Semisynthesis and degradation of the tubulin inhibitors epothilone
and tubulysin
G. H�fle*, N. Glaser, T. Leibold, U. Karama, F. Sasse, and H. Steinmetz
*GBF, Gesellschaft für Biotechnologische Forschung
mbH, Department of Natural Products, D-38124 Braunschweig, Germany
Abstract: The structure-activity relationships of epothilones
indicate that major modifications are only tolerated in the western
ring segment. In particular, C2 methyl of the thiazole ring appears
to be most flexible. Its broad modification started from epothilone
F, which was obtained from natural epothilone B by hydroxylation via
the N-oxide. Some of the prepared derivatives exhibit improved
esterase stability in addition to high cytotoxic activity. For these
and other favorable properties, amine (BMS-310705) was recently introduced
in clinical trials. In an alternative approach, modified side chains
were introduced by replacement of the C12,C15 ring segment via ring-opening
olefin metathesis (ROM) of epothilone C in the presence of ethylene
to 12,13-seco-epothilone C, introduction of a synthetic building
block followed by ring-closing olefin metathesis (RCM), and epoxidation
to the 16-alkyne analog of epothilone A.
The structure of the tetrapeptide tubulysin D was confirmed by total
hydrolysis to N-methyl D-pipecolic acid,
L-isoleucine, tubuvaline (Tuv), tubuphenylalanine
(Tup), formaldehyde, and 3-methylbutyric acid. Mild acidic hydrolysis
to cyclo-tubulysin and
oxidative degradation to L-valine allowed the
assignment of the stereocenters of Tuv, hydrazinolysis, and comparison
with synthetic reference samples to that of Tup. The absolute configuration
of tubulysin D is: (R)-Mep, (2,3S)-Ile, (1'R ,3'R)-Tuv,
and (2S,4R)-Tup.
*Pure Appl.Chem. 75,
141-419 (2003). An issue of reviews and research papers based on
lectures presented at the 23rd IUPAC International Symposium on the
Chemistry of Natural Products, Florence, Italy, 28 July - 2 August 2002.