Synthesis of Methyl 5′‐Thio‐α‐isomaltoside via an Acyclic Monothioacetal and its Behavior toward Glucoamylase

Methyl 5′‐thio‐α‐isomaltoside ( 1 ), which contains the ring‐sulfur analogue of the nonreducing glucoside of isomaltose, was synthesized from gentiobiose through a novel ring opening‐recyclization approach. The nonreducing glucoside of per‐ O ‐benzylated phenyl 1‐thio‐β‐gentiobioside underwent O‐5′–C‐1′ bond cleavage with dimethyl‐boron 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 1 , 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°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.