Protein–carbohydrate
interactions are implicated in many
biochemical/biological processes that are fundamental to life and
to human health. Fluorinated carbohydrate analogues play an important
role in the study of these interactions and find application as probes
in chemical biology and as drugs/diagnostics in medicine. The availability
and/or efficient synthesis of a wide variety of fluorinated carbohydrates
is thus of great interest. Here, we report a detailed study on the
synthesis of monosaccharides in which the hydroxy groups at their
4- and 6-positions are replaced by all possible mono- and difluorinated
motifs. Minimization of protecting group use was a key aim. It was
found that introducing electronegative substituents, either as protecting
groups or as deoxygenation intermediates, was generally beneficial
for increasing deoxyfluorination yields. A detailed structural study
of this set of analogues demonstrated that dideoxygenation/fluorination
at the 4,6-positions caused very little distortion both in the solid
state and in aqueous solution. Unexpected trends in α/β
anomeric ratios were identified. Increasing fluorine content always
increased the α/β ratio, with very little difference between
regio- or stereoisomers, except when 4,6-difluorinated.
The regioselective protection of both methyl galactopyranoside anomers at the 2 and 3-positions as the butane diacetal (BDA) is well known. Here we describe the formation of an unexpected byproduct, which mainly occurs when α-methyl galactopyranoside is reacted with 2,3-butanedione under BF•OEt catalysis. The structure of the byproduct, which did not arise from anomerisation to the β-anomer or from BDA formation at the galactopyranoside 3,4-positions, was elucidated by NMR and X-ray crystallographic analysis, and proved to be the expected BDA protected galactopyranoside, but in which the stereochemistry of both its BDA acetal centres are inverted. Interestingly, the conformation of the resulting six-membered BDA ring was distorted to a skew boat conformation in order to maintain anomeric stabilisation.
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