Base-Mediated Tandem 1,6-Addition/Cyclization/Isomerization Reactions between para-Quinone Methides and Benzyl Chlorides: Approaches to Diverse Frameworks at Each Cascade Stage

Abstract

Base-mediated stereospecific tandem reactions using para-quinone methides and carbene-like benzyl chlorides are developed. DBU-mediated 1,6-addition/cyclization/isomerization reactions produce triarylsubstituted alkenes in 43–89% yields and Z/E ratios of 5:1 to 35:1 in favor of the Z-isomers. Single-step 1,6-conjugate additions are realized with the mediation of cesium carbonate, and different triaryl chloroethanes are obtained in yields of 41–54% and excellent diastereoselectivities of more than 20:1. In addition, tandem 1,6-addition/cyclization reactions are achieved using tuned 2,4-disubstituted benzyl chlorides to afford diaryl spirocyclopropyl para-dienones in yields of 35–83% and diastereoselectivities of more than 20:1 in favor of the anti-isomers. The transformations from triaryl chloroethanes and spirocyclopropyl para-dienones into triarylsubstituted alkenes are also demonstrated to support the proposed tandem mechanisms.