Appel–Lee synthesis of glycosyl inositols, substrates for inositol dehydrogenase from Bacillus subtilis

We recently reported that inositol dehydrogenase (EC 1.1.1.18) from Bacillus subtilis can catalyze the highly stereoselective oxidation of 1l-4-O-substituted myo-inositol derivatives, as well as disaccharides melibiose and isomaltose, but not gentiobiose or maltose, consistent with the requirement of an α-(1→6) linkage. We believed that the enzyme might therefore catalyze efficient stereoselective oxidation of the appropriate α-linked glycosyl inositols. We have synthesized α-d-glucopyranosyl-(1→4)-(dl)-myo-inositol and α-d-galactopyranosyl-(1→4)-(dl)-myo-inositol using the Appel–Lee protocol to couple benzyl-protected glycosyl donors to protected inositols. This method failed in our hands using glycosyl donors derived from d-mannose and 2-azido-2-deoxy-d-glucose. When myo-inositol 1,3,5-monoorthoformate is used as the acceptor, the reaction is regiospecific for the 4/6-position. We report here the mildest conditions known for the removal of the orthoformate group. 2-Azido-2-deoxy-α-d-glucopyranosyl-(1→4)-(dl)-myo-inositol was synthesized using the trichloroacetimidate derivative as the donor, and all three pseudo-disaccharides were substrates for inositol dehydrogenase. The glucopyranosyl and galactopyranosyl derivatives displayed apparent second-order rate constants comparable to that of myo-inositol.