By Cope rearrangement

J 7i-Cycloadducts at the C4 — C5 azepine positions are also formed with 1,2,3,4-tetra-chloro-5,5-dimethoxycyclopentadiene260 261 and with hexachlorocyclopentadiene.261 The reactivity of ethyl l//-azepine-l-carboxylate towards cyclopentadienones has been studied in terms of frontier molecular orbital theory which predicts that dimethyl 2-oxo-4,5-diphenyl-cyclopenta-1 (5),3-diene-1,3-dicarboxylate (20) should be more reactive towards the 1/f-azepine than other more common cyclopcntadienone derivatives.262 In fact, in refluxing benzene, the cyclopentadienone and ethyl 1/f-azepine-l-carboxylate (1) form a mixture of the [4 + 2] n-endo,anti-adduct 22, produced by Cope rearrangement of the initially formed [2 + 4] 7T-adduct 21, and the c.w-adduct 23, a rare example of a [6 + 4] rc-cycloadduct. At room temperature, only the [6 + 4] adduct 23 and a small amount of the [2 + 4] adduct 21 are obtained, the latter rearranging to the [4 + 2] adduct 22 on warming.262 Other [4 + 2] 7r-adducts with cyclopentadienones have been prepared similarly.263... 

Thermolysis of the functionally substituted 1,2,6-trienes 28 and 31a-d leads by Cope rearrangements to dienes 29, 30 and 32a-d (equations 9 and 10), respectively26. The reactions of aldehyde 28 occur at a relatively high temperature (>170 °C) to furnish both 29 and 30. Product 29 can be cyclized to 30 by heating. The similar thermolysis of the Shiff base 33 obtained from aldehyde 28 proceeds via two steps to afford the separable analogous products 34 and 35 (equation ll)27. 

Cyclic diphosphanes, from ring expansion by Cope rearrangement, 33 300-301 Cyclic iminoboranes, as reaction intermediates, 31 130... 

By the low-temperature irradiation experiments, Noh and Kim reinvestigated the photocycloaddition of furan to 1-cyanonaphthalene [139], which was first reported by Pacetal. [100,101], Irradiation of a mixture through Pyrex filter at 78°C yielded exclusively endo-(4 + 4) cycloadduct (endo-26) with a small amount of syn-(2 + 2) cycloadduct. Formation of 81 was proposed by Cope rearrangement of exo-(4 + 4) cycloadduct (exo-26)... 

Furans do not undergo [4+4] dimerization, however irradiation of a mixture of 2,5-dimethylfuran with an excess of cyclopentadiene leads to a mixture of two cross products 132 and 133 (Sch. 30) [84]. The alkene position in 133 was not determined, but 133 could have been formed by Cope rearrangement of the cis [4+4] adduct 134 that was not observed. 

An interesting synthesis of citral reported by a Russian group starts with 6-methyl-A -heptene-2-one (4) prepared by Cope rearrangement of the acetoacetate ester (3). The ketone (4) was converted in very high yield into the diethyl ketal (.5) by... 

The meso- and Aisomers 2 and 3, prepared by Cope rearrangement of 1, have been separated. ... 

Synthesis from Isobutene and Formaldehyde. 3-Methyl-3-buten-l-ol, obtained from isobutene and formaldehyde [43], isomerizes to form 3-methyl-2-buten-l-ol [58]. However, it is also converted into 3-methyl-2-butenal by dehydrogenation and subsequent isomerization [59], [60]. Under azeotropic conditions in the presence of nitric acid, 3-methyl-2-buten-l-ol and 3-methyl-2-butenal form an acetal (shown below) [61], which eliminates one molecule of 3-methyl-2-bu-ten-l-ol at higher temperatures. The intermediate enol ether undergoes Claisen rearrangement followed by Cope rearrangement to give citral in excellent yield [62] ... 

Such an epimerization is observed in the desulfurization of vinyl sulfides. Thus, treatment of 41 with Raney nickel produces meso-42 together with a smaller amount of tetraester 43815,816. Formation of 43 can either arise by Cope rearrangement via a six-centered boatlike transition state or by epimerization of meso-42 to ( )-42 under the reaction conditions followed by rearrangement. As rearrangement of meso-42 gives (f ZTtetraester 44 as the only... 

The [2,3] sigmatropic rearrangement of sulfides 45 followed by Cope rearrangement yields vinyl sulfides with the usual selectivity, i.e., ( ,Z)-products from the me-w-substrates, and (E,E)-products from the rar-substrates815. 

Thio-Claisen rearrangement by a-allylation of a,//-unsaturated thioamides followed by Cope rearrangement provides a method for the preparation of unsaturated thioamides 88s03. 

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