A novel N-acetyl-5-N,4-O-carbonyl-protected dibutyl sialyl phosphate donor for sialylation of both primary and sterically hindered secondary acceptors to prepare sialosides with high yield and α-selectivity is disclosed. Methods for making disaccharide building blocks comprising α(2→3), α(2→6), α(2→8), α(2→8)/α(2→9) alternate, and α(2→9) sialosides are provided. methods for one-pot synthesis of complex sialosides are disclosed. Libraries of sialosides and methods for using the libraries for detection and receptor binding analysis of surface glycoproteins or pathogens and cancer cells are disclosed. Methods for distinguishing between hemagglutinin (HA) from various strains of influenza are provided.