Competitive Stereochemical Control Operative during Conrotatory Electrocyclization of Helically Equilibrating Diquinanyl-Substituted 1,3,5,7-Octatetraenyl Bisenolates

Activation of the squarate ester cascade by adding the lithiated bicyclo[3.3.0]octene 20 and
vinyllithium sequentially to 1 results in the isolation of the four tetracyclic products 21−24. The
structures of the topographically complex products were deduced by 2D NMR spectroscopy and
X-ray diffraction studies. The mechanistic insights gained by these findings are discussed. The
product distribution is telltale evidence for predominant 1,2-addition of the second alkenyl anion.
Product stereochemistry is in turn diagnostic of the preferred mode of conrotatory ring closure
operating within equilibrating helical intermediates of opposite pitch. A competing pathway for
the elimination of methanol in these highly functionalized intermediates has been observed for
the first time.