Determination of coupling rate constants of N-t-Butyloxycarbonyl-amino acid-active esters

Abstract

To determine the relative coupling rates of five common!y used actlve esters in peptide synthesis, i.e  pentafluorophenyl (OPfp), pentachlorophenyl (OPcp), p-oitrophenyl (ONp), 2,4,5 trichlorophenyl (OTcp) and sllccinimldyl (OSu), of three Boc-ammo acid (Ala, Phe and Cys (Bzl)), second-order coupling rate constants (Kcl) were determined with L-vahne methyl ester (Val-OMe) in THF 23°C. As expected, the active ester group had a significant effect on the Kc value, while the side chain of the amino acid had a less significant effect. The active esters Boc-amino acids of couple slower than the corresponding Z-amino-acid-active esters in most cases.