Electrocyclic reactions oxyallyl-cyclopropanone

Other chemists prefer a pericyclic description of the ring-closure step. The same enolate simply loses chloride to give an oxyallyl cation —a dipolar species with an oxyanion and a delocalized allylic cation. This species can cyclize in a two-electron disrotatory electrocyclic reaction (Chapter 36) to give the same cyclopropanone. We shall return to this discussion in the next chapter but, whatever the mechanism, there is no doubt that a cyclopropan one is an intermediate. 

What is happening here is that the oxyallyl cation is in equilibrium with the cyclopropanone by an electrocyclic reaction (Chapter 36) and the alcohol is capturing this unstable ketone by nucleophilic li addition. Hemiacetals of cyclopropanones form spontaneously in alcoholic solution (Chapter 6)... 

What is happening here is that the oxyallyl cation is in equilibrium with the cyclopropanone by an electrocyclic reaction (Chapter 36) and the alcohol is capturing this unstable ketone by nucleophilic addition. Hemiacetals of cyclopropanones form spontaneously in alcoholic solution (Chapter 6) because of the strain in the ketone. The anion of the hemiacetal decomposes by cleavage of a C-C bond to release what would be the more stable of the two carbanions, that is, the benzylic carbanion. This carbanion is not actually formed as it is protonated by the alcohol as it leaves. 

This looks as though each of the C—C bonds is independently the result of both HOMO/LUMO interactions, with an endo selectivity as well. In the presence of dienes, these species behave as allyl cations (see p. 259) and undergo clean [4 + 2] cycloadditions, as in the reaction of the oxyallyl 6.372 giving the tricyclic ketone 6.373, which is similar to the diene 6.369. Normally, oxyallyls are in equilibrium by disrotatory electrocyclic ring closures with cyclopropanones and with allene oxides, but the presence of the five-membered ring in these particular examples makes these pathways counter-thermodynamic. 

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See also in sourсe #XX -- [ , ]

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