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center is favoured, with a resultant drastic acceleration of the reaction
rate. In fact, we found that oxidation of the free secondary hydroxyl
group at the anomeric center of the
D
-glucopyranose derivative
25
was
complete after 45 min, yielding smoothly the desired
D
-glucono-δ-
lactone
42
. As the chosen oxidation method occurs under very mild
conditions, and given the fact that reactions are complete in a very short
time, it is possible to oxidise very sensitive substrates.
In contrast to what is observed for monosaccharides of similar
configuration, the most stable cyclic form of
D
-gluconolactones is the
five-membered ring (
i.e.
, γ-lactone or 1,4-lactone). The γ-lactone is
indeed well suited to accommodate the structural requirements for its
formation with respect to bond lengths and angles. In particular, the
flattening of the structure caused by the sp
2
-hybridation of the lactone
carbon atom does not lead to conformational twisting, as it is the case for
six-membered δ-lactones (or 1,5-lactones). In the latter case, this
flattening causes the molecule to adopt a non-ideal half-chair or boat
conformation (Lehmann, 1996). As a result, the six-membered δ-lactones
have higher ring strain energies and hydrolyse faster than the
corresponding five-membered γ-lactones (Brown
et al.
, 1989). In our
case, the ring strain of the
D
-glucono-δ-lactone
42
is further increased by
the presence of the bivalent and relatively rigid hexabenzyloxydiphenoyl
unit, which opposes the twisting of the chair conformation. Due to the
substitution pattern, an otherwise possible conversion to the more stable
1,4-lactone is in this case impossible. These considerations led us to
believe that lactone
42
would be easily opened under mild conditions to
D
-gluconic acid derivative
43
. In fact, when the anomeric position of
25
was oxidized to give lactone
42
, we even observed its hydrolysis into
43
during silica gel chromatography (Fig. 5.9).
Subsequent attempts to selectively acylate the free hydroxyl function
at C-5 of
43
were unsuccessful in spite of several variations of the
reaction conditions. Neither the use of benzylated gallic acid
22
in the
presence of DCC and DMAP, nor the use of galloyl chloride
7
in the
presence of DMAP, led to the formation of the desired product. The
trimethylsilyltriflate (TMSOTf)-catalysed acylation conditions reported
by Procopiou
et al.
(1996, 1998) using benzylated gallic acid anhydride
(Pacheco and Grouiller, 1965, Schmidt and Klinger, 1957) were also to
