Normal Cope Rearrangement

If the diene has a 3-hydroxy substituent (C-3) the rearrangement is called the Oxy-Cope rearrangement. It is different from the normal Cope rearrangement, in two respects ... 

A search for possible degenerate behavior in bicyclo[3.3.0]octa-2,6-dienes164 has not produced clear results due to structural misassignments.126 The normal Cope rearrangement of 112 can be redirected through coordination to rhodium (I) to... 

Jain and McCloskey have found that acid-catalysed cyclisation of costu-nolide (234) on Amberlite cation exchange resin gives a good yield of a- (235) and ]8-cyclocostunolides (236). Furthermore, the same authors have shown not only that dihydrocostunolide (237) undergoes the normal Cope rearrangement but also that at elevated temperatures the bicyclic compounds (238), (239), and (240) are obtained. 

The equilibrium of the normal Cope rearrangement favors the cyclopentane ring to a large extent. For example, at 220 C the ratio of ( ,Z)-1 and as-2 is 5 95. The same equilibrium is reached from (Z,Z)-1. but this stereoisomer must rearrange through a boat transition state921. 

In contrast to the normal Cope rearrangement, the irreversible oxy-Cope rearrangement allows cyclononene derivatives to be synthesized from 1.2-divinylcyclopentanes, e.g., formation of ( >5922 and 7923. 

The Normal Cope Rearrangement. Sigmatropic rearrangements are not confined to hydrogen shifts occurring within a n framework. Thermolysis of 1,5-dienes results in a six jr-electron reorganization, a... 

R = R = H) underwent equilibration with its cis-fuscd isomer and only a trace of the cycloheptadiene was observed presumably steric interactions make the Cope rearrangement very difficult. The results suggest an initial epimeriza-tion of irons- to cis-fused isomers followed by a normal Cope rearrangement. An analysis of the kinetics of conversion of ( — )-trans-divinylcyclopropane into cyclohepta-1,4-diene indicates that the ring-opening is not a concerted pro-cess. 

The dienone intermediate (53a), as well as enolising to the phenol (52a), is itself capable of undergoing a Cope rearrangement to yield a second dienone (cf. 56a), whose enol is the p-substituted phenol (c/ 57a). Enolisation normally predominates, but where (51) has o-substituents, i.e. (54a), o-enolisation cannot take place, and only the p-phenol (57a) is then obtained. That this product is indeed formed not by direct migration of the allyl group, but by two successive shifts, is suggested by the double inversion of the position of the, 4C label in the allyl group that is found to occur ... 

The Cope rearrangement of alkoxide ions is much faster (1010 - 1017 times) compared with the normal oxy-Cope rearrangement. This is also called anionic oxy-Cope rearrangement and occurs under mild conditions. 

The Cope rearrangement mechanism can be also strongly affected by other substituents. Thus, the normal electrocyclic process in the thermal isomerization of divinyl aromatics has been suppressed relative to the thermolysis of l,2-bis(trifluorovinyl)naphthalene 438 (in benzene, at 193 °C, 24 h)231. Three major products 440-442 were isolated from the reaction mixture, but none of them was the expected product 439. Also formed in low... 

In the Lewis acid catalysed reactions of a,/J-unsaturated carbonyl compounds with dienes, sometimes the products of a [2 + 4]-cycloaddition, where the carbonyl compounds function as heterodienes, were isolated. It was proposed that the intermediate of the [2 + 4]-cycloaddition is formed first in this case, followed by a Cope rearrangement which leads to the normal Diels-Alder product (Scheme 7). 

Cope rearrangement is known to take place in semibullvalene (S4)368 and bull-valene ( 5)369. This process is quite slow compared to the electron-diffraction process, and the bond distances of (84) and (85) are therefore found to be similar to normal single and double bonds. 

Attempts have been made to find reaction sequences which allow the introduction of more than four atoms into a ring by a Cope rearrangement. Two of these methods should be mentioned, both quite different. The first method uses an enlarged Cope system , which forms bigger rings than the normal Cope system. In the second method the product of one Cope rearrangement can be easily transformed into the starting material for a second Cope shift sequence. 

The normal equilibrium can be reversed, of course, by using the oxy Cope rearrangement of a 1,2-di-vinylcyclopentanol one example was shown in Scheme 7 (62 - 63) in which irreversible tautomeriza-tion of the enol gave a fused cyclononenone. Two additional examples are shown in equations (67) and (68), the latter a key step in a synthesis of phoracantholide. ° The fused divinylcyclopentane shown in equation (69) undergoes anionic oxy-Cope rearrangement to a bridged cyclononenone. 

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