Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Bicyclo heptene system

CIDNP effects observed for norcarene [147, 178] supported a radical cation, 87 +, which spin and charge are delocalized between the olefinic group and the Walsh orbital of the (more highly substituted) internal cyclopropane bond. The bicyclo-heptene system, 87 +, appears to be more flexible than 86 + either the internal or the lateral cyclopropane bond can align with the alkene p-orbitals. Delocalization of spin and charge into the more highly substituted bond is preferred. [Pg.776]

Electrocyclic closure of butadiene units encased within cycloheptane rings has been used to obtain bicyclo[3.2.0]heptene systems (Scheme 5)12. For example, irradiation of eucarvone 21 led to the formation of adduct 22 in 52% yield via a disrotatory ring closure123. This adduct was used as a key intermediate in the synthesis of the pheromone grandisol, 23, which proceeded in 20% overall yield from 22. In their synthesis of a-lumicolchicine. Chapman and coworkers utilized a photochemically initiated four-electron disrotatory photocyclization of colchicine to produce /Murnicolchicine 24a and its /-isomer 24b in a 2 1 ratio12b. These adducts were then converted, in a second photochemical step, to the anti head-to-head dimer a-lumicolchicine 25. [Pg.268]

The bicyclo[3.2.0]heptene system can lead to the formation of 7-membered ring compounds. This method will be depicted by the following examples. [Pg.121]

Ring contraction of appropriately substituted 1,6-di- or tetrahalocycloheptatrienes or -cyclo-heptanones provides access to the bicyclo[4.1.0]heptene system. Thus l-bromo-6-chlorocyclo-hepta-l,3,5-triene reacted with butyllithium directly to alford benzocyclopropene (la) in 50% yield, presumably via a norcaradiene intermediate. The 2-methyl derivative lb was... [Pg.2869]

The enol ether double bond contained within the ds-fused dioxa-bicyclo[3.2.0]heptene photoadducts can also be oxidized, in a completely diastereoselective fashion, with mCPBA. Treatment of intermediate XXII, derived in one step from a Patemo-Buchi reaction between 3,4-dimethylfuran and benzaldehyde, with mCPBA results in the formation of intermediate XXIII. Once again, consecutive photocycloaddition and oxidation reactions furnish a highly oxygenated system that possesses five contiguous stereocenters, one of which is quaternary. Intermediate XXIII is particularly interesting because its constitution and its relative stereochemical relationships bear close homology to a portion of a natural product known as asteltoxin. [Pg.321]

An example of the vinylogous reactivity is the reaction of 52 with cyclopentadiene (Tab. 14.9) [77]. Rhodium(II) acetate-catalyzed decomposition of 52 in dichloro-methane, yields a 2 1 mixture of the bicyclic system 53 derived from the [3-1-4] cycloaddition, and the bicyclo[2.2.1]heptene 54 resulting from electrophihc attack at the vinylic position followed by ring closure. When Rh2(TFA)4 is used as the catalyst, bicy-clo[2.2.1]heptene 54 becomes the dominant product, while the reactivity of the vinyl terminus is suppressed using a hydrocarbon solvent as observed in the Rh2(OOct)4-cat-alyzed reaction in pentane, which affords a 50 1 ratio of products favoring the [3-1-4] cycloadduct 53. [Pg.314]

Michelotti, F. W., and W. P. Keaveney Coordinated Polymerization of the Bicyclo[2,2,l]-heptene-2 Ring System in Polar Media. Paper presented at New York Meeting Sep. 1963, Amcr. chem. Soc. Division of Polymer Chem. Polymer Preprints Vol. 4, No. 2, 293. [Pg.95]

The results of the hydroformylation of internal olefins are reported in Table 9. In the case of (Z)- and (E)-2-butene, the same fare of the unsaturated carbon atom is formylated with either a rhodium- or platinum (—)-DIOP-containing catalytic system. With the rhodium catalyst, when an acyclic olefin is used as the substrate, the same fare is always attacked, and it is only the notation but not the geometric requirement that is different for (E)-l-phenyl-1-propene. The only exception is represented by bicyclo[2,2,l]heptene. However, using (—)-CHIRAPHOS instead of (—)-DIOP, also bieyelo[2,2,l]heptene behaves like internal butenes. No regularity is observed for the cobalt or ruthenium (—)-DIOP catalytic systems. With the same system, only in 3 cases out of 15 the face of the prochiral atom preferentially formylated has different geometric requirements. [Pg.97]

Many examples of polycyclic alkene osmylations have been reported in the literature in connection with the syntheses of specific target molecules such as alkaloids, prostanoids, or steroids. However, carefully determined diastereomer ratios are usually not available. Due to the varied nature of these substrates, it is not possible to formulate definite rules for diastereo-face differentiation except in specific cases. Thus, for example, the exclusive exo reactivity of bridged systems such as bicyclo[2.2.1]heptene derivatives (norbornene-type) is well known as Alder s rule of exo addition63. [Pg.72]

Weiterhin konnen an das Thiophen-System C-C-Mehrfachbindungen cyeloaddicrt werden. Mit 2,3-Dimethyl-maleinsaureanhydrid ergibt die sensibilisierte Photolyse 6,7-Dimethyl- 2-thia-bicyclo[3.2.0]hepten-(3)-6,7-dicarbonsaure-anhydrida ... [Pg.558]

In general, excellent diastercoselectivities are observed in the catalytic hydrogenation of bicyclic systems. Bicyclo[2.2.1]heptenes, such as 1, are hydrogenated to the corresponding saturated bicyclo[2.2.1]heptanes yielding nearly exclusively the ewfo-c/.y-product, independent of the catalyst, the reaction conditions and the solvent53- 54. [Pg.973]

Reaction of several bicyclo[2,2,l]heptenes with phenyl(trichloromethyl)mer-cury reveals the fact that born-2-ene (281) (the synthesis of which has just appeared in Organic Syntheses ) is completely unreactive to this substance, because steric hindrance prevents the cxo-attack required by torsional strain. The labelled bornyl-isobornyl Grignard mixture (282) undergoes cis-exo eliminative transfer to phenyl isopropyl ketone, although to a lesser extent than in the norbornyl system, this probably being a reflection of the fact that the gem-dimethyl group reduces the energy difference between exo- and e/ido-transfer. ... [Pg.63]

Cyclopentadiene is a popular moiety to release in a retro Diels-Alder reaction, as the cyclohexene portion of the bicyclo[2.2.1]heptene is locked in a boat conformation, as required in the transition state for the retro (and indeed forward Diels-Alder) reaction. Other bridged bicyclic systems are also more prone to undergo retro Diels-Alder reaction. The bridged adduct from fiiran and maleic anhydride readily undergoes the retro reaction and, although the endo isomer is formed at a much faster rate, the reversible nature of the reaction leads to the accumulation of the more stable exo isomer. [Pg.200]

Another system in which the effects of steric resistance to reagent approach have been studied systematically is the bicyclo[2.2.1]heptene ring system. The stereochemistry of a number of reactions has been studied with the parent hydrocar-... [Pg.118]

In fact, the intramolecular carbonylation of ene—yne systems (e.g. l-heptene-5-yne) affords bicyclo-[3.3.0]-l-octene-3-ones which are suitable precursors for these natural products [5j]. [Pg.113]


See other pages where Bicyclo heptene system is mentioned: [Pg.260]    [Pg.260]    [Pg.136]    [Pg.328]    [Pg.136]    [Pg.373]    [Pg.914]    [Pg.442]    [Pg.139]    [Pg.129]    [Pg.651]    [Pg.1001]    [Pg.460]    [Pg.173]    [Pg.1052]    [Pg.632]    [Pg.367]    [Pg.31]    [Pg.91]    [Pg.242]    [Pg.209]    [Pg.143]    [Pg.219]    [Pg.213]    [Pg.182]    [Pg.16]    [Pg.149]    [Pg.329]   
See also in sourсe #XX -- [ Pg.651 , Pg.652 , Pg.659 ]




SEARCH



1-Heptene

2- Hepten

5- -bicyclo heptene

Bicyclo hepten

Bicyclo heptenes

Bicyclo system

Heptenal

© 2024 chempedia.info