Synthesis of the Dolastatin 3 Isomer cyclo-[L-Pro-L-Leu-L-Val-(-R,S)-(gln)Thz-(gly)Thz]

With application of mixed carbonic anhydride and dicyclohexylcarbodiimide-N-hydroxysuccinimide peptide bond forming methods culminating in a 2,4,5-trichlorophenol active ester promoted cyclization reaction, synthesis (cf., Scheme I) of cyclo-[L-Pro-L-Leu-L-Val-(R,S)-(gln)Thz-(gly)Thz] was completed. Comparison of this cyclic peptide with the Indian Ocean sea hare cell growth inhibitory constituent dolastatin 3 clearly showed that the natural peptide possesses a closely related structure. The high resolution (400 MHz) ¹H NMR and ¹³C NMR spectra of the synthetic peptides were evaluated in detail and suggested that dolastatin 3 may contain (R)-(gln)Thz and/or a modified Pro-Leu-Val sequence.