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.
Pd(OAc)(2)/3 is an efficient catalyst system for the base-free oxidative Heck reaction that outperforms the currently available catalysts for the more challenging substrates studied. The catalyst system is highly selective, and works at room temperature with dioxygen as the oxidant.
Palladium-catalyzed alcohol oxidation allows the chemo- and regioselective modification of unprotected 1,4 linked glucans. This is demonstrated in the two-step bisfunctionalization of 1,4 linked glucans up to the 7-mer. Introduction of an anomeric azide is followed by a highly regioselective mono-oxidation of the terminal C3-OH functionality. The resulting orthogonal bis-functionalized oligosaccharides are a viable alternative to PEG-spacers as demonstrated in the conjugation of a cysteine mutant of 4-oxalocrotonate tautomerase with biotin.
Palladium-catalyzed
oxidation of isopropyl N-acetyl-α-d-glucosamine (GlcNAc) is used to prepare the rare sugars allosamine,
lividosamine, and related compounds with unprecedented selectivity.
The Passerini reaction applied on 3-keto-GlcNAc provides an entry
into branching of the carbon skeleton in this compound.
Palladium-catalyzed oxidation can single out the secondary hydroxyl group at C3 in glucose, circumventing the more readily accessible hydroxyl at C6 and the more reactive anomeric hydroxyl. Oxidation followed by reduction results in either allose or allitol, each a rare sugar that is important in biotechnology. Also, N-acetylglucosamine is selectively oxidized at C3. These results demonstrate that glucose and N-acetylglucosamine, the most readily available chiral building blocks, can be versatile substrates in homogeneous catalysis.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.