Studies on sensitivity to racemization of activated residues in couplings of N-benzyloxycarbonyldipeptides.

A series of 24 peptides Z-Gly-Xaa(R)-OH where Xaa = 15 different residues and R = H, NH2, tBu, Bzl, Trt, Mtr, and StBu were coupled with valine benzyl ester in dimethylformamide or dichloromethane at +5 degrees. The accompanying racemization was determined by analysis of the epimeric products by normal phase high-performance liquid chromatography (HPLC) for Xaa(R) = Met, Cys(StBu) and Lys(Z) and by reversed-phase HPLC after removal of benzyl-based protecting groups for Xaa(R) = Ser(tBu), Thr(tBu) and Arg(Mtr). The coupling methods examined included mixed anhydride (MxAn) at -5 degrees, and N,N'-dicyclohexylcarbodiimide (DCC), benzotriazol-1-yl-tris(dimethylamino)phosphonium hexafluorophosphate (BOP) and O-benzotriazol-1-yl-N,N,N',N'-tetramethyluroniumhexafluorophosp hate (HBTU) in the presence of 1-hydroxybenzotriazole (HOBt). Very few couplings gave stereochemically pure products. The order of sensitivity to racemization of residues depended on the method of coupling and the solvent. It varied most when comparing MxAn to HOBt-assisted reactions; it varied moderately when comparing HOBt-assisted reactions. There was less variation in comparing BOP and HBTU reactions that are initiated by the same mechanism. Leu, Nle, Phe, Asn, Lys(Z) and Asp(OBzl) are identified as the residues least sensitive to racemization. DCC-HOBt generally led to less epimerization than the other methods.