SYNTHESIS AND ENZYME-INHIBITORY ACTIVITY OF NOVEL GLYCOSIDASE INHIBITORS CONTAINING SULFUR AND SELENIUM

The syntheses of novel methyl maltoside analogues containing sulfur in the nonreducing ring and either oxygen, sulfur, or selenium atoms in the interglycosidic linkage are described. The compounds are substrate analogues for glucosidases and are of interest as potential inhibitors of these enzymes. The syntheses rely on the use of the 3,4,5,6-tetra-O-acetyl-5-thio-alpha-D-glucopyranosyl trichloroacetimidate 6 as a glycosyl donor and methyl 2,3,6-tri-O-benzoyl-4-X-alpha-D-glucopyranoside (X = OH, SH, SeH) as glycosyl accepters. The glycosylation reactions are catalyzed by triethylsilyl triflate. The 5'-thio-4-X-disaccharides are obtained as alpha:beta mixtures of 100:0 (X = O 8), 36:1 (X = S 17, 18), and 4,5:1 (X = Se 23, 24), in yields of 85%, 55%, and 57%, respectively. The notable alpha-selectivity is attributed to the greater thermodynamic stability of the alpha-isomers. In accordance with this conclusion, rearrangement of the orthoester 12, formed as a side product in the reaction to give the 5'-thiomaltoside, under acid catalysis affords the alpha-disaccharide. A reaction employing 6 and methyl 2,3,6-tri-O-benzyl-alpha-D-glucopyranoside 13 leads to a loss of stereoselectivity and gives a 1:1.2 alpha:beta mixture of the disaccharides 14 and 15. Deprotection of 8, 17, and 23 by transesterification gives the pure methyl maltoside heteroanalogues 1, 2, and 3, respectively. Kinetic studies indicate that 1, 2, and 3 are competitive inhibitors of the binding of maltose by glucoamylase G2, with K-i values of 1.34, 2.04, and 0.80 mM, respectively. The K-i values for the phenyl selenoglycosides of alpha-D-glucose 4 and alpha-D-5-thioglucose 5 are 5.88 and 4.01 mM, respectively, and the K-m values for the substrates maltose and 4-nitrophenyl alpha-D-glucopyranoside are 1.2 and 3.7 mM, respectively.