2142
R. N. de Oli6eira et al. / Tetrahedron Letters 43 (2002) 2141–2143
Scheme 1. Glycosidations of 1 with alcohols (2a–f) in the presence of Montmorillonite K-10.
spectroscopic means. We also examined other cata-
lysts. This includes silica gel and ferric chloride, but
either no reaction occurred or there was an extensive
decomposition with these acids. Compounds 3a3 and
3f2c are known, although they were prepared differ-
ently. Unsaturated glycosides 3d and 3e have also
been synthesized recently using conventional heating
and their structures established.11 A comparison of
the physical and spectroscopic properties of 3a,d,e,f
with those obtained previously gave satisfactory
results.
imidoalkyl or 1,2,4-oxadiazol-5-yl functions as agly-
cones by microwave irradiation is reported. The
anomeric configurations of 2,3-unsaturated glycosides
3b,d,e have been determined by nuclear Overhauser
enhancement spectroscopy (NOESY) experiment which
confirmed the spatial interactions between H-1 and
H-4.
Acknowledgements
In order to confirm the configuration at C-1 in these
unsaturated glycosides, we chose compounds 3b,d,e and
obtained their two-dimensional spectra by applying the
One of us (R.N.O.) thanks CAPES for the fellowship.
We are also grateful to Conselho Nacional de Desen-
volvimento Cient´ıfico e Tecnolo´gico (CNPq) and to
Pernambuco State Foundation of Science and Technol-
ogy (FACEPE) for financial help, to Dr. Jean-Pierre
Ferezou (Far-Manguinhos: Fundac¸a˜o Oswaldo Cruz)
for obtaining the specific rotations. Our thanks are also
due to Ricardo Oliveira for obtaining the NOESY
spectra of compounds 3b,d,e.
nuclear
Overhauser
enhancement
spectroscopy
(NOESY) technique. The spectra clearly showed the
spatial interaction between H-1 and H-4 and demon-
strated that H-1 and H-4 are on the same side.
Microwave
method
(solvent-free
conditions):
Microwave irradiation was performed in the domestic
multimode oven.12 An intimate mixture of 1 (1
mmol), 2a–f (1.5 mmol) and Montmorillonite K-10
(25 mg, 50% w/w of 1) was put in an open glass test
tube, and then irradiated in the domestic microwave
oven for a specified time, as indicated in Table 1,
cooled and dissolved in ethyl acetate giving a color-
less solution. Filtration to remove Montmorillonite
K-10 and solvent evaporation left a solid mass which
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was chromatographed over
a silica gel column
employing n-hexane–ethyl acetate (9:1) as eluent. The
fractions containing 3 (Rf:0.6) were combined and
solvent evaporated to provide chromatographically
pure products. These were further purified by crystal-
lization and recrystallization from ethanol. For com-
parison purposes, the synthesis of 3a–f have also been
carried out by conventional heating.13
In conclusion, a rapid, clean and efficient synthesis of
2,3-unsaturated glycosides 3a–f containing N-phthal-