The chemical ligation of selectively S-acylated cysteine peptides to form native peptides via 5-, 11- and 14-membered cyclic transition states

Abstract

Cysteine and C-terminal cysteine peptides are selectively S-acylated at 0-20 degrees C by N-(Pg-alpha-aminoacyl)benzotriazoles to give N-Pg-S-acyl-isodi-, -isotri-, and -isotetra-peptides isolated in good yields. N-Fmoc-S-acyl-isopeptides are Fmoc deprotected to afford free S-acyl-isopeptides isolated in high yields. S-Acyl-isodi-, S-acyl-isotetra-, and S-acyl-isopentapeptides undergo chemical ligation; migration of the cysteine S-acyl groups to the N-terminal amino acids via 5-, 11-, and 14-membered transition states giving the corresponding native di-, tetra-, and penta-peptides. By contrast, the Sacyl-isotripeptide prefers intermolecular acylation from one molecule to another over an 8-membered intramolecular transition state. The developed methodology allows convenient isolation of stable, unprotected S-acyl cysteine peptides including the first isolation of S-acyl-isopeptides, which should facilitate the investigation of ligation by physical organic chemistry techniques.

Study of Chemical Ligation Via 17‐, 18‐ and 19‐Membered Cyclic Transition States

“…This result is consistent with our previous work in which we have shown similar S fi N acyl migration by an intramolecular mechanism (23,24) and this was supported by competitive experiments (23) and computational arguments (30).…”

“…Compound S3 was converted into the benzotriazolide S4 at −15 °C, and S4 was then reacted with S2b at 20 °C in the presence of triethylamine to generate protected tripeptide Fmoc‐Gly‐Leu‐Cys‐OH S5 (91%). Tripeptide S5 was S ‐acylated by Cbz‐Ala‐Bt S1b in the presence of KHCO 3 to provide N ‐protected mono‐isotetrapeptide S6, which, after deprotection by DBU, yielded the mono‐isotetrapeptide 1 in 85% yields (Scheme S1 in Appendix S1; 22, 25–27).…”

“…According to a generally accepted notion, the subsequent acyl shift should proceed via entropically favored 5‐ and 6‐member ring intermediates to achieve useful rates. However, our group has tested the feasibility of chemical ligation of S‐acylated cysteine peptides via 5‐, 8‐, 11‐, and 14‐membered cyclic transition states (22–24). We now report migration of the acyl group from S ‐acylated cysteine isopeptides containing α ‐, β ‐ and γ ‐amino acids via 17‐, 18‐ and 19‐membered cyclic transition states.…”

α‐Substitution Effects on the Ease of S→N‐Acyl Transfer in Aminothioesters

“…A study by Seitz used a modified NCL protocol where the reaction vessels were incubated in a thermo mixer at 35 °C (12). We showed that NCL via 11‐membered transition state was achieved (∼14%) when the reaction mixture was subjected to microwave irradiation of 50 W at 50 °C (15).…”

Cysteinoyl‐ and Cysteine‐containing Dipeptidoylbenzotriazoles with Free Sulfhydryl Groups: Easy Access to N‐terminal and Internal Cysteine Peptides

“…Cysteine peptides are diagnostically and therapeutically important in many areas of biomedical research (1–6) and thus attractive targets for drug discovery. Consequently, interest in cysteine peptide synthesis has increased significantly (7–9), including the preparation of unsymmetrical disulfides (10,11), formation of oligodeoxynucleotide (ODN)‐peptide covalent linkages (12), and the synthesis of cyclic peptides (13).…”