53. Acid-Catalyzed Cyclization Rections of Substituted Acetylenic Ketones: A New Approach for the Synthesis of 3-Halofurans, Flavones, and Styrylchromones

Article abstract of DOI:10.1002/hlca.19890720305

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.