Synthesis and applications of cyclopeptides and depsipeptides

A solid phase protocol has been devised for the synthesis of linear precursors to cyclic depsipeptide analogues of dolastatin D. t-Butyldimethylsilyl groups were used for hydroxy group protection, with deprotection being carried out by t-butyl ammonium fluoride. HATU and PyBrop were successful in coupling highly hindered residues and in depside bond formation. Cyclic peptide analogues, cyclo[Arg-Gly-Asp-D-Phe-Lys(or Tyr)] have been synthesised and modified for use as carrier molecules for the transport of radio isotopes (In-111 and I-125) into blood platelets as prototypes for medical imaging.