Intramolecular Cycloadditions of Mesoionic Carbonyl Ylides with Alkynes. Synthesis of 5,6‐Dihydro‐4H‐cyclopenta‐ and 4,5,6,7‐Tetrahydrobenzo[b]furan Derivatives

Intramolecular cycloaddition reactions of acetylenic isomünchnones, formed in situ by rhodium acetate catalyzed decomposition of N‐(diazoacetyl)‐N‐methylalkynamide derivatives 16a–c, 18c, and 18d have been studied. The intermediate cycloadducts fragmentate spontaneously under the reaction conditions (110°C) to afford the annulated furans 19a–c, 20c, and 20d.