Anionic ring opening of norbornenes fused to heterocycles

The 2-hydroxy and 2-oxo derivatives of 1,2,3,4-tetrahydro-1,4-methanophenazine were prepared and found to evolve in basic media through the opening of their bicyclo[2.2.1]heptene moiety, affording 2,3-dihydro-1H-cyclopenta[b]quinoxaline derivatives with two-carbon 1-substituents that depend on the starting compound. In the case of 2-hydroxy starting compounds, ring-opening occurs regardless of the orientation of the hydroxyl group, and in methanolic solution is spontaneous, though slow, even in the absence of added base (at least in the case of the endo derivative). It is presumably favoured by the steric strain of the heteroaryl-fused bicyclo[2.2.1]heptene moiety, and is hypothesized to involve the base-promoted formation of anionic intermediates that are stabilized by the π-deficient nature of the quinoxaline system.     

Nickel(0)-induzierte c—c-verknüpfung zwischen alkenen und kohlendioxid

The oxanickelacyclopentanone-derivate (I) can easily be prepared from norbornene (bicyclo[2.2.1]heptene), CO₂ and nickel(0) compounds.     

On the synthesis of β-bromohydrine ethers via intermolecular alkoxyl radical addition to bicyclo[2.2.1]heptene

Primary, secondary, and tertiary alkoxyl radicals add exo-selectively to the olefinic π-bond in bicyclo[2.2.1]heptene to afford exo-2-alkoxybicyclo[2.2.1]hept-3-yl radicals, which are trapped with BrCCl₃ preferentially from the endo face to furnish β-bromohydrine ethers in 23-67% yield.     

Enantiomeric discrimination of cyclic β-amino acids using (18-crown-6)-2,3,11,12-tetracarboxylic acid as a chiral NMR solvating agent

(18-Crown-6)-2,3,11,12-tetracarboxylic acid is an excellent chiral NMR solvating agent for cyclic β-amino acids with cyclopentane, cyclohexane, cycloheptane, cyclopentene, cyclohexene, bicyclo[2.2.1]heptane, and bicyclo[2.2.1]heptene rings. The crown ether was added to the neutral β-amino acids in methanol-d₄. A neutralization reaction between the crown ether and β-amino acid forms the ammonium ion needed for favorable association. Enantiomeric discrimination of the two hydrogen atoms α to the amine and carboxylic acid moieties of the β-amino acid was observed with every substrate studied. Trends in the order of the enantiomeric discrimination of certain hydrogen atoms for substrates of similar structures correlate with the absolute configuration.     

(η3-allyl)palladium(II) catalysts for the addition polymerization of norbornene derivatives with functional groups

Cycloaliphatic polyolefins with functional groups were prepared by the Pd(II)-catalyzed addition polymerization of norbornene derivatives. Homo- and copolymers containing repeating units based on bicyclo[2.2.1] hept-5-en-2-ylmethyl decanoate (endo/exo-ratio = 80/20), bicyclo[2.2.1]hept-5-ene-2-carboxylic acid methyl ester (exo/endo = 80/20), bicyclo[2.2.1]hept-5-ene-2-methanol (endo/exo = 80/20), and bicyclo[2.2.1]hept-5-ene-2-carboxylic acid (100% endo) were prepared in 49–99% yields with {(η³-allyl)Pd(BF₄)} and {(η³-allyl)Pd(SbF₆)} as catalysts. The catalyst containing the hexafluoroantimonate ion was slightly more active than the tetrafluoroborate based Pd-complex.     

Synthesis of bi- and tricyclic acrylates in the presence of boron trifluoride etherate

The addition of acrylic acid to bicyclo[2.2.1]heptene hydrocarbons and tricyclo[5.2.1.0^2,6]deca-3,8-diene catalyzed with BF₃·O(C₂H₅)₂ was studied and bi- and tricyclic esters of acrylic acid were synthesized that were reactive monomers for preparation of macromolecular compounds.     

Hydrosilylation of bicyclo[2.2.1]heptene derivatives by trichlorosilane with the use of Ni catalytic systems A study of the stereochemistry of the process

Hydrosilylation of endo- and exo-cyanonorbornenes, endo-methylnorbornene and camphene by trichlorosilane on a Ni catalytic system was investigated. Based on the spectral data obtained from ¹³C and ¹H NMR studies, using Eu(fod)₃ as the shift reagent and by employing the double resonance technique, the stereochemistry of the process has been studied. Stereoselective exo-addition of hydrosilane to bicyclo[2.2.1]heptene derivatives has been noted.     

On the exceptional reactivity of the norbornene double bond

The anomalous reactivity of the bicyclo [2.2.1] heptene double bond is interpreted in terms of hyperconjugative effects, leading to an exceptionally low endo out-of-plane deformation potential.     

Steroid photochemistry : The photocycloaddition of an enone and dienone to norbornene and norbornadiene

The photocycloaddition of a steroidal 4-en-3-one and a 4,6-dien-3-one to bicyclo[2.2.1]heptene and bicyclo[2.2.1]heptadiene has been studied. The photocycloaddition to norbornene furnished only the trans-4α,5β-cyclobutane, while the enone gave a mixture of cis and trans-[2+2] adducts as well as the first example of hydrogen transfer in this series to form a 4-(2-norbornanyl 1)-conjugated enone. The photocycloaddition of the dienone to norbornadiene formed the 4-(7-norbornenyl)-dienone by a 1,4-hydrogen shift as the major product, together with lesser amounts of the unrearranged trans-4α,5β-[2+2]-cyclobutane. On the other hand, the cycloaddition of the enone to norbornadiene yielded the 4α-(7-norbornene)-β,γ-enone as the major product; accompanied again by the unrearranged trans-[2+2]-adduct. In addition, the 4α,5α-cis-[3+2] adduct was obtained and was formed by rearrangement of the norbornenyl radical portion of the intermediate diradical.     

Stereochemistry of cyclobutanone resulted from cycloadditions of t-butylcyanoketene to bicyclo[2.2.1]heptene derivatives, as evidence for a π2s+π2a reaction mode

The stereochemistry of the cyclobutanones 1-7, resulted from the reaction of t-butylcyanoketene with bicyclo[2.2.1]heptene, bicyclo[2.2.1]heptadiene, 1,4 - dihydro - 1,4 - methanonaphthalene, 1,4 - dihydro - 9 - (1 - methylethylidene) - 1,4 - methanonaphthalene, 1,4 - dihydro - 1,4 - epoxynaphthalene, l,4,4a,8b - tetrahydro - 1,4 - methanobiphenylene (l,4,4a,8b) and 1,4,4a,8b - tetrahydro - 1,4 - methanobiphenylene(1,4,4aβ,8bβ) was established as having the cyclobutanone ring exo and the t-Bu group in the configuration. These findings represent a stereochemical argument in favour of a _π2_s + _π2_a reaction mode of t-butylcyanoketene to the above mentioned bicyclo[2.2.1]heptene derivatives. Observations regarding preservation of the original configurations of alkenes as well as the geometrical distorsion of the cyclobutanones are shortly discussed.     

Oxabicyclo[3.2.1]octane derivatives as highly reactive dienophiles: synthesis of bicyclo[5.n.0] systems

[reaction: see text] We have developed highly versatile, homochiral oxabicyclo[3.2.1]octadiene building blocks for the synthesis of natural products. We have found that these bridged alkenes undergo exceptionally facile Diels-Alder reactions and react faster than several well studied bicyclo[2.2.1]heptene dienophiles. The reaction proceeds with high levels of stereochemical control and in very good to excellent yields, providing access to bicyclo[5.4.0]undecane and bicyclo[5.3.0]decane systems. This reactivity is attributed to strain and homoconjugation effects.     

Domino ring-opening metathesis-ring-closing metathesis of bicyclo[2.2.2]octene derivatives: scope and limitations

Domino metathesis involving ring-opening metathesis-ring-closing metathesis (ROM-RCM) of a bicyclo[2.2.2]octene derivative having an appropriate alkene chain, expected to produce a 7/6 fused bicyclic system, provided a decalin system in contrast to ROM-RCM of the corresponding bicyclo[2.2.1]heptene analogues, which as expected produced the 7/5 fused bicycles. The expected 6/7 bicyclic system could, however, be made through RCM of the elusive ROM product prepared from the same bicyclo[2.2.2]octene analogue by a nonmetathetic route. A rationale to explain the difference in reactivity pattern between these two systems toward ROM-RCM has been forwarded.     

One-pot palladium-catalyzed synthesis of 2,3-disubstituted bicyclo[2.2.1]heptanes and bicyclo[2.2.1]hept-5-enes

A new and highly stereoselective palladium-catalyzed synthesis is reported, based on two subsequent insertions of the bicyclo[2.2.1]heptene system into an aryl or vinylpalladium bond, formed in situ from aryl or vinyl bromides.     

Synthetic studies toward longeracemine: a SmI2-mediated spirocyclization and rearrangement cascade to construct the 2-azabicyclo[2.2.1]heptane framework

Longeracemine, a member of the Daphniphyllum family of alkaloids contains a novel carbon framework featuring a highly functionalized 2-azabicyclo[2.2.1]heptane core as part of an overall 5/6/5/5/6/5 skeleton. A synthetic intermediate containing the core of longeracemine has been efficiently prepared by employing a stereoselective SmI₂-mediated cascade reaction to advance a 7-azabicyclo[2.2.1]heptadiene to a 2-azabicyclo[2.2.1]heptene that is functionally poised for conversion to the natural product.

A synthetic intermediate containing the core of longeracemine, that is functionally poised for conversion to the natural product, has been efficiently prepared by employing a stereoselective SmI₂-mediated cascade reaction.     

Neighbouring effect in the course of the ozonolysis of a hindered bornene derivative

In the course of the ozonolysis of the bicyclo[2.2.1]heptene anhydride 1, the three bis-lactones 3-5 have been obtained (the structures were confirmed by X-ray crystallographic analysis).     

Reaktionen Gespannter C—C Einfachbindungen Mit Übergangsmetallen : XI. Eisencarbonylderivative Des Homosemibullvalen

The light-induced reaction of homosemibullvalene with Fe(CO)₅ under kinetic control gives iron carbonyl compounds resulting from exo-attack at the bicyclo[3.1.0]hexene unit and a bis-allyl complex from endo-attack at the divinylcyclopropane unit; an iron carbonyl complex derived from the bicyclo[4.1.0]heptene part has been obtained on thermodynamically-controlled reaction with Fe₂(CO)₉ in refluxing hexane.     

Pyridazines. LXX. Reactions of azidoazolopyridazines with unsaturated compounds

Cycloaddition reactions of azidoazolopyridazines with unsaturated esters, cyclic enol ethers, styrene, 2-vinyl-pyridine, bicyclo[2.2.1]heptene, dicyclopentadiene and dehydrobenzene were investigated. The reaction proceed via the intermediate Δ² -1,2,3-triazolines which were in most cases termolabile and decomposed further to give the final products which were enamines, imines of fused aziridines.     

Photoreaction of 5-dicyanomethylenebicyclo[2.2.1]hept-2-ene

Irradiation of 5-dicyanomethylenebicyclo[2.2.1lhept-2-ene induces new types of photoreactions, i.e., unprecedented skeletal rearrangement leading to 7,7-dicyano-6-methylenebicyclo[3.1.1]hept-2-ene, and novel 1,3-carbon shifts to bicyclo[3.2.0]- and bicyclo[4.1.0]heptene.     

Inductive Hydroxyhalogenation of Bicyclo[2.2.1]heptenes and Synthesis of Amino Alcohols from the Reaction Products

A method of synthesis of aminobicycloheptanols by the hydroxyhalogenation of bicyclo[2.2.1]- heptene and its alkyl derivatives in a cycloolefin?oxidant?hydrogen halide system to induce electrophilic intermediates followed by substitution of the halogen atoms by amino groups of different compositions and structures. It was found that the selectivity of the reaction depends on the formation/consumption rate ratio for the electrophilic intermediate. A high selectivity with respect to hydroxyhalides takes place, when the consumption rate of the latter intermediate is higher than its formation rate. An increase in the pH of the medium becomes higher than 10.5 decreases the yield of amino alcohols and changes the reaction direction due to acceleration of dehydrohalogenation and formation of the corresponding epoxides.     

Total chemical synthesis of 2-ethenyl-3,5-dimethylpyrazine and 3-ethenyl-2,5-dimethylpyrazine

The aroma compounds 2-ethenyl-3,5-dimethylpyrazine 1 and 3-ethenyl-2,5-dimethylpyrazine 2 were synthesized via a new chemical route. The key steps of the synthesis involve cyclocondenzation of 1-[bicyclo[2.2.1]5-hepten-2-yl]-1,2-propanedione and 1,2-propanediamine, aromatization of the resulting 5,6-dihydropyrazines, and subsequent Retro-Diels-Alder reaction to generate pyrazines 1 and 2. Pyrazine 1, a powerful odorant, was obtained in large excess (8:2) when endo-1-[bicyclo[2.2.1]5-hepten-2-yl]-1,2-propanedione was used as intermediate substrate.