C O M M U N I C A T I O N S
Scheme 3
Supporting Information Available: Experimental procedures, 1H
NMR spectra, and X-ray structural information for compound 15â (PDF
and CIF). This material is available free of charge via the Internet at
97%) of methyl 2-azido-3-O-benzyl-2-deoxy-R-D-glucopyrano-
side,14 was coupled with 17 to provide the R-linked trisaccharide
19 (89%) as a single isomer. Further chain-elongation sequence,
involving removal of the O4-NAP using DDQ and subsequent
glycosylation with the disaccharide donor 17, posed no problems
and furnished the pentasaccharide 20 (58%), exclusively. While
reaction of compound 19 with HBF4‚Et2O15 gave the corresponding
6′-alcohol in excellent yield, simultaneous removal of two acetyl
groups in 20 proved testing and compelled us to rethink our present
strategy.
References
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Levulinoyl (Lev) esters were thought to be better options for
our purpose. Direct levulinolysis of 15r did not work. Alternatively,
deacetylation16 of 16 afforded the 1,6-diol (81%), which was reacted
with Lev2O in pyridine to get the ester 21 (97%). A similar reaction
sequence of anomeric deprotection and imidate formation led to
the glycosyl donor 22 (53% in two steps), which was coupled with
18 in a likewise manner to construct the R-linked trisaccharide 23
(84%). The elongation cycle was then repeated thrice to assemble
the penta-, hepta-, and nonasaccharides 24, 25, and 26, respectively.
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TEMPO,17 individually, furnished the acids 27-30 in good overall
yields. The corresponding O-sulfates, obtained by consecutive
deacetylation and O-sulfonation of 27-30, underwent hydrogenoly-
sis to reduce the OBn, O-2-NAP, and N3 groups and subsequent
N-sulfonation to provide the desired target molecules 2-5, respec-
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In summary, we successfully developed a straightforward
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Acknowledgment. We thank Professor Chun-Chen Liao for his
helpful discussions. This work was supported by the National
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