A novel strategy is developed to expand the scope of chemoenzymatic synthetic products by designing a chemoenzymatic synthon. The synthon is enzymatically converted to carbohydrate analogs which are readily derivatized chemically to produce desired targets. The strategy is demonstrated for synthesizing glycosides containing 7,9‐di‐N‐acetyllegionaminic acid (Leg5,7Ac2), a bacterial nonulosonic acid (NulO) analog of sialic acid. A versatile library of a2‐3/6‐linked Leg5,7Ac2‐ glycosides is built using chemically synthesized 2,4‐diazido‐2,4,6‐trideoxy mannose as a chemoenzymatic synthon for highly efficient one‐pot multienzyme (OPME) sialylation followed by a downstream chemical conversion of the azido group to an acetamido group. The overall yields of the syntheses are 34‐52% in 10 steps from commercially available D‐fucose, representing significant improvements over previous methods. Free Leg5,7Ac2 monosaccharide is also synthesized using a sialic acid aldolase‐catalyzed reaction.