Coupling Carbohydrates to Proteins for Glycoconjugate Vaccine Development Using a Pentenoyl Group as a Convenient Linker

Carbohydrates are important molecular targets in the development of vaccines against cancer, viral and bacterial infections, and many other diseases. However, carbohydrates are usually poorly immunogenic and cannot induce a T cell-dependent immune response that is necessary for effective immunity. To overcome this problem, carbohydrate antigens have to be coupled to an immunogenic carrier molecule, such as a protein, to improve their immunogenicity. To this end, many carbohydrate–protein coupling methods have been developed. A recently established method is based on the introduction of an azido group to carbohydrate antigens during their syntheses, and after the carbohydrate antigens are synthesized, the azido group can be selectively reduced to a free amino group, to which a 4-pentenoyl group can be readily and regiospecifically attached. Thereafter, the C=C bond of the pentenoyl group is ozonolyzed to generate a reactive aldehyde functionality, through which the carbohydrate antigens are linked to carrier proteins by reductive amination. Since the azido group is orthogonal to most transformations involved in carbohydrate synthesis, it can be introduced at an early stage of the synthesis. Moreover, since the pentenoyl group, as well as its aldehyde derivative, is attached to the carbohydrate antigens after they are synthesized, this would significantly simplify the synthetic design of complex carbohydrates, including the design of protecting tactics.