Synthesis of the AC Ring of Paclitaxel: Formation of the C Ring by [3+2] Cycloaddition with a Preformed A Ring

The stereoselective formation of the C ring was accomplished by nitrile oxide [3+2] cycloaddition of an intermediate with the A ring already constructed. We found that stereoelectronic effect to a 1,1-substituted alkene moiety is important in the reaction.