Two-step multi-enzyme chemoenzymatic synthesis of diasialyl oligosaccharides containing different sialic acid forms and various sialyl linkages. Enzymes: E. coli aldolase, Escherichia coli K12 sialic acid aldolase; NmCSS, Neisseria meningitidis CMP-sialic acid synthetase; PmST1, Pasteurella multocida sialyltransferase for the formation of α2–3-linked sialosides; Pd2,6ST, Photobacterium damsela α2–6-sialyltransferase for the formation of α2–6-linked sialosides; CstII, Campylobacter jejuni sialyltransferase for the formation of α2–8-linked sialosides. Compounds: Man, mannose; ManNAc, N-acetylmannosamine; CTP, cytidine 5′-triphosphate; Sia, sialic acid.