The invention relates to cis- and trans-12, 13-cyclopropyl and 12,13-cyclobutyl epothilones of formula I to IV
1
wherein Ar is a radical represented by the following structure:
2
and the other radicals and symbols have the meanings as defined herein; to their chemical synthesis and biological evaluation; their use in the treatment of neoplastic diseases and to pharmaceutical preparations containing such compounds. The compounds described herein are potent tubulin polymerization promoters and cytotoxic agents.