SYNTHESIS OF A PHOSPHONIC ACID ANALOG OF N-ACETYL-2,3-DIDEHYDRO-2-DEOXYNEURAMINIC ACID, AN INHIBITOR OF VIBRIO-CHOLERAE SIALIDASE

The synthesis of the phospha analogue 10 of DANA (2) is described. Bromo-hydroxylation of the known 11 (--> 12 and 13) followed by treatment of the major bromohydrin 13 with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) gave the oxirane 14 (Scheme 1). Depending on the solvent, TiBr4 transformed 14 into 16 or into a 15/16 mixture. Reductive debromination of 16 (--> 17), followed by benzylation provided 18. Oxidative decarboxylation (Pb(OAc)4) of the acid, obtained by saponification of 18, yielded the anomeric acetates 19 and 20. While 19 was inert under the conditions of phosphonoylation, the more reactive imidate 22, obtained together with 23 from 19/20 via 21 (Scheme 2), gave a mixture of the phosphonates 24/25 and the bicyclic acetal 26. Debenzylation of 24/25 and acetylation led to the acetoxyphosphonates 27/28. Since beta-elimination of AcOH from 27/28 proved difficult, the bromide 34 was prepared from 27/28 by photobromination and subjected to reductive elimination with Zn/Cu (--> 35; Scheme 3). This two-step sequence was first investigated using the model compounds 30 and 31. Transesterification of 35, followed by deacetylation gave 10, which is a strong inhibitor of the Vibrio Cholerae sialidase.