Synthesis of the ABC ring system of paclitaxel: Formation of the 8-membered B ring by intramolecular alkylation

The formation of the B ring of the ABC ring system of paclitaxel was accomplished by intramolecular alkylation of the protected cyanohydrin ether in the presence of the 7-alkoxy group on the C ring. A significant effect of the protective groups at the 1,2-positions was observed in this alkylation. Acyclic protection provided the desired compound 3c and 3d in moderate yields, whereas cyclic protection led to 3a in low yield.