Communications
precursor.[9] In terms of chemical synthesis, the preparation of
(2,6)-[7a] and (2,3)-linked disialylated N-glycans[7b] have been
reported.
Polymer-supported synthesis is considered to be a prom-
ising technology that should facilitate, and ultimately auto-
mate, the synthesis of oligosaccharides. To date, however,
there are only a few reported syntheses of branched complex
structures such as sialylated N-glycans.[10] As part of our
efforts to develop methodologies for the efficient construc-
tion of glycan chains based on soluble polymer-support
technology,[11] we set out to synthesize disialylated com-
pounds 1–4 as well as monosialylated saccharides 5 and 6 in a
divergent manner. Low molecular weight (average MW ꢀ 750)
monomethyl polyethylene glycol (PEG) was chosen as the
polymer support. For purification, the PEG-supported prod-
uct can be adsorbed on silica gel, washed to remove the non-
PEG supported materials (e.g. excess donor), and then
retrieved by elution with polar solvents.[11e] With a unique
resin capture–release purification, which uses a chloroacetyl
group as the purification handle, fully assembled oligomers
can be distinguished from shorter products. A nitro-modified
Wang-resin-type linker, which has been shown to endure most
of the typical glycosylation conditions,[12] was used.
Scheme 2. Reagents and conditions: a) TFA, Et3SiH, CH2Cl2, ꢁ408C,
96%; b) (ClAc)2O, pyridine, CH2Cl2, 96%; c) NBS, DAST, CH2Cl2, 98%;
d) AgOTf, [Cp2HfCl2], molecular sieves (4 ꢀ), toluene, 458C, 85%;
e) NBS, DAST, CH2Cl2, 84%. TFA=trifluoroacetic acid,
Cp=cyclopentadienyl.
The synthetic plan for the preparation of target glycans 1–
6 is depicted in Scheme 1b. With hexasaccharide 7 as the
common precursor, the scheme involves the use of glycosyl-
transferases to introduce the terminal Neu5Ac residues and
penultimate Gal of the (1,6) branch. An initial glycosylation
with either (2,6)- or (2,3)-sialyltransferase should provide
monosialylated heptasaccharide 5 or 6, which can then serve
as substrates of sequential galactosylation–sialylation to
afford 1–4. Thus, simply by changing the type of glycosyl-
transferase, all positional isomers of Neu5Ac2Gal2GlcNAc2-
Man3, 1 (2,6/2,6), 2 (2,3/2,6), 3 (2,6/2,3), and 4 (2,3/2,3), as well
as monosialylated 5 and 6, can be prepared from the single
precursor 7. Compound 8 was designed as a protected
hexasaccharide that would be assembled from Man3 9,
GlcNAc 10, and LacNAc 11 derivatives.
As shown in Scheme 2, lactosamine fluoride 11 was
prepared from galactosyl donor 15[13] and glucosamine
component 19.[14] Removal of the trityl group of 15 afforded
16, which was reprotected with a chloroacetyl (ClAc) group to
give 17. Conversion of the phenylthio group to fluoride was
carried out by using N-bromosuccinimide (NBS) and diethyl-
aminosulfur trifluoride (DAST)[15] to afford 18 in nearly
quantitative yield (a/b = 46:54). Coupling with 19 was then
conducted through activation with [Cp2HfCl2] and AgOTf[16]
in toluene to afford 20, which was then converted into fluoride
11.
with 2-O-Ac- and 2-O-ClAc-protected donors 13 and 14. The
initial glycosylation with chloride 13[21] proceeded smoothly
under standard conditions to afford disaccharide 27 with
excellent purity in 98% yield,[22] then desilylation produced
28. Subsequent glycosylation with methylthiomannoside
14,[11d] when conducted through activation with NIS and
TMSOTf (0.2–0.4 equiv) in CH2Cl2 (ꢁ208C), gave a mixture
of the desired product 9 [dH = 5.18 (d, J = 1.7 Hz), 4.99 ppm
(d, J = 1.7 Hz)] and the corresponding orthoester 30 [dH =
5.25 (d, J = 2.4 Hz), 5.19 ppm (s)] in variable ratios (1:5–
1:0.7). In contrast, the use of a stoichiometric amount of
TfOH with NIS at low temperatures drastically suppressed
formation of the orthoester and provided 9 as the major
product. Capture–release purification was then carried out by
using Merrifield resin loaded with N-protected (Boc or Fmoc)
Cys (29a,b)[11d] in the presence of excess iPr2NEt to specif-
ically capture 9. We found that Boc-protected 29a was more
expedient than the Fmoc-protected 29b for the chemo-
selective reaction. Notably, orthoester 30 did not react under
these conditions and remained in the solution phase, along
with unreacted 9. Removal of the Boc group was carried out
with 10% TFA in CH2Cl2. Treatment with 10% piperidine in
THF initiated the cyclo-release to afford product 31 (60%
overall yield from 12).
The synthesis of the trimannose core is depicted in
Scheme 3. Phenylthiomannoside 21[17] was silylated and
chloroacetylated to give 22 in 93% yield. The nitro-modified
Wang-resin-type linker was introduced with 23[18] under
activation with N-iodosuccinimide (NIS) and triflic acid
(TfOH),[19] to afford 24 in 92% yield. Removal of the allyl
group afforded phenol 25, to which PEG was introduced
under standard Mitsunobu conditions to provide 12. Follow-
ing the selective deprotection with hydrazinedithiocarbonate
(HDTC),[20] the resultant 26 was glycosylated sequentially
The construction of hexasaccharide 8 is shown in
Scheme 4. For the elongation of the a(1,6) branch, thioglyco-
side 10[23] was used as the donor to afford 32,[22] which was
deacetylated to give 33. Reaction with LacNAc donor 11 with
[Cp2HfCl2] and AgOTf gave hexasaccharide 8 in 92% yield,
which was subjected to the same capture–release purification
used for 31 to afford 34 in 81% yield (from 31).
Cleavage from the linker and global deprotection are
depicted in Scheme 5. After acetylation of 34, the nitro group
was reduced to an amine function by using Mo(CO)6.[24]
4220
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2005, 44, 4218 –4224