Toward fully synthetic, homogeneous glycoproteins: advances in chemical ligation.

Traditionally, in the pharma sciences, there has been an unstated but operative bifurcation into small molecules and biologics. Small molecules were seen to be, at the discovery level, in the province of chemistry, based on targets provided through biology. By contrast, "biologics" were seen to arise solely from the province of biology exploiting its accessible replicative mechanisms. Our laboratory has been dedicated to the proposition that explosive advances in chemical synthesis have been such as to render so called "biologics" as being accessible to chemical synthesis. In this article, we focus particularly on the area of glycopeptides. Chemical synthesis, in principle, offers an advantage, in that it can lead to homogeneous glycopeptides characterized by a single glycoform of the glycosidic domain mounted at a particular amino acid in the polypeptide domain. In support of this defining goal, a variety of new methods have been developed. The key problem addressed is that of ligation. In this article, we review how insights available from mechanistic organic chemistry have been used to create an imposing framework for the synthesis of structures which would, in an earlier day, have been seen to be strictly in the realm of chemically inaccessible "biologics".