Highly Enantioselective Proton-Initiated Polycyclization of Polyenes

This report describes the synthesis of a range of chiral
polycyclic
molecules (tricyclic to pentacyclic) from achiral polyene precursors
by enantioselective proton-initiated polycyclization promoted by the
1:1 complex of o,o′-dichloro-BINOL
and SbCl5. Excellent yields (ca. 90% per ring formed) and
enantioselectivety (20:1 to 50:1) were obtained. The process is practical
as well as efficient, because the chiral ligand is both readily prepared
from R,R- or S,S-BINOL and easily recovered from the reaction mixture by
extraction.