5-Diketones, internal condensation

The enone lactone 410 could also be obtained in a single operation from the ozonolysis in methanol of hemi-ketal [3 (118). In this reaction, 41 produced first the diketone 14 which underwent an internal aldol condensation to 415 which is nicely set up to give K) vi a the intermediate 416. 

Pentanedione is in equilibrium with two enolate ions after treatment with base. Enolate A is stable and unreactive, while enolate B can undergo internal aldol condensation to form a cyclobutenone product. But, because the aldol reaction is reversible and the cyclobutenone product is highly strained, there is little of this product present when equilibrium is reached. At equilibrium, only the stable, diketone enolate ion A is present. 

Substituted cyclopentenones ate formed by internal aldol condensation of y-diketones in the presence of basic catalysts. 

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