Organocatalyzed Microwave-Assisted Competing Cyclization of Cyanopropargylic Alcohols with Carboxylic Acids: 4-Cyano-3(2H)-furanones versus 4-Cyano-[(Z)-3-cyanomethylene]-2,3-dihydro­furans

Abstract

It is shown for the first time that the reaction of cyanopropargylic alcohols with carboxylic acids in the presence of triethylamine (5–100 mol%) under microwave irradiation (MeCN, 100 °C, 1.2 atm) follows two competing pathways to afford 4-cyano-3(2 H )-furanones (1:1 adduct) and 4-cyano-[( Z )-3-cyanomethylene]-2,3-dihydrofurans (2:1 adduct) in moderate to good yields. The ratio of the products depends on both the reaction conditions (the reactant ratio, catalyst content) and the reactant structure, the 2:1 adduct formation being favored by an excess of cyanopropargylic alcohol, lower catalyst content, and bulky substituents at the carboxylic functions. Both competing reactions proceed via the same intermediates, keto esters, which either intramolecularly cyclize to the 1:1 adducts, or intermolecularly add the second molecule of cyanopropargylic alcohol to give the 2:1 adducts.