Cyclopentadienes rearrangements

The labile cycloadduct 262 of azodibenzoyl to cyclopentadiene rearranges to the fused oxadiazine 263 on heating. The process involves dissociation of 262 into its components, followed by a Diels-Alder reaction in which the azo compound functions as a hetero diene (equation 142)135. 

According to new data for the vinylcyclopropane-cyclopentadiene rearrangement,372 particularly concerning the stereochemistry of the process, the [1,3] sig-matropic carbon shift proceeds through all four stereochemical reaction paths,... 

Methyl-1,3-cyclopentadiene rearranges to give a mixture of 5-methyl-1,3-cyclopentadi-ene, l-methyl-l,3-cyclopentadiene, and 2-methyl-1,3-cyclopentadiene. Show how these products are formed. 

The cycloaddition of glyoxylic acid with cyclopentadiene in water at pH 6 and 60 °C is slow and occurs with low yield and low diastereoselectivity [18] (Scheme 6.17). Proton (pH = 0.9) [18], copper salts [27] and Bi(OTf)3 [28] accelerate the reaction and increase the diastereoselectivity. The lactones 28 and 29 originate from endo and exo cycloadducts 27, respectively. The proposed rearrangement is depicted in Scheme 6.17 for the major endo adduct 30. A competitive ene reaction that originates 28 and 29 cannot be excluded [28]. 

Note that the first example bears out the stereochemical prediction made earlier. Only the two isomers shown were formed. In the second example, migration can > continue around the ring. Migrations of this kind are called circumambulatory rearrangements. Such migrations are known for cyclopentadiene, pyrrole, and phosphole derivatives.[1,5] Hydrogen shifts are also known with vinyl aziridines." ... 

Staudinger observed that the cycloaddition of ketenes with 1,3-dienes afforded cyclobutanones from a formal [2+2] cycloaddition [52] prior to the discovery of the Diels-Alder reaction. The 2+2 cycloadditions were classified into the symmetry-allowed 2+2 cycloaddition reactions [6, 7], It was quite momentous when Machiguchi and Yamabe reported that [4+2] cycloadducts are initial products in the reactions of diphenylketene with cyclic dienes such as cyclopentadiene (Scheme 11) [53, 54], The cyclobutanones arise by a [3, 3]-sigmatropic (Claisen) rearrangement of the initial products. 

CuS04-catalyzed decomposition of the (l-sila-cyclopentadienyl)diazomethane 398 did not furnish defined products. The desired rearrangement reactions to a silabenzene and a l-ethylidene-l-sila-2,4-cyclopentadiene, both trapped by /-butanol, were brought about by irradiation of 398, however 388... 

In certain cases the initial Diels-Alder adducts of ADC compounds are labile. For example, the adduct (121) from cyclopentadiene and azodibenzoyl rearranges in quantitative yield on heating in aqueous methanol to give the 1,3,4-oxadiazine 122.207 Solvent has little or no effect, and a concerted [3,3] rearrangement as shown in Scheme 17 seems the most likely explanation. The rearrangement has been extensively studied by Mackay and coworkers,208 and it shows great dependence on substitution effects. 

Enol ether additives were used to probe the protonation of 3-cyclopen-tenylidene (127). Treatment of A-nitroso-A-(2-vinylcyclopropyl)urea (124) with sodium methoxide generates 2-vinylcyclopropylidene (126) by way of the labile diazo compound 125 (Scheme 25). For simplicity, products derived directly from 126 (allene, ether, cycloadduct) are not shown in Scheme 25. The Skat-tebpl rearrangement of 126 generates 127 whose protonation leads to the 3-cyclopentenyl cation (128). In the presence of methanol, cyclopentadiene (130) and 3-methoxycyclopentene (132) were obtained.53 With an equimolar mixture of methyl vinyl ether and methanol, cycloaddition of 127 (—> 131)... 

A simple preparation of tropolone (253) was reported, in which the rearrangement of the 1 1 adduct (252) of dichloroketene and cyclopentadiene readily provide tropolone (253) by catalyzed with KOAc—HOAc 84). 

In both cyclic and acyclic dienes which can achieve the necessary geometry the [1, 5] shift is commonly observed because the activation energy is lowest for the transition states involving minimal distortion. This is particularly so in cyclopentadienes and indenes. The preference for [1, 5] over [1, 3] shifts is demonstrated by thermal rearrangement of 1-duterioindene. At 200°C, deuterium became scrambled over all three non-benzenoid carbons. 

In the reaction of benzylideneaniline with cyclopentadiene, the imine functions as an azadiene to yield the rearranged Diels-Alder adduct 77 (equation SI)44,453. In a study of the effect of various Lewis acids (ZnCl2, TiCU, Et2AlCl and SnCU) on diastereoselective cycloadditions of Danishefsky s diene to the imines 79, obtained from the chiral aldehydes 78 (R = MeO or Cl), it was found that SnCLj was the most effective, giving the optically active products in high yields and excellent ee values (equation 52)46. 

Photooxygenation of a-terpinene 155 in the presence of eosin (equation 87) produces ascaridole 156, a constituent of the essential oil Chenopodium ambrosioides L 1. The ewrfo-peroxide 157 derived from cyclopentadiene is a crystalline solid, stable at —100 °C78 above this temperature it rearranges to a mixture of the bis-epoxide 158 and the epoxyaldehyde 159 (equation 88)79,80. 

The action of 2,4,6-trichlorobenzenediazocyanide on cyclopentadiene results in an unstable cycloadduct, which over several days undergoes a trisaza-Cope rearrangement to the fused benzimidazole 261 (equation 141). By contrast, the analogous adduct to cyclohexadiene is stable134. 

Cyclopentadiene forms a mixture of the 1,2- and 1,4-adducts in equal proportions. However, the 1,2-isomer rearranged completely into the thermodynamically more stable 1,4-isomer after prolonged standing in the solvent (alcohol or dichloroethane). 

The formation of the bridged product 191 was investigated using the cyclopentadiene system as a model. Thus, the salt of the tosylhydrazone 198 was prepared and thermolyzed in order to examine three possible variants of rearrangements (equation 62)75. Analysis of the reaction products 200-202 and their transformations [e.g. the pyrolysis of bicyclic triene 202 to cA-8,9-dihydroindene 203 (equation 63) rather than to product 200 or 201] allows one to conclude that the mechanism involves a transformation of carbene 188 into diradical 204 which can be the precursor of all the products observed (equation 64)75. An analogous conversion takes place via radical 205 in the case of carbene 199 (equation 65). 

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