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M. Vojtech et al. / Tetrahedron Letters 49 (2008) 3112–3116
3115
H
+
NO2Na
H
O
Me
OH
HO
HO
+
OH
HO
H
OH
OH
+
N
H
OH
O
HO
H
N
MeOH
HO
OH
OH
OH
OH
HO
OH
HO
OH
4d
5d
6d
NaOMe
MeOH
+
- H , - NOH, - H2O
NO2
OH
OH
HO
HO
O
HO
OMe
O
HO
OMe
+
OH
OH
OH
HO
OH
HO
OH
2d
3d
1d
major (1,2-cis
)
minor (1,2-trans)
Scheme 3.
The stereoselectivity of the transformation, which is
controlled by the structural configurations of individual
starting compounds, may even change stereospecifically
as demonstrated by a 5 min treatment of 4d with methano-
lic H2SO4 at ꢀ50 °C. The only product of this transforma-
tion was methyl b-D-mannofuranoside (2d) isolated in a
30% yield; the remainder was starting 1-deoxy-1-nitro-D-
mannitol (1d). On the other hand, if the reaction tempera-
ture was increased to 0 °C, a significant amount of methyl
a-D-mannopyranoside appeared in the reaction mixture.
This may result from the consecutive reactions also accom-
panying the Fischer procedure for the preparation of
glycosides.5
The high preference for the observed five-membered ring
closure over six-membered ring closure during the methan-
olysis of the protonated nitronic acids of 1-deoxy-1-nitro-
alditols is in good agreement with a difference in the
relative rate constants for closing five- and six-membered
lactone rings from the respective 4-bromobutanoate and
5-bromopentanoate of about two orders.24 However, a
much more convenient model for studying the competitive
ring-closure addition reactions, which are so common in
carbohydrate chemistry, including those occurring in this
novel preparation of methyl glycofuranosides, is strong
acid-catalyzed methanolysis of 1,2-dideoxy-1-nitro-L-arabi-
no-hex-1-enitol to L-arabinofuranosylmethanal dimethyl
acetals.16 This model reaction proceeds cascade-wise and
thus is able to quench reactive intermediates of the cycliza-
tion step forming stable compounds. This study, applied to
the starting compounds of different configurations is in
progress in our laboratory.
lyzed methanolysis of their respective nitronates at ꢀ30 °C.
In addition to the regiospecificity, which results from pre-
ferential five-membered ring closure, the new reaction is
also stereoselective. The stereoselectivity of the reactions
led to the conclusion that lysis of the protonated aci-nitro
form involves a two-step process, that is nucleophilic addi-
tion to the protonated carbon–nitrogen double bond in the
first step, followed by the bimolecular nucleophilic substitu-
tion of the nitrogen-containing residue in the second step.
Acknowledgements
This work was supported by the APVV-51-046505 and
VEGA-2/6129/26 Grants. The funds for the NMR
measurements were provided by the Slovak State
Programme Project No. 2003SP200280203.
References and notes
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Bishop, C. T.; Cooper, F. P. Can. J. Chem. 1963, 41, 2743; (c)
In conclusion, we have developed a new and simple
method for the preparation of methyl glycofuranosides
from acyclic 1-deoxy-1-nitroalditols using strong acid-cata-