Catalytic Asymmetric Conjugate Addition of Indoles to para-Quinone Methide Derivatives

A catalytic asymmetric conjugate addition of indoles to o-hydroxyphenyl substituted p-quinone methides has been established in the presence of chiral phosphoric acid, which afforded chiral indole-containing triarylmethanes in generally high yields (54-98%) and good enantioselectivities (90:10-96:4 enantiomeric ratio). The control experiments indicated that o-hydroxyphenyl substituted p-quinone methides had a high possibility to transform into o-quinone methides in the presence of chiral phosphoric acid, and the formation of o-quinone methides might be a necessity for the reaction. This reaction will not only contribute to the research field of catalytic asymmetric transformations of p-quinone methides and o-quinone methides but also provide a useful method for the construction of enantioenriched triarylmethane frameworks.