Cyclooctadienes Cope rearrangement

Not all Cope rearrangements proceed by the cyclic six-centered mechanism. Thus cis-1,2-divinylcyclobutane (p. 1131) rearranges smoothly to 1,5-cyclooctadiene, since the geometry is favorable. The trans isomer also gives this product, but the main product is 4-vinylcyclohexene (resulting from 8-33). This reaction can be rationalized as proceeding by... 

The Cope rearrangement produces a number of interesting phenomena in ring systems. Thermal dimerization of butadiene yields as one product 1,5-cyclooctadiene (Equation 12.104). Formally, this dimerization is a 4 + 4 cycloaddition its actual course (Equation 12.105) is a 2 + 2 addition (presumably biradical) followed by a [3,3]-rearrangement of the eij-divinylcyclo-butane.171... 

Ethenolysis is synthetically very useful. The reaction of stilbene (68) with ethylene is attracting attention as a potential commercial process for styrene (65). The a,co-dienes 46 are formed from cyclic alkenes 43 and ethylene. Ethenolysis of the bicyclo[2.2.0]hexene 71, formed from 69 via 70, afforded the 1,5-diene 72, which underwent Cope rearrangement to give the cyclooctadiene 73 [24],... 

The Cope rearrangement was used in the total synthesis of (-)-asterisca-nolide (14), a novel sesquiterpene natural product4 (Scheme 1.4e). Ring-opening metathesis of the cyclobutene 15 with ethylene in the presence of the ruthenium catalyst 165 proceeded smoothly to provide the cyclooctadiene 18 via Cope rearrangement of the intermediate dialkenyl cyclobutane (17). 

Bridged cyclooctadienes result from Cope rearrangement of fused substrates, following the theme seen in Scheme 12. A general route to the requisite substrates is the [2 + 2] cycloaddition of vinylketenes to cyclic 1,3-dienes, illustrated in equations (61) and (62) the vinylketenes may be formed by dehydroha-logenation of unsaturated acid chlorides or by thermolysis of cyclobutenones. ... 

The Cope rearrangement, like the Claisen rearrangement, is a no mechanism reaction and thus does not involve ionic or radical intermediates. For practical purposes the result is that Cope rearrangements are independent of catalysts and of the nature of the solvent, and that substituent effects are slight. Steric influences, however, are considerable cw-1,2-divinylcyclobutane rearranges to 1,5-cyclooctadiene within a few minutes at 120° ... 

Lanthanum(III) atoms have rarely been used to study the photo-reactivity in CPs. Michaelides and coworkers explored the photo-reactivity of muconate ligands bonded to Er(III) and Y(III) atoms. Muconate is another linear spacer ligand with two conjugated C=C bonds in the backbone which presents different possibilities of photoproducts, including mono [2-1-2] cycloaddition or [4-1-4] cycloaddition products. The transformation of cyclobutane to cyclooctadiene by Cope rearrangement is another possibility. Further, it may also result in the formation of a highly strained ladderane structure by double [2-1-2] cycloaddition reaction. 

When the 1,3-dienes are largely or exclusively in the s-trans-conformation, such as with diene 83, pyridone 18 leads to three products, 87, 88, and 89." The two cyclobutane products 87 and 88 derive from Cope rearrangement of the highly strained [4-1-4]-photocycloaddition adducts 84 and 85, a mechanism precedented by photocycloadditions of 1,3-dienes with other aromatic species. Pyridine 89 may be formed from oxetane 86. When the 1,3-diene is delivered intramolecularly (90), the only photoproduct is 91, presumably "via an intermediate relating to 85. In contrast with the chemistry of the cis-pyridone dimers (Scheme 3), cyclobutane 91 thermally rearranges to the cyclooctadiene 92. ... 

Transannular hydride shifts, first detected by Cope and coworkers in solvolyses of cyclooctene oxide, have subsequently been found in a number of related systems, e.g. cyclooctadiene monoepoxides, CA o-bicyclo[3.3.1 ]non-2-ene epoxide and l-oxaspiro[2.6]nonane. In general these reactions do not involve skeletal rearrangements, and they will not be discussed in detail. 

Nakamura also demonstrated a very simple method for differentiating the ds-isomers 10 and 12 from the trans-isomers 9 and 11. Heating the dimers to 65°C results in a very fadle [3,3]-rearrangement of the ds-1,5-cyclooctadienes to q clobutanes. The C2 symmetric 10 can form only one product 13, whereas the meso-12 leads to nearly equal quantities of two different Cope products 14 and 15. frans-Isomers 9 and 11 are stable under these conditions. 

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