Rearrangements Cope-type

Thermal Cope-type rearrangements between C-vinyl and nitrogen substituents bearing appropriately labeled unsaturation are also known, e.g. (147) -> (148) (67JA60S, 71JOC3076). 

Diethenyloxiranes undergo thermal Cope-type rearrangements to oxacyclo-heptadienes (Scheme 16) (67AG(E)385) even when these are very strained intermediates (Scheme 17) (80JOC428). This process is particularly facile in fused-ring diethenyloxiranes like (16) (Chapter 5.17) and (17) (Chapter 5.07). 

A slight modification of the cyclopropyl conjunctive reagent transforms a cyclopentannulation into a cycloheptannulation. Thus, the 2-vinylcyclopropyllithium reagent 3, converted to its cuprate 4, generates a 1,2-divinylcyclopropane. Heating to only 180 °C leads to smooth Cope type rearrangement, driven by the release of the cyclopropyl strain, to create a perhydroazulene ring systerh of many sesquiterpenoids (Eq. 19) 20>. 

In principle, Cope-type rearrangements can occur in any 1,5-diene system consisting of six carbon and/or heteroatoms (equation 244). However, despite the apparent variety of potential possibilities, few examples of hetero-Cope rearrangements are known up to now. It should be noted that the structures depicted in equation 244 which can generally contain up to six heteroatoms are no longer real dienes. Nevertheless, we will briefly... 

In the solution proposed by Evans [25], however, the unsaturated ketone is condensed with the anion of an alkyl allyl ether, followed by a [3,3]-sigmatropic oxy-Cope type rearrangement (Scheme 5.22). In the retrosynthetic sense this means a [3,3]-sigmatropic rearrangement of the bis-enol form of the 1,6-dicarbonyl system. 

This behavior is in marked contrast to that of vinylcyclopropyl isocyanate, which in boiling benzene undergoes an irreversible Cope-type rearrangement to 3H- azepin-2-one (65LA(682)1>. 

Thermal propargyl rearrangement-Claisen rearrangement of propargyl vinyl ethers and Cope-type rearrangements of propargyl malonates [93]. 

The rearrangement exhibits some stereochemical preference for c/s-vinyl carbene complex (with respect to the metal) compared to the //ww-isomer. Thus, 2-methyl-2-m-vinyl cyclopropyl (methoxy) carbene chromium pentacarbonyl rearranges to 5-methyl-5-vinyl-2-methoxycyclopentenone approximately 4 times faster (THF, 52 °Q than the trans-isomer, which in turn rearranges faster than phenyl derivatives. This suggests that vinyl complexes undergo initial Cope-type rearrangement to form metallacycloheptadienes, which then rearrange to jt-allyl complexes. Subsequent CO insertion and reductive elimination leads to the vinylcyclopentenones (equation 89)150. 

Besides intermolecular reactions, curved-arrow notation is also useful in indicating bonding changes in intramolecular reactions and rearrangement. For example, Cope-type rearrangements are seen to involve changes in three pahs of bonded electrons. 

The study of Bergman, Myers-Saito and related biradical cyclizations using an unrestricted broken spin symmetry approach refined by single-point energy coupled-cluster calculations has been reviewed, and a simple rule outlined for predicting biradical involvement in such Cope-type rearrangements radicals were found to be probable... 

Quantum chemical studies of cyclizations of enediynes and enyneallenes have been reviewed.180 The intermediates are computationally tractable as a result of the unrestricted broken spin symmetry (UBS) approach using GGA functionals for the description of open-shell biradicals. The intermediacy of biradicals in Cope-type rearrangements, to which the Bergman and Myers-Saito reactions belong, are shown to be predictable using a very simple rule biradicals are likely to be intermediates if they are stabilized either by allyl resonance or by aromaticity. 

A new approach to the abe ring system of the pentacyclic C20 diterpenoid alkaloids has been reported by van der Baan and Bickelhaupt.51 For the model system of major interest, (134) was prepared from (133) by reaction with cyano-acetamide. Treatment of (134) with allyl bromide gave almost exclusively C-alkylation, to afford (135). On heating (135) at 100—110°C, a Cope-type rearrangement to (136) was effected. This compound was then N-alkylated with ethyl iodide-DMF, the product being (137). Treatment of (137) with IV-bromosucc-... 

An intramolecular cascade reaction initiated by the addition of an alkenyl radical to a furan was used to synthesize an indene <1998SL1215>. As illustrated in Scheme 31, radical fragmentation in the spiro-dihydrofuran radical 52 provided the intermediate triene 53, which underwent Cope-type rearrangement to form the product. A related reaction with 1-bromocyclohexene that led to unsaturated ketone product was also developed <2003EJ01729>. 

The control of furan versus oxepin manifolds has been addressed recently. Vinyloxiranes of type (139) yielded oxepins (140) at lower temperatures, whereas higher temperatures gave dihydrofurans (141 Scheme 29), indicating that the Cope-type rearrangement of cis-divinyloxirane may be controll by precisely defining the temperature profile of the flash vacuum pyrolysis. 

A detailed reexamination of the thermal rearrangement of 1-methyl-imidazole originally reported by Wallach has been extended to encompass a wide range of 1-substituted imidazoles. The reaction appears to be of potential synthetic utility since it leads easily to 2-alkyl- and 2-arylimidazoles. It is an irreversible reaction, uncatalyzed, intramolecular, and does not involve radicals [l,5]-sigmatropic shifts, as shown in Scheme 40, are probably implicated. The major product is the 2-substituted imidazole (134), but small amounts of the 4- (or 5-) isomer (135) are also formed. A 1-allyl substituent migrates with equal facility to both 2- and 4(or 5)-positions, suggesting that a Cope-type rearrangement may also be involved for this substituent. ... 

Increasing importance has to be attributed to modem tandem (or cascade ) techniques—reaction sequences that can be performed as a one-pot procedure because the first reaction step creates the arrangement of functional groups needed for the second to occur. Schemes 5-7 present some in situ preparations for iminium species, which can then react further with appropriate nucleophiles that are already present (preferably in the same molecule). Most elegantly, in situ generation of iminium ions for tandem processes was performed by a 3,3-sigmatropic (aza-Cope-type) rearrangement (Scheme 5), but also by initial Michael-type addition reactions to vinyl-substituted Atio... 

Sperling, D., Reissig, H.-U., Fabian, J. [1,3]-sigmatropic rearrangements of divinylcyclopropane derivatives and hetero analogs in competition with Cope-type rearrangements. A DFT study. Eur. J. Org. Chem. 1999,1107-1114. 

A similar reaction was observed when 2-methylene-l,l-diphenylcyclopropane (8, = H) was heated to 135°C. The isolated 2-methyl-3-phenylindene (9, R = R = H) was probably formed via a 2,7a-dihydro-2-methylene-3-phenyl-liy-indene and subsequent 1,5-hydrogen shift.An analogous reaction occurred when 2-ethylidene-l,l-diphenylcyclopropane (8, R = H R = Me) and 2-(l-methylethylidene)-l,l-diphenylcyclopropane (8, R = R = Me) were submitted to thermolysis. These reactions can be rationalized by both a diradical mechanism and a concerted Cope-type rearrangement, followed by a hydrogen shift to restore the aromatic system. 

The initial isomerization product is now the (trimethylallyl)allene 30 which subsequently participates in a Cope-type rearrangement leading to the enyne 31. 

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