ABSTRACT
Mild and efficient methods for site-specific methylation of peptide backbone amides are important tools for chemists seeking to modulate the pharmacokinetic properties of peptide drugs. The Mitsunobu reaction was used to selectively methylate N-trifluoroacetamide (Tfa) protected peptides on-resin. The Tfa group was removed quickly and completely by reduction with excess NaBH4, and it was shown to be orthogonal to many of the protecting groups used in solid-phase peptide synthesis.
Subject(s)
Acetamides/chemistry , Amides/chemistry , Fluoroacetates/chemistry , Peptides/chemical synthesis , Methylation , Peptides/chemistry , Peptides/pharmacokinetics , Solid-Phase Synthesis TechniquesABSTRACT
Backbone N-methylation is common among peptide natural products and has a substantial impact on both the physical properties and the conformational states of cyclic peptides. However, the specific impact of N-methylation on passive membrane diffusion in cyclic peptides has not been investigated systematically. Here we report a method for the selective, on-resin N-methylation of cyclic peptides to generate compounds with drug-like membrane permeability and oral bioavailability. The selectivity and degree of N-methylation of the cyclic peptide was dependent on backbone stereochemistry, suggesting that conformation dictates the regiochemistry of the N-methylation reaction. The permeabilities of the N-methyl variants were corroborated by computational studies on a 1,024-member virtual library of N-methyl cyclic peptides. One of the most permeable compounds, a cyclic hexapeptide (molecular mass = 755 Da) with three N-methyl groups, showed an oral bioavailability of 28% in rat.
Subject(s)
Peptides, Cyclic/metabolism , Peptides, Cyclic/pharmacokinetics , Animals , Biological Availability , Chemistry, Pharmaceutical , Combinatorial Chemistry Techniques , Computer Simulation , Drug Discovery/methods , Male , Methylation , Molecular Structure , Peptides, Cyclic/chemistry , Rats , Structure-Activity RelationshipABSTRACT
A mild and effective method was developed to convert peptides immobilized on the 2-chlorotrityl and Wang resins directly to C-terminal esters. After conventional Fmoc peptide synthesis, treatment with anhydrous HCl (0.2-3 M) in a variety of alcohols was shown to produce the corresponding peptide esters in good yield and purity. Under the mildest conditions investigated, acid-sensitive protection groups such as N-Boc, trityl, tert-butyl ether, tert-butyl ester, and Pbf remain intact.
Subject(s)
Esters/chemistry , Peptides/chemistry , Resins, Synthetic/chemistry , EsterificationABSTRACT
Despite the vast number of techniques developed for the cyclization of small peptides, cyclization efficiency remains problematic in peptides that lack turn-promoting structures. Here we demonstrate the utility of click chemistry as a macrocyclization tool in the solid-phase synthesis of cyclic tetra-, penta-, hexa-, and heptapeptides. On-resin cyclization is completed at room temperature within 6 h, resulting in predominantly monomer with small amounts of cyclomultimer byproducts.