[4+2]-Cycloaddition and 1,4-Addition of ortho-Quinone Methides by a Chiral Crotyl Silane

Anhydrous FeCl3 in the presence of 2,6-lutidine promotes the substrate-controlled enantioselective [4 + 2]-cycloaddition and crotylation reaction between an enantioenriched (S,E)-crotyl silane and in situ generated ortho-quinone methides (oQMs). The reaction produces both the chiral chroman and crotylation products in a ratio reflective of the electronic nature of the parent oQM with overall combined yields up to 96%. A ring-opening and elimination sequence was subsequently developed to provide direct access to the crotylation products, containing two contiguous tertiary carbon stereocenters, in good yields and enantioselectivities.