Acid‐Catalyzed Cyclization Reactions of Substituted Acetylenic Ketones: A new Approach for the Synthesis of 3‐Halofurans, Flavones, and Styrylchromones

Acetylenic acetals of type I (Scheme 1) and acetylenic ketones of type III (Scheme 1) , 37 and 38 (Scheme 7) are versatile synthetic precursors for the synthesis of various heterocycles by acid‐catalyzed cyclization reactions. By this way, substituted 3‐halofurans of type II and IV (Scheme 1) and flavones and styrylchromones (Scheme 7) can be synthesized in good‐to‐excellent yields. The high degree of regioselectivity in the synthesis of the 3‐halofurans (Scheme 4) is the result of the regioselective β‐addition of HX (X = Cl, Br, I) to the acetylenic aldehyde and acetylenic ketone moieties. A possible mechanism is depicted in Scheme 5 . Since 3‐halofurans can easily be metalated and substituted, this approach constitutes a new synthesis of highly substituted furans.