J. Boutet et al. / Tetrahedron Letters 49 (2008) 5339–5342
5341
emphasize that the acid-sensitivity of the 4,6-O-isopropylidene
acetal prevented any transfer of the efficient methodology used
to prepare 4,6-O-benzylidene-3-O-chloroacetyl-2-deoxy-2-trichlo-
roacetamido-
In summary, two new 2-deoxy-2-trichloroacetamido-
a-D
-glucopyranosyl trichloroacetimidate.47
D-gluco-
pyranosyl trichloroacetimidate donors bearing a 4,6-O-isopropyli-
dene protecting pattern are disclosed. Most interestingly, the
synthesis of 3-O-acetylated 2 (56% on a 10 g scale) proceeded
through an easily scalable five-step process. The high b-D-glucosa-
mine donor potency and compatibility of 2 with possibly sterically
hindered acceptors were demonstrated for simple primary alco-
hols as well as for a di- and a tetrasaccharide. Analogue 23, pro-
tected at O-3 with a levulinoyl group, is proposed as a suitable
alternative to 2 to answer the need for acetyl orthogonality as fre-
quently encountered when dealing with bacterial polysaccharides.
Trichloroacetimidate 23 is readily accessible from 2 (63%) via allyl
glycoside 15.
Scheme 4. Donor 2: an efficient precursor to pentasaccharide 17.
Acknowledgement
This work was supported by the ANR (Grant NT05-1_42479 and
EMPB-013-02), the Institut Pasteur, the KOSEF (fellowship to
T.H.K.), and the MENRT (fellowship to J.B.). The authors thank F.
Bonhomme (URA CNRS 2128) for running the NMR and Mass
spectra.
Scheme 5. Donor 2 as precursor to branched trisaccharide 20.
pattern (Scheme 5). Nevertheless, coupling of donor 2 (1.4 equiv)
with the known disaccharide acceptor19 19 gave the branched tri-
References and notes
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Scheme 6. Efficient synthesis of donor 23 from 15.