in a preferred embodiment , taxol is treated with mesyl chloride to prepare a taxol derivative with a contracted a - ring , which has comparable activity to taxol in a tubulin depolymerization assay , and which shows cytotoxicity against kb cells in a cell culture assay . in an alternate preferred embodiment , taxol is treated with triethyloxonium tetrafluoroborate to produce a taxol derivative with an opened oxetane ring . in another alternate preferred embodiment , reaction of taxol with acetyl chloride yields a taxol derivative in which the oxetane ring is opened and the a - ring is contracted . all of the aforementioned products show in vivo activity in kb cell culture assays . further , the preferred compounds have different in vivo activities , which makes them ideal to form a range of standards for use in biological testing of other compounds .