Bridging Converts a Noncytotoxic nor-Paclitaxel Derivative to a Cytotoxic Analogue by Constraining It to the T-Taxol Conformation

Abstract
The synthesis of the bridged A-nor-paclitaxel 4 has been achieved from paclitaxel in a key test of the T-Taxol conformational hypothesis. Although the unbridged A-nor-paclitaxel 3 is essentially noncytotoxic, the bridged analogue 4 is strongly cytotoxic. This result provides strong evidence for the T-Taxol conformation as the bioactive tubulin-binding conformation of paclitaxel.