Total Synthesis of Taxol. 3. Formation of Taxol's ABC Ring Skeleton

The synthesis of Taxol's ABC ring system has been achieved. The Shapiro coupling of an aldehydic C ring synthon (8) with an anionic A ring synthon derived from hydrazone 9 gave, diastereoselectively, A-B conjugate 10. Functional group manipulations and McMurry ring closure produced the highly functionalized ABC ring system 17. Extensive attempts to optimize the McMurry reaction revealed a single predominant side reaction leading to byproducts 19 and 20. Resolution of the C9,C10-diol (±)-17 via its camphanyl esters provided the ABC ring system as its natural isomer (+)17.