The ruthenium-catalyzed intermolecular cyclocoupling of ketones (or aldehydes), alkenes (or alkynes), and CO, which leads to gamma -butyrolactones, is described. The reaction represents the first example of the catalytic synthesis of heterocycles via an intermolecular carbonylative [2 + 2 + 1] cycloaddition. A wide variety of ketones, such as alpha -dicarbonyl compounds and N-heterocyclic ketones, can be used in this cycloaddition. The addition of phosphines is quite effective in reactions of alpha -dicarbonyl compounds. Of the phosphines examined, P(4-CF3C6H4)(3) represents the additive of choice. Cyclic olefins, unpolarized terminal olefins, and internal alkynes can be successfully used in the synthesis of highly functionalized lactones. The introduction of a CF3 group to the aromatic portion of an aromatic keto ester accelerates the reaction of the keto ester with ethylene, while the introduction of a MeO group enhances the rate of the reaction of N-heterocyclic ketones with ethylene. The rate of the reaction increases with increasing pressure of ethylene or a lower pressure of CO relative to the reaction of a keto ester. However, these pressure-rate relations are reversed for the reaction of an N-heterocyclic ketone with ethylene. Such differences can be rationalized by assuming that the rate-limiting step in the catalytic cycle is different for these reactions.