Side reactions in solid-phase peptide synthesis and their applications.

Side reactions in peptide synthesis indicate steps needing improvement as well as opportunities for structural diversification in combinatorial design. Among the side reactions observed in this study, transesterification of Boc-Glu(OBzl) occurred in TMAH-catalyzed resin attachment, leading to Boc-DKKREE(OMe) in solid-phase synthesis of Boc-DKKREE. Acetylation of Boc-Arg(NO2)-resin occurred during resin capping with Ac2O/Et3N, leading to GPR (Ac) in GPR synthesis. His- and Lys-modification occurred during GHRPLDKKREE cleavage from resin by Pd(OAc)2-catalyzed hydrogenation in DMF. To verify these side reactions, model experiments were performed, which indicated rapid transesterification of Boc-Glu(OBzl) in methyl, isopropyl, or tert-butyl alcohol into the corresponding ester by TMAH, but studies of acetylation showed that both Boc-Arg(NO2) and Boc-Arg(Tos) were stable to Ac-Im treatment, but were modified by Ac2O/Et3N. Since transfer hydrogenation of Boc-His(Bzl) and Boc-Lys(Z) in HCOOH or ammonium formate did not generate the formylated side-products of catalytic hydrogenation, DMF and not its decomposed product, HCOOH, appeared involved in side-chain modification. Elimination of the side reactions, by using Cs-derived Boc-Glu(OBzl)-resin for peptide synthesis and catalytic hydrogenation in NMP-HOPr for peptide cleavage, increased the GHRPLDKKREE yield by 1/3. On the other hand, the side reactions provided modified peptides, whose bioassays revealed different importance of the modified side-chains.