piperidines and aspartimide

The Fmoc-based SPPS of H-Xaa-Asp(OBzl)-Yaa-Gly-NH(2) sequences results in side reactions yielding not only aspartimide peptides and piperidide derivatives, but also 1,4-diazepine-2,5-dione-peptides. Evidence is presented to show that the 1,4-diazepine-2,5-dione derivative is formed from the aspartimide peptide. The rate of this ring transformation depends primarily on the tendency to aspartimide and piperidide formation, which is influenced by the nature of the amino acid following the aspartic acid beta-benzyl ester (Xaa). However the bulkiness of the amino acid side chain preceeding the aspartic acid beta-benzyl ester (Yaa) is also important. Under certain conditions the 1,4-diazepine-2,5-dione peptide derivative may even be formed dominantly, which is a highly undesirable side reaction in peptide synthesis, but which provides a new way for the synthesis of diazepine peptide derivatives with targeted biological or pharmacological activity.

Topics: Aspartic Acid; Esters; Molecular Structure; Peptides, Cyclic; Piperidines

The sequence dependence of base-catalysed aspartmide formation during Fmoc-based SPPS was systematically studied employing the peptide models H-Val-Lys-Asp-Xaa-Tyr-Ile-OH. The extent of formation of aspartimide and related by-products was determined by RP-HPLC. Considerable amounts of by-products were formed in the case of Xaa = Asp(OtBu), Arg(Pbf), Asn(Mtt), Cys(Acm) and unprotected Thr. Aspartimide formation could be diminished by incorporation of Asp(OMpe) or by employing milder methods for Fmoc cleavage, e.g. hexamethyleneimine/N-methylpyrrolidine/HOBt/NMP/DMSO 4:50:4:71:71 (v/v/w/v/v).

Topics: Amino Acids; Aspartic Acid; Bridged Bicyclo Compounds, Heterocyclic; Chromatography, High Pressure Liquid; Fluorenes; Peptide Biosynthesis; Peptides; Piperidines

With the objectives of developing new protecting groups for the beta-carboxyl group of aspartic acid that are resistant to base-catalyzed aspartimide formation and of evaluating the importance of sterical factors in the design of such protecting groups, four new alkyl ester derivatives of aspartic acid were synthesized. The beta-3-pentyl, beta-4-heptyl, beta-2,6-dimethyl-4-heptyl and the recently described beta-2,4-dimethyl-3-pentyl esters of Boc-aspartic acid were incorporated into model peptides, and the resin-bound protected peptides were treated with 20% piperidine for 10 h. The levels of aspartimide-related side products were compared with the previously reported beta-cyclohexyl, beta-menthyl and beta-2-adamantyl esters of aspartic acid. The results show that bulky, acyclic, aliphatic protecting groups (in particular the 2,4-dimethyl-3-pentyl ester) are significantly more resistant to base-catalyzed aspartimide formation than comparably rigid cyclic alkyl esters that under the same reaction conditions form several-fold more aspartimide-related side products. Using elevated temperatures to overcome difficult couplings leads to the formation of significant amounts of aspartimide when aspartic acid is protected with the cyclohexyl group, but the 2,4-dimethyl-3-pentyl protecting group offers excellent protection under these conditions. The use of the 2,4-dimethyl-3-pentyl protecting group will allow the use of orthogonally removable base-labile protecting groups in Boc chemistry and suggests a design of protecting groups for other nucleophile-sensitive trifunctional amino acids in both Boc and Fmoc chemistry.

Topics: Aspartic Acid; Chromatography, High Pressure Liquid; Esters; Oligopeptides; Piperidines; Temperature; Time Factors