Divinylcyclopentanes, rearrangement

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. 

In this series, as well as the divinylcyclopentanes, the equilibria can be shifted completely in favor of the 10-membered ring by using the irreversible oxy-Cope rearrangement of 1,2-divinylcyclohexanols the prototype (64 65) was shown in Scheme 7. This reaction type has found wide application in the... 

Ring strain effects may also play an important role in the positioning of the equilibrium in Cope rearrangements. Whereas in the equilibrium of strain-free 1,2-divinylcyclopentane and 1,2-di-vinylcyclohexane derivatives the substrate side is favored, the equilibrium of bridged bicyclic systems and particularly that of cyclopropane- and cyclobutane-derived substrates usually lies more or less completely on the less-strained product side, as shown by the following examples a) cyclopropane derivatives formation of l756. 2757 375s 759, 4760. and 5761. 

The position of the Cope equilibrium in 3,4 rearrangements of 1,2-divinylcycloalkanes largely depends on the ring size. In the case of 1,2-divinylcyclopropanes and cyclobutanes, the equilibrium lies on the side of the enlarged rings. For 1,2-divinylcyclopentanes, cyclohexanes, and cycloheptanes, the equilibrium of the Cope rearrangement is reversed, but use of the irreversible... 

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. 

As already discussed for 1,2-divinylcyclopentanes, formation of the 10-membered ring can be forced by using the irreversible oxy-Cope rearrangement. 

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