Synthesis of Cyclic Tetrapeptides via Ligation of Peptide Hydrazides

Abstract

Naturally occurring or man-made cyclic tetrapeptides have unique rigid skeletons and therefore, represent an interesting class of candidate bioactive molecules in drug discovery. However, efficient chemical synthesis of cyclic tetrapeptides often presents a difficult problem due to the large strain involved in this category of target compounds. To overcome this problem we describe in the present study the use of peptide hydrazides for the preparation of highly strained 12-membered all-L cyclic tetrapeptides. The new synthetic route starts with the easy Fmoc solid phase synthesis of a linear N-Cys-tetrapeptide hydrazide precursor. Upon quick activation by NaNO 2 at pH 3 and -10 ℃ for 20 min followed by treatment with a pH 7 buffer containing an external thiol (4-mercaptophenylacetic acid, 40 equiv.) at room temperature, the N-Cys-tetrapeptide hydrazide precursor is converted in situ to an N-Cys-tetrapeptide thioester. This thioester undergoes a fast intramolecular thioester exchange reaction to generate a 13-membered thiolactone intermediate. Then an S-to-N acyl shift is expected to take place to create an amide bond, which affords the desired cyclic tetrapeptide in a modest overall yield (ca. 40%). Finally, desulfurization of the cyclic peptide product can be carried out to produce the target cyclic tetrapeptide that does not contain any Cys residue. Through detailed 1 H, 13 C, and TOSCY NMR and HPLC analyses, it is found that the new method for tetrapeptide synthesis can be carried out at relatively high substrate concentrations (0.8-1.0 mmol/L) without causing the formation of much cyclic octapeptide byproduct. Our test also showed that the reaction can generate the desired cyclic tetrapeptide in an epimerization free manner. By using the new method we have successfully prepared several cyclic tetrapeptides including cyclo(Ala-Leu-Ala-Leu), cyclo(Ala-His-Gly-Trp), and cyclo(Ala-Val-Gly-Ile) in 18%-25% overall yields. We expect that our method of cyclic tetrapeptide synthesis may find applications in the development of cyclic peptide libraries for bioactivity screening.

Ligation Technologies for the Synthesis of Cyclic Peptides

“…Zhang and Tam proposed that the absence of oligomerization at higher concentrations resulted from the ring-chain tautomeric equilibrium of the transthioesterification step between the thioester and the thiol on N-terminal Cys. This observation was consistent with a recent study by Liu and co-workers on the synthesis of highly strained cyclic tetrapeptides by NCL . In Liu’s work, they found that cyclic monomers were formed exclusively at a concentration up to 1 mM and that significant cyclic dimer formation occurred at a concentration of 3 mM.…”

Quantitative Structure-Activity Relationships of the Trisubstitued Triazines Bearing Aminopyrimidine Group

“…Natural charge: The atom natural charge of compounds are given in Table-3. These data show that, the negative charge is mainly concentrated in the C(1) of pyrimidine ring, N(2), N (5) and N(6) of triazine ring, and C(7), C(8), C (11) and C(12) of benzene ring. These atoms make the electronegative area, they could combine with positive area of receptor.…”

“…For pesticide molecules, too low-E LUMO or too high-E HOMO means that the molecule itself activity is too strong, it is easy to be metabolized in organism. The effect of pesticide is difficult to control, so the E LUMO or E HOMO of the pesticide molecule should be suitable to estimate expected value [10][11][12] .…”

An update on new methods to synthesize cyclotetrapeptides

“…Thus, four cyclic tetrapeptides containing all L-amino acids cyclo(L-Cys-L-Leu-L-Ala-L-Leu), cyclo(L-Cys-L-His-L-Gly-L-Trp), cyclo(L-Cys-L-Val-L-Gly-L-Ile) and cyclo(L-Cys-L-His-L-Trp-L-Gly) have been successfully prepared in moderate yields (∼40%) through intramolecular native chemical ligation (NCL) (Scheme 6). 56,57 Fmoc-SPPS on a hydrazine 2-chlorotrityl resin (26) was used to prepare the peptide hydrazide (27) and its conversion to acyl azide (28) in solution followed. The peptide hydrazide was used as a precursor to the corresponding peptide thioester (29), which undergo NCL (30) to afford the corresponding cyclotetrapeptide (31).…”