2-Cycloalkenones dimerization

In the case of cycloheptenone and larger rings, the main initial photoproducts are the -cycloalkenones produced by photoisomerization. In the case of the seven- and eight-membered rings, the double bonds are sufficiently strained that rapid reactions follow. In nonnucleophilic solvents dimerization occurs, whereas in nucleophilic solvents addition occurs. ... 

The synthetic implications of this discovery were slow to be exploited. Base-initiated dimerizations of 2-cycloalkenones, known to give crystalline solids,3233 remained puzzling for some time before conjugate additions were suggested to account for some of the possible products 34 indeed, the product of base-catalyzed dimerization of 4,4-dimethyl-2-cyclopentenone, which proceeds via a double Michael addition sequence, was not identified until 1969 (Scheme 2).35 An unanticipated cyclopropanation reaction of acrylaldehyde36 37 using ethyl bromomalonate and proceeding by means of a similar Michael addi-tion-Sw enolate alkylation represents an early synthetic use of tandem vicinal difunctionalization. 

Cycloalkenone-2-carboxylates tautomerize to conjugated dienols in the presence of either acids or bases. Iron(III) catalysts have also been found to promote enone-dienol equilibration, and, at room temperature, dimerization . Thus, treating 87 with 1 mol% iron(in) chloride hexahydrate in methylene chloride at room temperature affords 88 in 81% yield (equation 46). The cyclohexadiene-cyclohexanone is in a rapid equilibrium with its triendiol tautomer, 89 (equation 47). 

---