Taxol is a natural substance represented by the following chemical structural formula, which can be obtained in small amounts from the bark or other parts of Taxus brevifolia. 
It is known that taxol has antitumor activity, and its mechanism of action is believed to be based on its ability to inhibit depolymerization of microtubules during cell division. At the time of the discovery of taxol this mechanism of action was different from the conventional antitumor agents, so it became of great interest for its potential clinical application as an antitumor agent.
Taxol can be obtained from natural sources, but only in very small amounts. However, taxol derivatives can now be synthesized using a taxol precursor, 10-O-deacetylbaccatine III (“10-DAB III”), which can be obtained from leaves and other parts of Taxus plants in relatively larger amounts. One such taxol derivative, docetaxel, is marketed by Sanofi under the tradename Taxotere® and has been approved for the treatment of various cancers, including breast cancer.
Recently, in U.S. Pat. No. 6,646,123, inventors at Daiichi Pharmaceutical Co. reported on a series of pentacyclic taxane compounds. These pentacyclic taxanes were obtained by reduction of the 9-position ketone of known taxanes to form a 9-position hydroxyl group which was then linked to the 10-position hydroxyl group to form a cyclic acetal. The resulting compounds have strong antitumor activity.
Additional studies on pentacyclic taxanes are reported in U.S. Pat. No. 6,677,456 (Daiichi Sankyo). These compounds have oral antitumor activity and therefore the potential to eliminate the toxic side effects associated with the use of Cremophor EL (a polyoxyethylated castor oil) and polysorbate 80 to solubilize taxanes for intravenous administration. One such compound is tesetaxel, having the following structure.

There is a continuing need for efficient and cost-effective synthesis schemes for preparing orally available taxol derivatives, such as tesetaxel, and for intermediates useful in such syntheses.