A Catalytic One-Step Process for the Chemo- and Regioselective Acylation of Monosaccharides

An organocatalytic method for the chemo- and regioselective acylation of monosaccharides has been developed. Treatment of octyl β-d-glucopyranoside with isobutyric anhydride in the presence of 10 mol % of a C 2-symmetric chiral 4-pyrrolidinopyridine catalyst (1) at −50 °C gave the 4-O-isobutyryl derivative as the sole product in 98% yield. Thus, chemoselective acylation, favoring a secondary hydroxyl group in the presence of a free primary hydroxyl group, and regioselective acylation, favoring one of three secondary hydroxyl groups, took place with perfect selectivity. A competitive acylation between octyl β-d-glucopyranoside and a primary alcohol (2-phenylethanol) with 1.1 equiv of isobutyric anhydride in the presence of 1 gave the 4-O-isobutyrate of octyl β-d-glucopyranoside with 99% regioselectivity in 98% yield, which indicates that acylation of the secondary hydroxyl group at C(4) of the carbohydrate proceeds in an accelerative manner. A possible mechanism, involving multiple hydrogen-bonding between 1 and the monosaccharide, is proposed for the chemo- and regioselective acylation.