Ozonolysis of cyclopentadiene derivatives. Competitive participation of [3 + 2] and [3 + 4] cycloadditions of carbonyl oxide moieties to α,β-unsaturated carbonyl groups

Reactions between cyclopenta-1,3-dienes 1a-i and ozone, conducted in a variety of solvents including diethyl ether, pentane, CCl₄, CH ₂Cl₂, CF₃CH₂OH, AcOH, and MeOH, afforded predominantly monomeric ozonolysis products consisting of either bicyclic endoperoxides 5 containing a 1,2,4-trioxepine ring, or unsaturated bicyclic ozonides 6, or mixtures of compounds 5 and 6. From their molecular structures, the novel bicyclic endoperoxides 5 are considered to result from intramolecular recombination of the carbonyl oxide and enone moieties, generated specifically from only one of the two possible decomposition modes of the primary ozonide, via stepwise [3 + 4] cycloaddition processes. The product composition was found to be sensitive to the nature of the substituents and the substitution pattern in the cyclopentadiene substrate, and the ozonolysis solvent. In general, protic solvents tended to assist the formation of the endoperoxides 5. The isomeric peroxides 5 and 6 could, in several instances, be interconverted by treatment with acid catalysts like CF₃CO ₂H, or even silica gel.