Tubulysin D

The total syntheses of tetrapeptides tubulysins D (1 b), U (1 c), and V (1 d), which are potent tubulin polymerization inhibitors, are described. The synthesis of Tuv (2), an unusual amino acid constituent of tubulysins, includes an 1,3-dipolar cycloaddition reaction of chiral nitrone D-6 derived from D-gulose with N-acryloyl camphor sultam (-)-9 employing the double asymmetric induction, whereas the synthesis of Tup (20), another unusual amino acid, involves a stereoselective Evans aldol reaction of (Z)-boron enolate generated from (S)-4-isopropyl-3-propionyl-2-oxazolidinone with N-protected phenylalaninal and a subsequent Barton deoxygenation protocol. We accomplished the total syntheses of tubulysins U (1 c) and V (1 d) by using these methodologies, in which the isoxazolidine ring was used as the effective protective group for γ-amido alcohol functionality. Furthermore, to understand the structure-activity relationship of tubulysins, we synthesized tubulysin D (1 b) and cyclo-tubulysin D (1 e) from 2-Me and 20, and ent-tubulysin D (ent-1 d) from ent-2-Me and ent-20, respectively. The preliminary results regarding their biological activities are also reported.

The synthesis and biological evaluation of stereoisomers in tubulysin D are described. The stereoselective synthesis of all possible stereoisomers of C-11 and C-13 positions in tubulysin D was achieved by employing 1'-epi-Tuv-Me, 3'-epi-Tuv-Me, and ent-Tuv-Me and their biological properties were evaluated. It is clear that the stereochemistries of the C-11 and C-13 positions in tubulysin D have no practical impact on the inhibition of tubulin polymerization but play a role in the potent antiproliferative activities.

A practical and stereoselective synthesis of the tubuvaline-tubuphenylalanine (Tuv-Tup) fragment of tubulysin is achieved involving the opening of aziridine, crucial MacMillan alpha-hydroxylation on both fragments, and an epoxide-opening reaction.