Acid-catalysed hydrolyses of 3,4,6-tri-O-methyl-1,2-O-(1-alkoxyethylidene)-α-D-glucopyranoses
Acid-catalysed hydrolyses of 3,4,6-tri-O-methyl-1,2-O-(1-alkoxyethylidene)-α-D-glucopyranoses (I) or (II) yield 2-O-acetyl-(III) and 1-O-acetyl-3,4,6-tri-O-methyl-α-D-glucopyranose (IV) exclusively as determined by t.l.c., g.l.c., and n.m.r. However, the relative amounts of (III) and (IV) formed, as determined by n.m.r. for hydrolyses of (I), exhibited drastic dependence on the solvent and the acid concentration. Hydrolyses of (I) catalysed by dilute 2,4,6-trinitrophenol (<0·01N) yielded (III) as the major product (93%) in deuterium oxide while in [2H6]ethanol–deuterium oxide (5 : 1 v/v) or [2H6]acetone–deuterium oxide (5 : 1, v/v)(IV) was the major product (ca. 70%). Hydrolysis of (I) in deuterium oxide with 0·004N-H2SO4 yielded predominantly (III)(83%) while 0·8N-H2SO4 yielded predominantly (IV)(79%). A reaction mechanism based on the 1,2-acetoxonium ion as the effective intermediate is suggested for the hydrolyses.