Anomerization of Methyl Glycosides by Acid-Catalysed Methanolysis: Trapping of Intermediates

Intermediate oxocarbenium ions or other intermediates produced during acid catalyzed anomerization of some methyl glycosides were trapped by reduction with 4-methylmorpholine borane. The glycosylium ion intermediate formed on anomerization of methyl 4-O-methyl-α-D-glucopyranoside gave, as expected, only a mixture of the α- and β-D-glucopyranosides, as this reaction is slow compared to the degradation of the reducing agent. The more reactive methyl 2-deoxy-4-O-methyl-α-D-arabino-hexopyranoside gave two reduction products, namely, 1,5-anhydro-2-deoxy-4-O-methyl-D-arabino-hexitol and 2-deoxy-1,4-di-O-methyl-D-arabino-hexitol, indicating alternative reaction pathways, involving either exo or endo C-O cleavage. The aldofuranosides, when necessary methylated on O-5 in order to avoid formation of pyranosides, gave 1-O-methylalditols, indicating protonation of the ring oxygen, followed by endo C-O cleavage. The methyl 2-deoxy-5-O-methyl-α-D-arabino-hexofuranosides, however, gave a mixture of the 1,5-di-O-methylalditol and the 5-O-methyl-1,4-anhydroalditol. The methyl fructosides and 1-deoxyfructosides, both furanosidic and pyranosidic, gave mixtures of 2,5- and 2,6- anhydrohexitols, indicating that the anomerization of these glycosides also proceeds via exo C-O cleavage.