New chemo-enzymatic route toward N-acetylneuraminic acid derivatives with alkyl groups at C-7 hydroxyl group

Based on chemo-enzymatic regio- and stereoselective reactions, new routes toward C-4 substituted N-acetyl-d-mannosamine (ManNAc) and the corresponding sialic acids from d-glucal were established. Lipase-catalyzed regioselective transformation of d-glucal and related substrates furnished precursors on which carbamate and alkyl substituent were properly introduced at C-3 and at C-4, respectively. Cyclic carbamate formation through rhodium-nitrenoid intermediates with iodobenzene pivalate and tert-butyl alcohol proceeded in manno-configured at C-2 as well as α- at C-1, exclusively. Ring opening and deprotection under mild conditions furnished the target ManNAc derivatives, which were the substrates for aldolase-catalyzed reactions.