An efficient method has been developed for the salicylaldehyde ester-mediated ligation of unprotected peptides at serine (Ser) or threonine (Thr) residues. The utility of this peptide ligation approach has been demonstrated through the convergent syntheses of two therapeutic peptides--ovine-corticoliberin and Forteo--and the human erythrocyte acylphosphatase protein (∼11 kDa). The requisite peptide salicylaldehyde ester precursor is prepared in an epimerization-free manner via Fmoc-solid-phase peptide synthesis.andmark advances in the field of synthetic protein chemistry have enabled the preparation of complex, homogeneous proteins (1-3), including those that carry specific posttranslational modifications (PTMs) (4-11). Of particular significance, Danishefsky and coworkers have obtained glycosylated human folliclestimulating hormone (7) and erythropoietin (11), solely through chemical synthesis. In general, protein synthesis consists of two key phases: (i) solid-phase peptide synthesis (SPPS) allows for the generation of moderately sized peptide segments (up to ∼30 amino acids) (12); and (ii) chemical ligation serves to chemoselectively join these synthetic peptide fragments (13-16). In the 1970s, Kemp and coworkers conceptually devised a promising peptide ligation strategy, involving prior capture followed by acyl transfer, which has laid the foundation for the development of chemical ligation in the convergent peptide synthesis (17)(18)(19)(20)(21)(22). A milestone advance in the field was the discovery, by Kent and coworkers, of native chemical ligation (NCL) (13), in which a C-terminal peptide thioester and an N-terminal cysteine-containing peptide--both in side-chain unprotected forms--are selectively coupled to generate a natural peptidic linkage (Xaa-Cys) at the site of ligation. Because the Kent NCL approach exploits the unique nucleophilicity of the cysteine thiol group, the relatively low abundance of cysteine residues in natural proteins can pose a significant challenge to NCL-based protein synthesis efforts. In an effort to expand the scope of NCL, a number of research groups have developed variants, wherein β-or γ-thiol containing amino acids are temporarily installed on the peptide N terminus, thus promoting NCL-like ligation. Subsequent desulfurization serves to restore the natural amino acid at the site of ligation (23)(24)(25)(26). Whereas this NCLdesulfurization strategy has been widely used in protein synthesis, a menu of complementary thiol-independent ligation approaches (27-34) may offer new opportunities for convergent protein synthesis. Nevertheless, these thiol-independent ligations require the installation of unique reaction functionalities, often tediously, on both sides of the N-terminal peptide segment and the C-terminal peptide segment to induce a chemoselective coupling reaction.Along these lines, our laboratory has been pursuing the development of methods for ligation at N-terminal serine and threonine residues to generate natural Xaa-Ser/Thr linkages directly, using natural serin...

A total synthesis of daptomycin, the first natural product antibiotic launched in a generation, was achieved. This convergent synthesis relies on an efficient macrocyclization via a serine ligation to assemble the 31-membered cyclic depsipeptide. The difficult esterification by the nonproteinogenic amino acid kynurenine was accomplished via the esterification of a threonine residue by a suitably protected Trp ester, followed by ozonolysis. This synthesis provides a foundation and framework to prepare varied analogues of daptomycin to establish its structure-activity profile.

Syntheses of MUC1 glycopeptides (40-mer and 80-mer) are described. The convergent synthesis was achieved by native serine ligation using side-chain unprotected glycopeptide segments.

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