Structure-activity relationships in side-chain-modified epothilone analogues - How important is the position of the nitrogen atom?

Side-chain-modified epothilone analogues 1 b, 2 b, and 3 b were prepared through stereoselective total synthesis to assess the importance of N-atom positioning in the side chain for tubulin polymerization and anti proliferative activity. Surprisingly, 1 b, 2b, and 3b appear to induce tubulin polymerization with activities similar to those of 1 a, 2a, and 3a, respectively. Substantial differences in antiproliferative activity were observed between 1 a and 2 a, and 1 b and 2b, but not between 3 a and 3b.