Convenient synthesis of 4-thiolactose, 3,4-dithiolactose and related thiooligosaccharides and disulfides. Inhibitory activity of the glycomimetics against a β-galactosidase

The ring-opening reaction of sugar 3,4-epoxides by 2,3,4,6-tetra-O-acetyl-1-thio-beta-D-galactopyranose (7) as a nucleophile led to (1 -> 3)- and (1 -> 4)-thiodisaccharides. High regio- and diastereoselectivities were achieved in the synthesis of the per-O-acetyl derivative of the beta-D-Galp-S-(1 -> 4)-4-thio-alpha-D-Glcp-O-iPr (10). Analogues of the 4-thiolactoside 10 have been prepared, with the beta-D-Galp non-reducing end S-linked to D-Glcp, D-Gulp and D-Idop. A similar regioselective attack of 7 on C-4 of 2-propyl 3,6-di-O-acetyl-3,4-epithio-alpha-D-galactopyranoside (6) led to 2-propyl 3,4-dithiolactoside derivative 15. During this reaction the free 3-SH group of 15 underwent oxidative dimerization or oxidative coupling with the SH function of 7 to give the respective disulfides. Glycosylation of the thiol group of 15 using trichloroacetimidate derivatives of beta-D-Galp or beta-D-Galf afforded the corresponding branched dithiotrisaccharides. The free compounds were evaluated as inhibitors of the E. coli beta-galactoside. The bis(2-propyl 3,4-dithiolactosid-3-yl)-disulfide, obtained from 15, displayed the strongest inhibitory activity in these series of glycomimetics and proved to be a non-competitive inhibitor (K-i = 95 mu M).