Cyclopentadiene, adduct formation with

Cyclooctanone, condensation with diethyl carbonate, 47, 20 Cyclopentadiene, adduct formation with 1,2,3-benzothiadiazole 1,1-diox-ide, 47, 8... 

Cookson et al. first reported prior to 1965 that photochemical reaction of the Diels-Alder endo-adducts of cyclopentadiene or cyclohexa-1,3-diene with p-benzoquinone gave cage compounds by formation of a cyclobutane ring from the two double bonds. Irradiation of an ethyl acetate solution of the cyclopentadiene adduct 36 with a medium pressure Hg lamp for 6 h afforded cage 37 (90%). This photoreaction proceeded even in the soHd state, and the cage compound 37 was obtained in 80% yield after 80 h irradiation. Similarly, the 1 1 adduct 38 of cyclohexa-l,3-diene and p-benzoquinone was converted into cage 39 by irradiating the ethyl acetate solution for 10 h (80%) or the soHd state for 90 h (90%). This reaction was successfully appKed to the adduct 40 of cyclooctatetraene (COT) and p-benzoquinone (92%... 

A DFT study of the polar Diels-Alder reaction of 4-aza-6-nitrobenzofuroxan with cyclopentadiene found only one highly asynchronous transition state structure associated with the formation of the 4 + 2-adduct.178 4-Nitrobenzodifuroxan has been shown to be a highly reactive nitroalkene in Diels-Alder reactions with common dienes (cyclopentadiene) to produce stable NED adducts and with ethyl vinyl ether to produce IED adducts.179 Unlike a-acylfuran, 2-nitrofurans have been shown to be active dienophiles in thermal NED Diels-Alder reactions with a variety of buta-1,3-dienes, including Danishefsky s diene.180... 

The Arrhenius diagrams gi for the rate ccxistants pertaining to Uk reacticxi of (V,f) with NQ the values reported in Table 23. The comparison of our kinetic data with those available for the cyclopentadiene-NQ adduct shows that (V,f)-NQ adduct is less stable by 9.1 Kcal/mole, but its formation activation energy is lower by 5,7 Kcal/mole. Also the frequency factor is lower in the case of (VJF)-NQ formation, and aU the data suggest that the steric hindrance existing on tl cyclopentadienyl ring of (V/) exerts its influence on the di odation reaction rather than on the adduct formation. 

Thus photolysis of the tosylhydrazone sodium salt of 5//-dibenzo[a,c]cyclohepten-5-one (14) at — 60 °C in the presence of cyclopentadiene or furan with tetrahydrofuran as cosolvent gave the cyclopropene Diels-Alder adducts 17 and 18 in 73 and 47% yields, respectively. If the photolysis was stopped shortly after all the tosylhydrazone salt had decomposed, adduct 17 was the only isomer found in the cyclopentadiene reaction. In the formation of the furan adduct 18, the reaction was not so clean and a number of unidentified products were also formed. Unfortunately, adduct 18 is thermally unstable at the temperature necessary for thermal formation of carbene 15. No trace of adduct 18 was detected when the arylcarbene 19 was generated directly from its tosylhydrazone salt in the presence of furan. ... 

Block and Aslam98 have reported sulfene formation as in reaction 14. It proceeds readily at room temperature in two hours, and has certain advantages in comparison with the reaction of methanesulfonyl chloride with triethylamine. These include formation of the sulfene adduct with cyclopentadiene, bromomethanesulfonyl bromide with bromine,... 

Catalysts that yield highly syndiotactic polypropylene (86% racemic pentads) were also developed. One of them is /-propylene(ri -cyclopentadienyl-r fluorenyl)zirconium dichloride [291]. Initial disclosures of metallocene catalysts were followed by numerous publications in the literature that described similar materials for the polymerizations of either ethylene or propylene, or both, and for formation of various copolymers. Thus, for instance, Kaminsky et al. [292], reported preparaticm of a zirconium dichloride-type catalyst for copolymerization of cyclic olefins with ethylene. These cyclic olefins are cyclopentene, norbomene, and the hindered cyclopentadiene adducts of norbomene [292]. The catalytic system consists of a bridged indene derivative that is combined with methylaluminoxane ... 

The same researchers found that lanthanide triflates could catalyze aldol reactions and allylations in aqueous media [10]. However, Wang et al. exploited the potential of lanthanide triflates to act as stable Lewis-acid catalysts in the aqueous imino Diels-Alder reaction [11]. This variant of the aqueous heterocycloaddition protocol also expands the scope of such reactions. The use of higher aldehydes in such reactions generally met with limited success under the conventional protocol. For example, under the standard conditions, the reaction of hexanal and benzylamine hydrochloride with cyclopentadiene in water is sluggish and affords only 4% of the Diels-Alder adducts 26 and 27 in a 2.7/1 ratio. In sharp contrast, the addition of various lanthanide(lll) triflates (0.25 M) to this reaction results in substantial increases in both the rate and yield of Diels-Alder adduct formation. In particular, use of praseodymium(iii) triflate results in a 68% yield of adducts 26 and 27 ... 

Tetraarylbutatrienes react with cyclopentadienone across its center double bond to form the Diels-Alder adducts In contrast, the butatrienylsulfonium salts 30 (R = Me, Et X = Cl, Br, PFe) react with cyclopentadiene to give 1 1 mixtures of the isomeric cycloadducts 31 and 32 in high yields. In these reactions the terminal double bonds are involved in adduct formation 2. ... 

Vinylboranes are interesting dienophiles in the Diels-Alder reaction. Alkenylboronic esters show moderate reactivity and give mixtures of exo and endo adducts with cyclopentadiene and 1,3-cyclohexadiene (441). Dichloroalkenylboranes are more reactive and dialkylalkenylboranes react even at room temperature (442—444). Dialkylalkenylboranes are omniphilic dienophiles insensitive to diene substitution (444). In situ formation of vinyl-boranes by transmetaHation of bromodialkylboranes with vinyl tri alkyl tin compounds makes possible a one-pot reaction, avoiding isolation of the intermediate vinylboranes (443). Other cycloadditions of alkenyl- and alkynylboranes are known (445). 

Diels-Alder reaction of 2-bromoacrolein and 5-[(ben2yloxy)meth5i]cyclopentadiene in the presence of 5 mol % of the catalyst (35) afforded the adduct (36) in 83—85% yield, 95 5 exo/endo ratio, and greater than 96 4 enantioselectivity. Treatment of the aldehyde (36) with aqueous hydroxylamine, led to oxime formation and bromide solvolysis. Tosylation and elimination to the cyanohydrin followed by basic hydrolysis gave (24). 

Cyclic dienes such as cyclopentadiene and furan react with allenic ketones at a lower temperature (Table 12.7) [105, 106], Preferential formation of the endo adducts was observed for the both dienes. 

Ohfune and coworkers78 used Diels-Alder reactions between 2-trimethylsilyloxy-l,3-butadiene (63) and acrylate esters 64 to synthesize constrained L-glutamates which they intended to use for the determination of the conformational requirements of glutamate receptors. The reactions between 63 and acrylate esters 64a and 64b did not proceed. Changing the ethyl and methyl ester moieties into more electron-deficient ester moieties, however, led to formation of Diels-Alder adducts, the yields being moderate to good. In nearly all cases, the cycloadducts were obtained as single diastereomers, which is indicative of a complete facial selectivity (equation 22, Table 1). Other dienes, e.g. cyclopentadiene and isoprene, also showed a markedly enhanced reactivity toward acrylate 64g in comparison with acrylate 64a. 

Brimble and coworkers172 reported the asymmetric Diels-Alder reactions between quinones 265 bearing a menthol chiral auxiliary and cyclopentadiene (equation 73). When zinc dichloride or zinc dibromide was employed as the Lewis acid catalyst, the reaction proceeded with complete endo selectivity, but with only moderate diastereofacial selectivity affording 3 1 and 2 1 mixtures of 266 and 267 (dominant diastereomer unknown), respectively. The use of stronger Lewis acids, such as titanium tetrachloride, led to the formation of fragmentation products. Due to the inseparability of the two diastereomeric adducts, it proved impossible to determine which one had been formed in excess. 

Niggli and Neuenschwander294 studied the reaction of fulvene (461) with cyclopen-tadiene. The main product fraction consisted of three 1 1 adducts, as illustrated in equation 138. Diels-Alder Adducts 462 and 463 resulted from attack of cyclopentadiene at the endocyclic and exocyclic double bonds of fulvene, respectively. The formation of 464 was rationalized by a [6 + 4] cycloaddition reaction followed by two [1,5] hydrogen shifts. It was stated that due to the absence of electron-donating and electron-withdrawing groups on both triene and diene, fulvene may have reacted via its HOMO as well as its LUMO. 

Acylnitroso compounds 197 (R = Me, Ph or Bn) react in situ with 1-methoxycarbonyl-1,2-dihydropyridine to yield solely the bridged adducts 198 quantitatively. On the other hand, 1 1 mixtures of the regioisomers 199 and 200 were formed from the nitroso-formates 187 (R = Me or Bn) (equation 110)103. The chiral acylnitroso compounds 201 and 202, which are of opposite helicity, add to cyclohexadiene to give optically active dihydrooxazines in greater than 98% diastereomeric excess (equations 111 and 112)104. Similarly, periodate oxidation of the optically active hydroxamic acid 203 in the presence of cyclopentadiene, cyclohexa-1,3-diene and cyclohepta-1,3-diene affords chiral products 204 (n = 1, 2 and 3, respectively) in 70-88% yields and 87-98% de (equation 113)105. 

In the Mukaiyama aldol additions of trimethyl-(l-phenyl-propenyloxy)-silane to give benzaldehyde and cinnamaldehyde catalyzed by 7 mol% supported scandium catalyst, a 1 1 mixture of diastereomers was obtained. Again, the dendritic catalyst could be recycled easily without any loss in performance. The scandium cross-linked dendritic material appeared to be an efficient catalyst for the Diels-Alder reaction between methyl vinyl ketone and cyclopentadiene. The Diels-Alder adduct was formed in dichloromethane at 0°C in 79% yield with an endo/exo ratio of 85 15. The material was also used as a Friedel-Crafts acylation catalyst (contain-ing7mol% scandium) for the formation of / -methoxyacetophenone (in a 73% yield) from anisole, acetic acid anhydride, and lithium perchlorate at 50°C in nitromethane. 

Scheme 10.5 Selective formation of Qq adducts with integral fulvene, cyclopentadienide and cyclopentadiene moieties.

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