Synthesis of pentopyranosyl-containing thiodisaccharides. Inhibitory activity against beta-glycosidases.

Beta-(1-->4)-thiodisaccharides formed by a pentopyranose unit as reducing or non reducing end have been synthesized using a sugar enone derived from a hexose or pentose as Michael acceptor of a 1-thiopentopyranose or 1-thiohexopyranose derivatives. Thus, 2-propyl per-O-acetyl-3-deoxy-4-S-(beta-D-Xylp)-4-thiohexopyranosid-2-ulose (3) and benzyl per-O-acetyl-3-deoxy-4-S-(beta-D-Galp)-4-thiopentopyranosid-2-ulose (11) were obtained in almost quantitative yields. The carbonyl function of these uloses was reduced with NaBH(4) or K-Selectride, and the stereochemical course of the reduction was highly dependent on the reaction temperature, reducing agent and solvent. Unexpectedly, reduction of 3 with NaBH(4)-THF at 0 degrees C gave a 3-deoxy-4-S-(beta-D-Xylp)-4-thio-alpha-D-ribo-hexopyranoside derivative (6) as major product (74% yield), with isomerization of the sulfur-substituted C-4 stereocenter of the pyranone. Reduction of 11 gave always as major product the benzyl 3-deoxy-4-S-(Galp)-4-thio-beta-D-threo-pentopyranoside derivative 14, which was the only product isolated (80% yield) in the reduction with K-Selectride in THF at -78 degrees C. Deprotection of 14 and its epimer at C-2 (13) afforded, respectively the free thiodisaccharides 19 and 18. They displayed strong inhibitory activity against the beta-galactosidase from Escherichia coli. Thus, compound 18 proved to be a non-competitive inhibitor of the enzyme (K(i)=0.80 mM), whereas 19 was a mixed-type inhibitor (K(i)=32 microM).