Highly regio- and stereoselective synthesis of Z and E enol esters by an amine-catalyzed conjugate addition-rearrangement reaction of ynals with carboxylic acids

The broad synthetic utility of labile enol esters demands efficient methods for the stereo- and regioselective synthesis of both Z and E isomers. The available synthetic methods dominated by metal catalysis cannot meet the challenge. We wish to report a metal-free organocatalytic divergent approach to both E and Z isomers of enol esters from the same reactant pools with the same catalytic system. A process involves an amine-catalyzed conjugate addition of carboxylic acids to ynals, which triggers a rearrangement leading to enol esters. The reaction proceeds highly regio- and stereoselectively. Simple manipulation of reaction temperatures enabled us to produce Z isomers at 0 °C (Z:E (15-20):1), whereas E isomers were produced at 30 °C (E:Z (15-20):1). Furthermore, the mild reaction conditions accommodate a broad array of densely functionalized carboxylic acids for the process, including complex biologically relevant structures and ynals. Therefore, synthetically valued, structurally diverse enol esters are efficiently synthesized. Preliminary mechanistic studies suggest an amine-promoted conjugate addition-rearrangement pathway to be responsible for the formation of the enol esters. (Chemical Equation Presented).