A carbometallation-intramolecular cycloaddition strategy for the AB-taxanering and an enone accelerated cope rearrangement to bicyclo[2.2.2]octanones

The one pot transformation of the magnesium chelates, derived from alkynyl alcohols, into tetraene-diols for the construction of substituted bicyclo[5 .3.1]undecenes is described. This sequence affords a direct route to the AB -taxane ring system via a Lewis acid catalyzed intramolecular Diels-Alder r eaction. The cycloaddition gives a single diastereomer due to the preferent ial formation of the Et2AlCl chelate. This adduct undergoes a facile enone accelerated Cope rearrangement to generate a substituted bicyclo[2.2.2]octa none. This isomerization fails below 110 degrees C when the ketone is absen t.