A CONCISE ROUTE TO THE TETRACYCLIC CORE OF PHORBOL

The tetracyclic ketone 17, which possesses a framework closely related to phorbol, has been prepared in enantiomerically pure condition. Con densation of the dichlorocerate derived from 3 with 2-chlorocyclohexan one leads to chlorohydrins 4 and 5 in a 1:1 ratio. Treatment of 5 with excess vinylmagnesium bromide delivers 7, thereby setting the stage f or anionic oxy-Cope rearrangement to 9. Epoxidation of 9 and base-prom oted cyclization of the a-isomer (15) under kinetically controlled con ditions eventuates in the formation of 17. A companion set of reaction s was performed on 4 with the result that hemiketal 11 is formed follo wing transannular ring closure. Lastly, one of the first intramolecula rly competitive anionic oxy-Cope rearrangements is described.