The large number of taxol analogs that have been synthesized has allowed the establishment of several structure-activity relationships (SAR)34 that can be summarized as follows:

a. The hydroxyl group 1 is not essential and can be removed, epimerized, or esterified.

b. The oxetane ring 2 (or a small-ring analog) is essential for activity.

c. The presence of acyl substituents 3, 4, and 8 is essential. Other acetoxy and benzoyloxy groups present in the natural product may be replaced by other acyls or removed.

d. Removal of the hydroxyl 5 leads only to a slight decrease in activity.

e. A free hydroxyl group in the side chain (6) is required. Esterification is possible if the ester group is easily hydrolyzable, leading to a variety of water-soluble and cell-specific paclitaxel prodrugs.

f. The phenyl group at the end of the side chain (7), or a close analog, is required for activity.

g. Reduction of the carbonyl 10 leads to slightly improved activity.

In summary, the northern half of the molecule allows more structural variations than the southern portion. The 2'R-3'S-isoserine side chain is also a key element in the antitubulin activity.

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