Stereoselective entry into the D-GalNAc series starting from the D-Gal one: a new access to N-acetyl-D-galactosamine and derivatives thereof.

A new stereoselective preparation of N-aceyl-D-galactosamine (1b) starting from the known p-methoxyphenyl 3,4-O-isopropylidene-6-O-(1-methoxy-1-methylethyl)-beta-D-galactopyranoside (10) is described using a simple strategy based on (a) epimerization at C-2 of 10 via oxidation-reduction to give the talo derivative 11, (b) amination with configurational inversion at C-2 of 11 via a S(N)2-type reaction on its 2-imidazylate, (c) anomeric deprotection of the p-methoxyphenyl beta-D-galactosamine glycoside 14, (d) complete deprotection. Applying the same protocol to 2,3:5,6:3',4'-tri-O-isopropylidene-6'-O-(1-methoxy-1-methylethyl)-lactose dimethyl acetal (4), directly obtained through acetonation of lactose, the disaccharide beta-D-GalNAcp-(1-->4)-D-Glcp (1a) was obtained with complete stereoselectivity in good (40%) overall yield from lactose.