Chemical Ligation of S-Scylated Cysteine Peptides to Form Native Peptides via 5-, 11-, and 14-Membered Cyclic Transition States

Cysteine-containing dipeptides 3a-1, (3b+3b') (compound numbers in parentheses are used to indicate racemic mixtures; thus (3b+3b') is the racemate of 3b and 3b'), and tripeptide 13 were synthesized in 68-96\% yields by acylation of cysteine with N-(Pg-alpha-aminoacyl)- and N-(Pg-alpha-dipeptidoyl)benzotriazoles (where Pg stands for protecting group in the nomenclature for peptides throughout the paper) in the presence of Et3N. Cysteine-containing peptides 3a-1 and 13 were S-acylated to give S-(Pg-alpha-aminoacyl)dipeptides 5a-1 and S-(Pg-alpha-aminoacyl)tripeptide 14 without racemization in 47-90\% yields using N-(Pg-alpha-aminoacyl)benzotriazoles 2 in CH3CN-H2O (7:3) in the presence of KHCO3. (In our peptide nomenclature, the prefixes di-, tri-, etc. refer to the number of amino acid residues in the main peptide chain; amino acid residues attached to sulfur are designated as S-acyl peptides. Thus we avoid use of the prefix ``iso{''}.) Selective S-acylations of serine peptide 3k and threonine peptide 3l containing free OH groups were thus achieved in 58\% and 72\% yield, respectively. S-(Pg-alpha-aminoacyl)cysteines 4a,b underwent native chemical ligations to form native dipeptides 3f,i via 5-membered cyclic transition states. Microwave irradiation of S-(Pg-alpha-aminoacyl)tripeptide 15 and S-(Pg-alpha-aminoacyl)tetrapeptide 17 in the presence of NaH2PO4/Na2HPO4 buffer solution at pH 7.8 achieved chemical ligations, involving intramolecular migrations of acyl groups, via 11- and 14-membered cyclic transition states from the S-atom of a cysteine residue to a peptide terminal amino group to form native peptides 19 and 20 in isolated yields of 26\% and 23\%, respectively.