Synthesis of cyclic analogues of loop 4 of nerve growth factor

Cyclic peptides cyclo(-Gly-Asp-Glu-Lys-), cyclo(-Gly-Gly-Asp-Glu-Lys-) and cyclo(-Gly-Gly-Gly-Asp-Glu-Lys-) were synthesized as models of theβ-turn of nerve growth factor loop 4. The corresponding protected linear precursors were obtained in 52–83% yields by the solid-phase method with the use of the Fmoc/Bu ^t strategy and a chlorotrityl anchor group. The cyclization was carried out with benzotriazolyloxytris(dimethylamino)phosphonium (BOP) hexafluorophosphate, N-[(1H-benzotriazole-1-yl)-(dimethylamino)methylene]-N-methylmetanaminium-N-oxide (HBTU) hexafluorophosphate, and diphenylphosphorylazide (DPPA) at a dilution of 10^?3 M. The distribution of reaction products was studied for each cyclopeptide in dependence on the type of the coupling agent. The use of DPPA was shown to completely inhibit the formation of cyclodimers in the synthesis of five-and six-membered cyclopeptides; however, in the case of a four-membered peptide, an additional tenfold dilution of the reaction mixture was necessary to achieve the effect. The identification of several byproducts during the synthesis showed that the elongation of the polypeptide chain using the BOP reagent can be complicated by substantial racemization, and the cleavage of the chlorotrityl anchor group by 0.5% TFA in dichloromethane proceeds with insufficient selectivity and is accompanied by the premature Boc deblocking of the lysine side function.