Syntheses and Evaluation of Simplified Pretubulysin Analogues

The syntheses of new tubulysin analogues are described, in which the central amino acid, tubuvaline, is replaced by the rather simple building blocks phenyltubuvaline and phenoxytubuvaline. These analogues can be obtained in only two to three steps from easily accessible starting materials. Although the new derivatives are less active than the tubulysins, their activities towards U-2 OS tumor cells are still in the nanomolar range.