Convergent synthesis of dolastatin 15 (1), a cytostatic depsipeptide isolated from the Indian Ocean sea hare, has been described. For construction of the backbone, a single-step condensation of peptide fragment 2 and pyrrolidone fragment 3 was successfully performed using 2-chloro-1,3-dimethyl-2-imidazolinium hexafluorophosphate (CIP) developed by us as an efficient coupling reagent. Coupling of an N-methylamino acid on solid support was first achieved using CTP for the efficient synthesis of peptide fragment 2. The effectiveness of CIP for the coupling of N-methylamino acids in solution and on solid support were clarified by the syntheses of model di- and tripeptides.
Total synthesis of the proposed structure of Dolastatin 15
Efficient synthesis of dolastatin 15 (1) was accomplished following a convergent strategy. The pyrrolidinone cycle of 5 was obtained by thermic cyclization of the corresponding Meldrum's adduct 4. The methylation of the enol function was performed under Mitsunobu conditions. On the other hand, the peptidepart 10 was elongated by using PyCloP as coupling reagent. The ester linkage between both segments