A Practical and Efficient Strategy to 2‐Spirocyclic Benzofuran‐3(2 <i>H</i> )‐ones: Phase‐Transfer‐Catalyzed Double Michael Addition With Dienones

ABSTRACT An efficient phase‐transfer‐catalyzed double Michael reaction between benzofuran‐3(2 H )‐ones and dienones was developed. Using commercially available tetrabutylammonium bromide as the phase‐transfer catalyst and potassium prolinate or potassium carbonate as the base, a broad variety of 2‐spirocyclic benzofuran‐3(2 H )‐one derivatives were prepared with moderate to high yields under mild conditions. Gram‐scale reactions were further performed to verify the practical feasibility of the established method.