Recent Departures in the Synthesis of Peptides and Glycopeptides

Abstract

In this account, we describe the results of a research program directed to the proposition that chemical synthesis can play a valuable role in identifying biologic level molecules worthy of pharma level development. We recount our journey towards the chemical synthesis of homogeneous erythropoietin, the challenges we encountered, and our efforts to address deficiencies in the current "state of the art" of glycopeptide synthesis. Here we describe new methods for the synthesis of glycopeptides that have emerged from the erythropoietin adventure, including the development of unique C-terminal acyl donors, novel amide bond forming methods, and new ligation and coupling strategies.

Figure 1

Glycal assembly

Figure 2

Strategy for glycopeptide-glycopeptide coupling using a masked thioester

Scheme 1

^a Glycopeptide-glycopeptide coupling using a ortho-disulfide phenolic ester as a latent acyl donor

Figure 3

Strategy for a cysteine-free glycopeptide-glycopeptide ligation

Scheme 2

^a Synthesis of bifunctional glycopeptide 19 using a cysteine-free ligation strategy

Figure 4

Second-generation cysteine-free glycopeptide-glycopeptide ligation strategy

Scheme 3

^a Synthesis of glycopeptide 31 using a cysteine-free ligation strategy

Figure 5

A retrosynthetic route to glycopeptide 32 via reiterative cysteine-based and cysteine-free ligations

Scheme 4

^a Synthesis of trifunctional glycopeptide 32 via reiterative cysteine-based and cysteine-free ligations

Scheme 5

^a Synthesis of cyclic peptide 39 via an intramolecular cysteine-free ligation

Scheme 6

^a Synthesis of the erythropoietin Ala²²-Glu³⁷ glycopeptide domain (44) via cysteine-based ligation

Scheme 7

^a Synthesis of the erythropoietin Ala¹¹⁴-Arg¹⁶⁶ glycopeptide domain (48) via cysteine-free ligation

Figure 6

Direct fragment condensation via metal-mediated activation of an ortho-disulfide phenolic ester

Scheme 8

^a Synthesis of glycopeptide 52 via silver-mediated fragment condensation

Scheme 9

^a Synthesis of glycopeptide 55 via TCEP-mediated direct condensation

Scheme 10

^a Reiterative TCEP/AgCl (glyco)peptide coupling

Scheme 11

^a Synthesis of the erythropoietin glycopeptide domain Gln⁷⁸-Arg¹⁶⁶ (63) via reiterative TCEP/AgCl fragment coupling

Scheme 12

^a A para-nitrophenyl ester as acyl donor in native chemical ligation

Scheme 13

^a Peptide ligation with hindered penicillamine

Scheme 14

^a Fine tuning acyl donors in the synthesis of the erythropoietin Cys²⁹-Gly⁷⁷ glycopeptide domain (76)

Scheme 15

^a Glycopeptide synthesis via native chemical ligation and radical desulfurization

Scheme 16

^a Synthesis of the erythropoietin Ala¹-Gly²⁸ glycopeptide domain (87) via native chemical ligation and radical desulfurization

Scheme 17

^a Native chemical ligation at valine

Figure 7

Two-component coupling reaction of carboxylic acids and isonitriles

Scheme 18

^a Studies on the two-component coupling in building asparagine-linked glycopeptides

Figure 8

The versatility of the N-formyl mixed imide