Asymmetric Annulation of Donor–Acceptor Cyclopropanes with Dienes

An
efficient [4 + 3] cycloaddition reaction of D–A cyclopropanes
with dienes has been successfully developed. The reaction proceeds
well with various dienolsilyl ethers in the presence of Lewis acid,
delivering a variety of cycloheptenes and [n,5,0]­carbobicycles
with excellent stereoselectivity. The asymmetric version of this reaction
is also realized using a newly designed chiral Cy-TOX ligand, providing
a new approach to access optically active cycloheptenes and [n,5,0]­carbobicycles. Mechanisic study reveals that the reaction
involves a stepwise pathway, which undergoes an unusual ring opening
of five-membered [3 + 2] intermediate and sequential intramolecular
cyclization to afford the thermodynamically stable [4 + 3] annulation
product.