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and pyrogallol react with phenyl boronic acid when stoichiometric mixtures
are ball milled at 80 1C (Scheme 1.49).
46
Deprotection also follows the same
procedure by treatment with aqueous NaHCO
3
at 40 1C for 2 h.
1.3.4.3 Protection of Aliphatic 1,2-Diols
The cyclization reaction of aliphatic 1,2-diols with boronic acid is equally
versatile and waste-free. Ball milling of a stoichiometric mixture of aliphatic
1,2-diols (pinacol or 2,2-dimethylpropane-1,3-diol) and boronic acids at
room temperature provided the corresponding heterocycles quantitatively
(Scheme 1.50).
46,47
1.3.4.4 Protection of Polyhydroxy Functionality
Quantitative formation of five- and six-membered ring heterocycles in a
waste-free manner by sugar alcohols and cyclic polyalcohols with phenyl
boronic acid in the solid state eases the protection-deprotection step in
carbohydrate chemistry.
46
For example,
D
-mannitol reacts with three mol-
ecules of phenyl boronic acid in a ball mill to give the corresponding product
42, the structure of which has been further confirmed by X-ray crystal
structure analysis (Scheme 1.51). Similarly, the racemic tris-borolic ester 43
with one five-membered and two six-membered rings was formed by the
solid state reaction between cyclic hexol myo-inositol with phenyl boronic
acid (1 : 3) at 95 1C (Scheme 1.51). Deprotection is performed by dissolving
43 in 0.01(N) HCl for 1 h at room temperature.
HO
OH
B
OH
O
20-25 Hz
+
+
2H
2
O
B
ball milling
O
OH
100%
pyrocatechol
HO
OH
B
OH
O
20-25 Hz
+
B
+
2H
2
O
ball milling
O
OH
OH
OH
100%
pyrogallol
Scheme 1.49 Formation of five-membered boron containing heterocycles.
O
O
OH
OH
OH
OH
2H
2
O
+
R
R-B(OH)
2
R
+
2H
2
O
B
B
O
O
Scheme 1.50 Formation of five- and six-membered boron containing heterocycles.
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