Synthesis of Fluorescent 7,8,9‐Tri‐ O ‐acetyl‐ N ‐acetyl‐ and 4‐ O ‐Acetyl‐ N ‐acetylneuraminic Acid α‐Thioketosides

Two new fluorescent regioselective O ‐acetylated N ‐acetylneuraminic acid (Neu5Ac) thioketosides, 2α‐[4‐(dansylamino)phenylthio]‐7,8,9‐tri‐ O ‐acetyl‐ N ‐acetylneuraminic acid {2α‐[4‐(dansylamino)phenylthio]‐Neu5,7,8,9Ac 4 } ( 3 ) and 2α‐[4‐(dansylamino)phenylthio]‐4‐ O ‐acetyl‐ N ‐acetylneuraminic acid {2α‐[4‐(dansylamino)phenylthio]‐Neu4,5Ac 2 } ( 9 ) were synthesized. The synthesis of both derivatives started with the peracetylated benzyl ester 1 as precursor. The 7,8,9‐tri‐ O ‐acetylated compound was prepared by partial deacetylation of 1 with sodium methoxide or with hydrazine hydrate, followed by catalytic benzyl ester hydrogenolysis. – Complete Zemplén deacetylation of 1 gave the Neu5Ac benzyl ester thioketoside 4 , which was converted into the 8,9‐isopropylidene‐protected compound 5 . By carefully performed regioselective 4‐ O ‐acetylation with acetic anhydride/pyridine in dichloromethane we obtained the desired fluorescent 4‐ O ‐acetyl derivative 6 and the nonfluorescent sulfonacetamide 7 as byproduct. Acidic 8,9‐deprotection of 6 and finally catalytic hydrogenolysis of the benzyl ester 7 terminated this synthetic route, yielding 2α‐[4‐(dansylamino)phenylthio]‐Neu4,5Ac 2 ( 9 ). – The 4‐ O ‐acetylated derivative 9 could not be de‐ O ‐acetylated by influenza‐C virus esterase. However, the virus esterase transformed the tri‐ O ‐acetylated derivative 3 in small amounts into the Neu5Ac thioketoside 10 .