Erythropoietin is a signaling glycoprotein that controls the fundamental process of erythropoiesis, orchestrating the production and maintenance of red blood cells. As administrated clinically, erythropoietin has a polypeptide backbone with complex dishomogeneity in its carbohydrate domains. Here we describe the total synthesis of homogeneous erythropoietin with consensus carbohydrate domains incorporated at all of the native glycosylation sites. The oligosaccharide sectors were built by total synthesis and attached stereospecifically to peptidyl fragments of the wild-type primary sequence, themselves obtained by solid-phase peptide synthesis. The glycopeptidyl constructs were joined by chemical ligation, followed by metal-free dethiylation, and subsequently folded. This homogeneous erythropoietin glycosylated at the three wild-type aspartates with N-linked high-mannose sialic acid–containing oligosaccharides and O-linked glycophorin exhibits Procrit-level in vivo activity in mice.

A method by which to accomplish formal threonine ligation has been developed. The method accomplishes ligations of two peptide domains. We have also demonstrated the ability to successfully ligate two independent glycopeptide domains.

Recent developments in the use of isonitriles to furnish secondary and tertiary amide bond formations are applied to a novel total synthesis of the important cyclic polypeptide cyclosporine A. Specifically, the disclosed synthetic route demonstrates the utility of microwave-mediated carboxylic acid isonitrile couplings, thioacid isonitrile couplings at ambient temperature, and isonitrile-mediated couplings of carboxylic acids and thioacids with amines to form challenging amide bonds.

Our global goal is that of synthesizing complex polypeptides and glycopeptides in homogeneous form. Chemistry-derived access to homogeneous biologics could well have useful consequences in the discovery of drugs and vaccines. The key finding in this report is that thio acids can become highly competent acyl donors following even trace levels of oxidative activation, thereby undergoing amide bond formation upon reaction with N-terminal peptides. Though our data set does not establish the specific mechanism of this reaction, a framework to account for the fact that minute levels of oxidation actuate amide bond formation with high turnover is offered. An apparently general coupling of thio acids (including complex peptide thio acids with N-termini of complex peptides) has thus been realized. These ligations are conducted with minimal α-epimerization in the C-terminal group and allow for the coupling of N-terminal and C-terminal glycopeptides en route to homogeneous glycoproteins.