Solid-phase synthesis of protected peptides using new cobalt(III) ammine linkers†

Cobalt(III) ammine complexes of the type cis-[CoL4(4-AMB)O-AA-Boc](CF3SO3)2, where L4= bisethylenediamine (en)2 or tetraammine (NH3)4, and 4-AMB = 4-(aminomethyl)benzoic acid, have been synthesized and used as linkers to polystyrene resins for solid-phase synthesis of protected peptides. Boc/t-Bu-protected [Leu5]enkephalin was assembled on the two different Co(III) resins, and then cleaved from the resins by reduction of the Co(III) center in 93–96%; yield. HPLC-purified protected [Leu5]enkephalin was obtained in 67–69% overall yield and characterized by amino acid analysis and 1H NMR. Stepwise synthesis on the Co(en)2-resin was also used in the assembly of Boc-Asp(OcHex)-Arg(Mts)-Gly-Asp(OcHex)-Ala-Pro-Lys(2Cl-Z)-Gly-OH, a sequence from collagen α1 Type 1. The protected peptide was cleaved from the Co(III) resin in 74% yield, and the HPLC-purified nonapeptide was characterized by amino acid analysis, 1H NMR and liquid secondary-ion mass spectrometry (LSIMS). New routes are described for the synthesis of isomerically pure Co(III) anchor complexes. The Co(III) resins were found to be compatible with both the tert-butyloxycarbonyl (Boc) and the 9-fluorenylmethoxycarbonyl (Fmoc) Nα-protecting group strategies used in solid-phase peptide synthesis.