Regioselective Carbohydrate Oxidations: A Nuclear Magnetic Resonance (NMR) Study on Selectivity, Rate, and Side-Product Formation

Palladium/neocuproine
catalyzed oxidation of glucosides shows an
excellent selectivity for the C3-OH, but in mannosides and galactosides,
unselective oxidation was initially observed. For further application
in more-complex (oligo)­saccharides, a better understanding of the
reaction, in terms of selectivity and reactivity, is required. Therefore,
a panel of different glycosides was synthesized, subjected to palladium/neocuproine
catalyzed oxidation and subsequently analyzed by qNMR. Surprisingly,
all studied glucosides, mannosides, galactosides, and xylosides show
selective oxidation of the C3-OH. However, subsequent reaction of
the resulting ketone moiety is the main culprit for side product formation.
Measures are reported to suppress these side reactions. The observed
differences in reaction rate, glucosides being the most rapidly oxidized,
may be exploited for the selective oxidation of complex oligosaccharides.