxide transformations

Photooxygenation of the helimeric mixture of (-)-(M,7S)/(P,7S)-isocolchicine (6) with the superdienophile singlet oxygen has been studied. Cycloaddition occurred with high regioselectivity at the 7a,11-positions of the alkaloid and predominantly at the diene face anti to the amidic substituent at the stereogenic center C-7, leading to two endoperoxides 7 (syn) and 8 (anti) with an 1:7 ratio. The structure of the minor product 7 was established by X-ray analysis. Investigation of the triethylamine induced transformation of the predominant endoperoxide 8 furnished a mixture of two isomers (M,7S)-10a/(M,7S)-10b in a 2:1 ratio possibly with constitutional interconversion and with (M,7S)-9 as plausible intermediate. Treatment of this mixture with silica gel/ethyl acetate at ambient temperature surprisingly led to an atropenantiomerically pure colchicinoid (M,7S)-12 characterized by an eightmembered oxocine B-ring, the structure and absolute configuration of which could be determined by X-ray analysis. For the unprecedented formation of the novel colchicinoid (M,7S)-12 a plausible reaction pathway is suggested, involving a complete transfer of the (M) helical asymmetry of the intermediate (M)-11 into (S) asymmetry of the newly formed carbon center of (M,7S)-12. Prerequisite for such a scenario is the configurational stability of the intermediate pseudobiaryl (M)-11, under the conditions employed, allowing to transmit the axial chirality onto the chiral center of the product (M,7S)-12.     

Thiaozonide formation by singlet oxygen cycloaddition to 2,5-dimethylthiophene

Singlet oxygen reacts with 2,5-dimethylthiophene (1) exclusively by (4+2)-cycloaddition to yield the thiaozonide 2. The structure of this product is inferred from its ¹H- and ¹³C NMR spectra. Neat thiaozonide 2 decomposes violently at room temperature. In aprotic non-polar and in protic polar solvents it is slowly transformed into cis-sulfine 3c and cis-2,5-dione 4c, which rearranges to the trans-isomer 4t. A mechanism for the transformation reaction is proposed.     

Ruthenium-catalyzed (2 + 2) intramolecular cycloaddition of allenenes

We report a ruthenium-catalyzed (2 + 2) intramolecular cycloaddition of allenes and alkenes. We have found that the use of the ruthenium complex RuH(2)Cl(2)(P(i)Pr(3))(2), which has previously gone unnoticed in catalytic applications, is crucial for the observed reactivity. The reaction proceeds under mild conditions and is fully diastereoselective, providing a practical entry to a variety of bicyclo[3.2.0]heptane skeletons featuring cyclobutane rings.     

Novel ring chemistry of vitamin B(6) with singlet oxygen and an activated ene: isolated products and identified intermediates suggesting an operable [3 + 2] cycloaddition

Pyridoxine reaction with (1)O(2) in aqueous solution at neutral pH resulted in oxidation at the 2- and 6-positions of the pyridine ring and unprecedented ring contraction. Kinetic and low temperature studies provided observable intermediates by NMR spectroscopy. In addition, novel cycloaddition between pyridoxine and N-methylmaleimide, without N-alkylation and in water, suggest a common [3 + 2] cycloaddition with the 3-hydroxypyridine ring.     

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.     

Photooxygenation of pyrazin-2(1h)-ones

1-Alkyl-5,6-diphenylpyra2in-2(1H)-ones (1a-b) reacted with singlet oxygen in dichloromethane to afford the stable endoperoxide (1a-b), while in methanol to afford the 1:1-adduct (3a-b) of the endoperoxide (2a-b) and methanol.     

Ruthenium(II)-catalyzed [2 + 2 + 2] cycloaddition of 1,6-diynes with tricarbonyl compounds

In the presence of catalytic amounts of Cp*Ru(cod)Cl, unsymmetrical 1,6-diynes possessing a variety of functional groups reacted with electron-deficient tricarbonyl compounds at the ketone C=O double bonds to selectively afford dienones via electrocyclic ring opening of the expected alpha-pyrans. The intramolecular Michael addition of the cycloadducts having an acetyl and an alkylidenemalonate moiety gave bicyclo[3.3.0]octenone derivatives.     

Electro-initiated [2+2+2] cycloaddition of molecular oxygen,olefin, and 1,3-diketone

Electrochemical oxidation of 1,3-diketones in the presence of olefins under an atmosphere of oxygen gave the formal [2+2+2] cycloadducts, i.e. cyclic peroxides. A catalytic amount of electricity was sufficient for the reaction and an electro-initiated radical chain mechanism was suggested.     

Cycloheptatriene-norcaradiene-equilibrium cycloaddition reactions of the 1.2-benzocycloheptatriene with singlet oxygen and 4-phenyl-1.2.4-triazolin-3.5-dione

Cycloaddition reactions of the 1.2-benzocycloheptatriene with singlet oxygen and 4-phenyl-1.2.4-triazolin-3.5-dione (PTAD) have been studied. For the first time, addition of the PTAD to a cycloheptatriene derivative has been observed.     

Synthesis of C-arylglycosides via Ru(II)-catalyzed [2 + 2 + 2] cycloaddition

In the presence of catalytic amounts of Cp*RuCl(cod), the cycloaddition of 1,6-diynes with various C-alkynylglycosides proceeded at ambient temperature to afford C-arylglycosides in 46-93% yields.     

Theoretical study on the mechanism of the [2+1] thermal cycloaddition between alkenes and stable singlet (phosphino)(silyl)carbenes

The mechanism and the origins of the stereocontrol observed in the reaction between differently substituted alkenes and stable (phosphino)(silyl)carbenes giving cyclopropanes have been studied computationally. These cyclopropanation reactions proceed via asynchronous concerted mechanisms involving early transition structures with a significant charge transfer from the carbene to the alkene moiety. The geometric features of these transition structures preclude a significant overlap between the orbitals required for secondary orbital interactions between the reactants. The stereoselectivity observed experimentally stems from favorable electrostatic and steric interactions between the reactants leading to the stereoisomers in which the phosphanyl and carbonyl or aryl groups are cis to each other.     

Intramolecular cycloadditions of cyclobutadiene with dienes: experimental and computational studies of the competing (2 + 2) and (4 + 2) modes of reaction

Highly functionalized, cyclobutene-containing adducts are afforded through intramolecular cycloadditions between cyclobutadiene and tethered dienes. The cycloaddition displays the following reactivity trend: cyclobutadiene serves as a dienophile in intramolecular reactions when it is connected to the diene through a four-atom tether. In cases where a three-atom linker separates the two reaction partners, the cyclobutadiene can function as both a diene and dienophile, affording a mixture of vinylcyclobutane (2 + 2) and cyclohexene-containing cycloadducts (4 + 2). Theoretical studies provide insight into the factors influencing the various pericyclic pathways operative in this system. In cases where cyclobutadiene functions as a diene to generate vinylcyclobutanes, these (2 + 2) adducts can be converted into the corresponding (4 + 2) cyclohexenyl products through a [3,3]-sigmatropic rearrangement.     

High-pressure (2+2)cycloaddition of toluene-4-sulphonyl isocyanate to glycals

High-pressure (2+2)cycloaddition of toluene-4-sulphonyl isocyanate to glycals is examined. Reactions proceed regiospecifically to afford single products in case of all 3-substituted glycals. Upon heating or even after standing at room temperature adducts undergo retro-addition to give starting glycals. Various aspects of the cycloaddition are discussed, especially ret-ro-reaction and rearrangement of β-lactams to α,β-unsaturated amides.     

Ru(II)-catalyzed [2 + 2 + 2] cycloaddition of 1,2-bis(propiolyl)benzenes with monoalkynes leading to substituted anthraquinones

[2 + 2 + 2] cycloadditions of 1,2-bis(propiolyl)benzenes with monoalkynes were effectively catalysed by Cp*RuCl(cod) under mild conditions to give substituted anthraquinones in moderate to high yields.     

Rhodium(I)-catalyzed intramolecular pauson-khand-type [2 + 2 + 1] cycloaddition of allenenes

[reaction: see text] The novel [RhCl(CO)(2)](2)-catalyzed [2 + 2 + 1] cycloaddition of allenenes leading to the bicyclo[4.3.0]non-1(9)-en-8-one as well as the bicyclo[5.3.0]dec-1(10)-en-9-one skeletons has been developed. This method also provides a new procedure for the construction of the bicyclo[4.3.0]non-1(9)-en-8-one skeleton having an alkyl appendage at the ring juncture, which was hardly attained in a satisfactory yield by the Pauson-Khand reaction of the corresponding enynes.     

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.     

Organocatalytic enantioselective (3+2) cycloaddition using stable azomethine ylides

We have developed a highly efficient procedure for carrying out the catalytic enantioselective (3+2) cycloaddition between enals and stable azomethine ylides such as isoquinolinium and phthalizinium methylides. Under the optimized reaction conditions highly substituted chiral pyrroloisoquinolines and pyrrolophthalazines have been obtained in high yields and excellent diastereo- and enantioselectivities.     

Highly diastereoselective dioxetane formation in the photooxygenation of enecarbamates with an oxazolidinone chiral auxiliary: steric control in the [2 + 2] cycloaddition of singlet oxygen through conformational alignment

The photooxygenation of oxazolidinone-substituted enecarbamates leads to diastereomerically pure dioxetanes. The high diastereoselectivity is rationalized in terms of effective pi-facial control achieved by shielding one side of the double bond with the chiral auxiliary. The absolute configuration of the dioxetanes is assigned by derivatization to diols.