Several Lewis acids were shown to cleanly open the oxetane ring of taxol and baccatin derivatives. The reaction is shown to proceed via anchimeric assistance by the C-4 acetate group. Several minor products, including a novel derivative possessing a bridged C-ring, were also isolated. A mechanistic rationale is provided for all compounds formed. When taxol derivatives were treated with Lewis acids in methanol, ester cleavage reactions were observed. We provide conditions that are selective for C-10 acetate cleavage and for C-13 side-chain methanolysis.