Concise synthesis of 3-deoxy-d-manno-oct-2-ulosonic acid (KDO) as a protected form based on a new transformation of α,β-unsaturated ester to α-oxocarboxylic acid ester via diol cyclic sulfite

A concise synthesis of KDO (1) as the suitably protected form (2) from 2,3:5,6-di-O-isopropylidene-α-d-mannofuranose (3) was achieved in five steps (overall 65% yield). The key step is the efficient transformation of readily available α,β-unsaturated ester to α-oxocarboxylic acid ester. The newly β-elimination of the corresponding diol cyclic sulfite and the in situ trap (DBU/TMSCl) into enol silyl ether was developed to give the tautomeric equivalent of α-oxocarboxylic acid ester. The deprotection of acid labile TMS ether provided the desired product.