Synthesis of methyl 5'-thio-alpha-isomaltoside via an acyclic monothioacetal and its behavior toward glucoamylase

Methyl 5'-thio-alpha-isomaltoside (1), which contains the ring-sulfur analogue of the nonreducing glucoside of isomaltoss, was synthesized from gentiobiose through a novel ring opening-recyclization approach. The nonreducing glucoside of per-O-benzylated phenyl 1-thio-beta-gentiobioside underwent O-5'-C-1' bond cleavage with dimethylboron bromide and thiolacetic acid to give the acyclic monothioacetal 4 with the 1-thioglucopyranoside at the reducing end intact. The HO-5' group in 4 was inverted by a standard oxidation-reduction process with good efficiency. Recyclization under Mitsunobu condition allowed C-5'-S-1' bond formation with inversion of configuration at C-5', to give 1 after functional group interconversion. TLC analysis showed that II unlike isomaltose, was not hydrolyzed by glucoamylase from Rhizopus niveus. A fluorometric assay confirmed that the dissociation constant (K-d) for 1 with the enzyme was 39 mM at 20 degrees C, which is comparable with that for isomaltose. A binding assay involving fluorescence titration of the enzyme-1 complex with gluconolactone indicated that the disaccharide 1 was bound to the catalytic and noncatalytic subsites. Since isomaltose is known to bind only to the noncatalytic subsites, this result indicates a relatively high affinity of the 5-thioglucose moiety for the catalytic subsite.