Cycloaddition precursor cyclopentadiene

As was mentioned, cycloaddition of unactivated hydrocarbons, namely, that of cyclopentadiene, has practical significance. 5-Vinyl-2-norbomene is produced by the cycloaddition of cyclopentadiene and 1,3-butadiene546,547 [Eq. (6.96)] under conditions where side reactions (polymerization, formation of tetrahydroindene) are minimal. The product is then isomerized to 5-ethylidene-2-norbomene, which is a widely used comonomer in the manufacture of an EPDM (ethylene-propylene-diene monomer) copolymer (see Section 13.2.6). The reaction of cyclopentadiene (or dicyclopentadiene, its precursor) with ethylene leads to norbomene548,549 [Eq. (6.97)] 550... 

A useful observation on the cycloaddition of cyclopentadiene and methyl acrylate catalyzed by an aluminum complex of VAPOL 63 is the autoinduction of asymmetry in the presence of an aldehyde or ester (including malonates). With a complex derived from a bisoxazoline ligand and CuBtj-AgSbF the cycloaddition between cyclopentadiene and 2-phenylthioacrylic esters is highly stereoselective, providing chiral precursors of (15,4S)-norbomenone in good yields. 

Direct Wittig reaction of Ph PCHCOn,Me with the four unsubstituted D-aldopentoses followed by acetylation provides convenient preparative access to acyclic seven-carbon trans-2.3-unsaturated sugar derivatives. These products served as dienophiles for a detailed comparative study in Diels—Alder cycloaddition with cyclopentadiene. Related syntheses afforded analogous cis-dienophiles. Cycloaddition under uncatalyzed thermal conditions gave mixtures of the four possible stereoisomeric norbornene adducts. The endo, exo ratios, and diastereofacial selectivities of the adducts were determined by NMR spectroscopy and by chemical transformations, supplemented by selected X-ray crystallographic analyses. Different distributions of isomers were encountered when a Lewis acid was used to catalyze the cycloaddition. The reaction can be controlled to provide preparative access to selected isomers and thus constitutes a versatile method for chirality transfer from the precursor sugar to four new asymmetric centers in a carbocyclic framework. 

The intramolecular Diels-Alder cycloadditions of 1-aminobenzofurans provide a simple synthesis of benzo[/r]quinolines. A-Acetyl-2-azetine undergoes facile 4-1-2-cycloaddition with cyclopentadiene, 5,5-dimethoxy-1,2,3,4-tetrachlorocyclopentadiene, hexachloropentadiene, and diphenylisobenzofuran to produce exclusively endo-cyc o-adducts. The highly strained cyclic alkyne bicyclo[2.2.1]hept-2-en-5-yne (108), prepared from the heptavalent iodine precursor (107), has been trapped using 1,3-diphenylisobenzofuran to yield the adduct (109) (Scheme 41). The reaction of sodium dinitroxytrioxide (110) with 1,3-diphenylisobenzofuran gives an initial 4-1-2-cycloadduct (111), which rearranges to the final product (112) (Scheme 42). " The... 

The first models for asymmetric induction in [2 + 2] cycloadditions are ketimines derived from precursors such as (25), which react with both cyclic and acyclic alkenes to give adducts with high enantiomeric excesses. Full details have been reported on the regio- and stereo-specificity of the addition of dichloroketene to 1-substituted cyclohexenes. Methyl (phenylthiomethyl)-ketene (26) provides a further example of a ketene which, after cycloaddition to cyclopentadiene, undergoes facile opening of the cyclobutane that is thus formed to provide vicinally substituted cyclopentene derivatives. ... 

Due to the two electron-donating groups in the bicyclic product 150 and the unhydrolyzed precursor of 148, they should be quite reactive dienes in Diels-Alder reactions. However, such [4+2] cycloadditions were observed only for the cyclohexane-annelated cyclopentadienes 151b, which equilibrate with the more reactive isomers 154 by 1,5-hydrogen shifts (Scheme 33). The [4+2] cycload-... 

Microwave heating has also been employed for performing retro-Diels-Alder cycloaddition reactions, as exemplified in Scheme 6.94. In the context of preparing optically pure cross-conjugated cydopentadienones as precursors to arachidonic acid derivatives, Evans, Eddolls, and coworkers performed microwave-mediated Lewis acid-catalyzed retro-Diels-Alder reactions of suitable exo-cyclic enone building blocks [193, 194], The microwave-mediated transformations were performed in dichloromethane at 60-100 °C with 0.5 equivalents of methylaluminum dichloride as catalyst and 5 equivalents of maleic anhydride as cyclopentadiene trap. In most cases, the reaction was stopped after 30 min since continued irradiation eroded the product yields. The use of short bursts of microwave irradiation minimized doublebond isomerization. 

Compared with the reaction with cyclohexadiene, the diastereoselectivity of the Diels-Alder reaction of 122 with cyclopentadiene was found to be lower under a variety of reaction conditions. In the latter case, the cycloaddition afforded the [2.2.1] bicyclic salt 129, which was further converted to 130 — the precursor of a carbocyclic L-nucleoside [95] (Scheme 10.41). 

The Diels-Alder reaction of these chiral dioxinones with cyclopentadiene has provided a highly stereoselective route to carbocyclic C-nucleoside precursors (27) (Scheme 11) <89CPB2615>. The [2 + 2] cycloaddition of the same menthone-derived dioxinone with cyclopentene has also been observed <87CPB3539>. 

A series of [2.2.1]bicycloheptenyl (norbornene) functional prepol)nners have been prepared via the cycloaddition reaction of cyclopentadiene monomer with corresponding acrylics. When these materials are formulated with an appropriate multifuntional thiol crosslinker and photoinitiator and irradiated, a rapid, exothermic, crosslinking reaction takes place. When the acrylic precursors are organic resins, the derived polymers behave like toughened plastics. The choice of a norbornene functional polydimethylsiloxane precursors gives elastomeric products. 

DicWoroketene is particularly reactive, and reductive dechlorination of the product with zinc and acetic acid allows access to the cyclobutanone from formal addition of ketene itself. Thus, cycloaddition of dichloroketene with cyclopentadiene, followed by dechlorination and Baeyer-Villiger oxidation gave the lactone 173, a usehil precursor to various oxygenated cyclopentane products (3.117). Intramolecular cycloaddition reactions of ketenes can allow the formation of bicyclic and polycyclic products using otherwise unstable ketene intermediates. ... 

Cycloadditions and Rearrangements. The addition of 2-oxyallyl cations to furan provides a route to oxabicyclo[3,2,l]octanes complementary to the cycloaddition of cyclopropanones to furan. Careful experimental studies have led to yields of preparative importance both with furan and with cyclopentadiene. Following the route to azabicyclo-octanes, dipolar addition to the pyrylium betaine (99) affords oxa-analogues (Scheme 23). Also reported are the addition of fiiran to 1-cyanonaphthalene, the formation of various cycloadducts of tropone and tropolone (Scheme 24), and the phototransformations of (100) (Scheme 25) and (101) (Scheme 26). Thermal addition gives (102) from (103) and similarly other 8-oxabicyclo-octanes are prepared from acyclic precursors. ... 

We became interested in this area of chemistry because we wished to prepare some new and highly electrophilic 2//-azirines with potential for use as dienophiles in the Diels-Alder reaction. Vinyl azides appeared to be the most promising precursors. Previously there had been only one report of the cycloaddition of 2H-azirines to a simple diene (cyclopentadiene) although highly activated dienes such as tetraphenylcyclopentadie-none and 1,3-diphenylisobenzofuran had been used to intercept some transient 2H-azirines. Our investigations led to the preparation of several new 2ff-azirines. Cycloaddition reactions with these provided access to some novel fused-ring aziridines. An outline of the results is included in Sections 6.2 and 6.3. 

The cyclotrimerization of alkynes catalyzed by transition metals is a general method for building substituted benzenes from aliphatic precursors. Multiple bonds are formed in these reactions in a single operation. Although the reaction of thermal trimerization relates to allowed electro-cyclic processes, it is catalyzed by several transition metals, such as Co, Ni, Rh, Pd, Rh, and Ru [38]. Most recent publications show promise for the participation of transition metal complexes in [2+2+2] cycloaddition reactions based on zirconium, titanium, and indium [9]. This reaction has synthetic potential for using metallocyclopentadienes as intermediates in the cyclotrimerization of alkynes. The reaction mechanism is shown in Scheme 2.1 [3, 38]. Two alkyne molecules coordinated to the metal, that is, complex 2.1, couple to form cyclopentadiene 2.2. Next there is either addition of the alkyne to the metallocycle 2.3 to form the metallocycle... 

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