Taxol, carbocyclization

The basic structure of taxol is that of a tetracyclic compound -A, B, C and D rings-, in which the central B ring is an eight-membered carbocycle. In principle, the formation of this medium sized ring appears somewhat problematic because of both entropic and enthalpic factors. 

Taxol, the anti-cancer compound from yew trees14 (Taxol spp.) has two six- and one eight-membered carbocyclic rings 58. Nicolaou15 saw that two C-C disconnections would give two simple six-membered ring precursors 57 and 59. 

Garcia-Fandino R, Codesido EM, Sobarzo-Sanchez E, Cas-tedo L, Granja JR. Tandem RCM of dienynes for the construction of Taxol-type carbocyclic systems. Org. Lett. 2004 6 193 196. 

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