Synthetic 6-O-methylglucose-containing polysaccharides (sMGPs): Design and synthesis

Article abstract of DOI:10.1021/jo061990v

With the hope of mimicking the chemical and biological properties of natural 6-O-methyl-D-glucose-containing polysaccharides (MGPs), synthetic 6-O-methyl-D-glucose-containing polysaccharides (sMGPs) were designed and synthesized from α-, β-, and y-cyclode

Full text of DOI:10.1021/jo061990v

Synthetic 6-O-Methylglucose-Containing Polysaccharides (sMGPs):
Design and Synthesis
Malte Meppen, Yonghui Wang, Hwan-Sung Cheon, and Yoshito Kishi*
Department of Chemistry and Chemical Biology, HarVard UniVersity, 12 Oxford Street,
Cambridge, Massachusetts 02138
kishi@chemistry.harVard.edu
ReceiVed September 27, 2006
With the hope of mimicking the chemical and biological properties of natural 6-O-methyl-D-glucose-
containing polysaccharides (MGPs), synthetic 6-O-methyl-D-glucose-containing polysaccharides (sMGPs)
were designed and synthesized from R-, â-, and γ-cyclodextrins (CDs). The synthetic route proved to be
flexible and general, to furnish a series of sMGPs ranging from 6-mer to 20-mer. A practical and scalable
method was discovered selectively to cleave the CD derivatives and furnish the linear precursors to the
glycosyl donors and acceptors. The Mukaiyama glycosidation was adopted to couple the glycosyl donors
with the glycosyl acceptors. Unlike in the 3-O-methyl-D-mannose-containing polysaccharide (sMMP)
series, the amount of the Mukaiyama acid required in the sMGP series increased with an increase of
substrate size; that is, for large oligomers, more than one equivalent of the acid was required.
Introduction
as they were isolated as complex mixtures of closely related
polysaccharides. For this reason, we chose to use synthetic
polysaccharides structurally related to natural MMP and MG-
(L)P for two major reasons: (1) synthetic polysaccharides should
be available as chemically well-defined and homogeneous
materials and (2) synthetic polysaccharides should be structurally
In the preceding paper,1 we outlined the background of this
research program. Briefly, Mycobacterium smegmatis is known
to produce two series of polysaccharides, i.e., 3-O-methyl-D-
mannose-containing polysaccharides (MMPs) and 6-O-methyl-
D-glucose-containing (lipo)polysaccharides (MG(L)Ps) (Figure
1).2-4 Both MMP and MG(L)P affect profoundly the fatty acid
biosynthesis catalyzed by fatty acid synthetase I (FAS I) isolated
from Mycobacterium smegmatis.5 We are interested in gaining
mechanistic insights for the intriguing biological role(s) of MMP
and MG(L)P. However, we felt that naturally occurring MMP
and MG(L)P are not necessarily ideal substrates for our study,
(3) For isolation and structural characterization of MGLP, see: (a) Lee,
Y. C.; Ballou, C. E. J. Biol. Chem. 1964, 239, PC3602. (b) Saier, M. H.,
Jr.; Ballou, C. E. J. Biol. Chem. 1968, 243, 4332. (c) Smith, W. L.; Ballou,
C. E. J. Biol. Chem. 1973, 248, 7118. (d) Forsberg, L. S.; Dell, A.; Walton,
D. J.; Ballou, C. E. J. Biol. Chem. 1982, 257, 3555. Regarding the structural
heterogeneity of MGP, Ballou commented that at least two forms of MGP
containing 21 hexoses exist: Kamisango, K.; Dell, A.; Ballou, C. E. J.
Biol. Chem. 1987, 262, 4580. Also see: Tuffal, G.; Albigot, R.; Monsarrat,
B.; Ponthus, C.; Picard, C.; Rivie`re, M.; Puzo, G. J. Carbohydr. Chem.
1995, 14, 631.
(1) Hsu, M. C.-P.; Lee, J.; Kishi, Y. 2007, 72, 1931.
(2) For isolation and structural characterization of MMP, see: (a) Gray,
G. R.; Ballou, C. E. J. Biol. Chem. 1971, 246, 6835. (b) Maitra, S. K.;
Ballou, C. E. J. Biol. Chem. 1977, 252, 2459. (c) Weisman, L. S.; Ballou,
C. E. J. Biol. Chem. 1984, 259, 3457. (d) Weisman, L. S.; Ballou, C. E. J.
Biol. Chem. 1984, 259, 3464. (e) Ilton, M.; Jevans, A. W.; McCarthy, E.
D.; Vance, D.; White, H. B., III; Bloch, K. Proc. Natl. Acad. Sci. U.S.A.
1971, 68, 87.
(4) The structure of MG(L)P shown in Figure 1 is the revised structure
suggested by Rivie`re based on the structure of polysaccharide isolated from
Mycobacterium bovis BCG. See: Tuffal, G.; Albigot, R.; Rivie`re, M.; Puzo,
G. Glycobiology 1998, 8, 675.
(5) For reviews on MMP and MGLP/MGP, see: (a) Bloch, K. AdV.
Enzymol. 1977, 45, 1. (b) Ballou, C. E. Acc. Chem. Res. 1968, 1, 366. (c)
Ballou, C. E. Pure Appl. Chem. 1981, 53, 107.
10.1021/jo061990v CCC: $37.00 © 2007 American Chemical Society
Published on Web 02/03/2007
J. Org. Chem. 2007, 72, 1941-1950
1941

Meppen et al.
TABLE 1. Screening of Protection Groups of Glycosyl Donors and
Acceptorsa
reaction yield
time (h)
R/â
entry R1 R2
-E
(%)b
ratioc
1
2
3
4
Bz Bz -CO-C6H4-CO2Me-(o)
Bn Bz -CO-C6H4-CO2Me-(o)
Bn Bz -CO-CH2O(CH2)2OMe
Bn Bn -CO-CH2O(CH2)2OMe
37
39
4
7 (â)
1:4
86 (R) 13:1
81 (R) 16:1
76 (R)
12
6:1
a
FIGURE 1. Structures of mycobacterial polysaccharides 3-O-methyl-
D-mannose-containing polysaccharides (MMPs) and 6-O-methyl-D-
glucose-containing lipopolysaccharides (MGLPs)/6-O-methyl-D-glucose-
containing polysaccharides (MGPs).
Glycosidation conditions: 10 mol % of AgClO4-SnCl4, Et2O, 0 °C.
b
c
1
Isolated yield. Data based on H NMR analysis.
achieve the synthesis of 6-O-methyl-D-glucose-containing polysac-
charides (sMGPs). However, we opted to use the synthetic
strategy reported in the preceding paper, primarily because it
was proved to be effective for the case of synthetic 3-O-methyl-
D-mannose-containing polysaccharides (sMMPs).1
Results and Discussion
FIGURE 2. Structures of synthetic analogues of mycobacterial
polysaccharides, sMMP and sMGP.
In the preceding paper, we reported a convergent synthesis
of sMMP1 with use of the modified Mukaiyama glycosidation.9
This glycosidation is well suited for a highly convergent
oligosaccharide synthesis, particularly because of good chemical
yields even when using equal-sized donors and acceptors in a
molar ratio of ca. 1:1. The exceptionally high R/â-selectivity
(>50:1 ∼ >20:1) observed in the sMMP series was due to the
result of the selective â-to-R anomerization under the Mu-
kaiyama glycosidation conditions. Unfortunately, this beneficial
anomerization was not observed in the sMGP series.10
With this knowledge, we reevaluated the synthetic plan for
sMGP. As the preliminary studies suggested that the Mukaiyama
glycosidation9 was best suited for a convergent synthesis of the
sMMP/sMGP class of polysaccharides, our efforts were focused
on the identification of the best combination of the glucopyra-
nosyl donor and acceptor. Using the 6-O-methylglucopyranosyl
donors and acceptors shown in Table 1, we studied the R/â-
stereoselectivity and chemical yield, thereby demonstrating the
following: (1) the optimal protecting groups for the donor and
the acceptor are benzyl and benzoate groups, respectively, and
(2) the newer Mukaiyama ester and monomethyl phthalate1 are
approximately equal, except that the glycosidation rate with the
newer Mukaiyama ester9c was significantly faster than that with
monomethyl phthalate.
tunable for the needs of our investigation. Obviously, the most
unique structural feature of natural MMP and MG(L)P is the
polymeric form of O-methylated mannose and glucose. Thus,
we decided to incorporate this structural feature in the synthetic
polysaccharides and selected the polymers composed of 3-O-
methyl-D-mannose and 6-O-methyl-D-glucose (Figure 2).
For the past one and a half decades, we have witnessed a
remarkable development in chemical synthesis of oligosaccha-
rides. A number of glycosyl donors, including halo sugars,
pentenyl glycosides, thioglycosides, isopropenyl glycosides,
orthoesters, 1-O-acyl sugars, 1-O-pentenoyl sugars, trichloro-
acetimidates, glycosyl sulfoxides, glycosyl sulfones, glycosyl
thiocyanates, glycosyl dialkylphosphites, glycosyl phospho-
rodithioates, glycosyl tetramethylphosphorodiamidates, glycals
and 1,2-anhydrosugars, seleno glycosides, and glycosyl diaz-
irines, are now known to effect the glycosidation in a stereo-
selective manner in solution- and/or solid-phase syntheses.6
Many of these synthetic methods have successfully been applied
for the synthesis of a broad range of complex oligosaccharides.7
Related to this work, we should specifically quote the work
from the Koto laboratory, as they reported the synthesis of
glucose-containing linear oligosaccharides having R(1f4) and
R(1f6) linkages.8
Having identified the optimal protecting groups for the donor
and acceptor, we realized that a modification should be made
on the strategy used for the synthesis of sMMP (Scheme 1). In
the sMMP series, M-a served as the source of glycosyl donor
With a dramatic advancement in chemical synthesis of
oligosaccharides, we recognized several promising options to
(6) For examples, see the reviews cited in refs 9 and 10 in the preceding
paper.
(7) For examples, see the syntheses cited in ref 11 in the preceding paper.
(8) Koto, S.; Haigoh, H.; Shichi, S.; Hirooka, M.; Nakamura, T.; Maru,
C.; Fijita, M.; Goto, A.; Sato, T.; Okada, M.; Zen, S.; Yago, K.; Tomonaga,
F. Bull. Chem. Soc. Jpn. 1995, 68, 2331.
(9) (a) Mukaiyama, T.; Takashima, T.; Katsurada, M.; Aizawa, H. Chem.
Lett. 1991, 533. (b) Mukaiyama, T.; Katsurada, M.; Takashima, T. Chem.
Lett. 1991, 985. (c) Mukaiyama, T.; Matsubara, K.; Sasaki, T.; Mukaiyama,
T. Chem. Lett. 1993, 1373.
(10) Wang, Y.; Cheon, H.-S.; Lee, J.; Kishi, Y., in preparation.
1942 J. Org. Chem., Vol. 72, No. 6, 2007

Synthetic 6-O-Methylglucose-Containing Polysaccharides
SCHEME 1. Divergent Synthetic Strategies of sMMP
(Panel A) and sMGP (Panel B)
SCHEME 2. Synthetic Plan for the Glycosyl Donor G-b
and the Glycosyl Acceptor G-c from r-, â-, and γ-CDs
SCHEME 3 a
a Reagents and conditions: (a) TBSCl, pyr, rt (a 83%; b 83%; c 78%);
(b) (1) BzOTf, pyr, rt (a 95%), or BzCl, pyr, 50 °C (b 93%; c 90%); (2)
aqueous HF, CH2Cl2-CH3CN, rt (a 83%; b 80%; c 82%); (c) Ag2O, MeI,
MS (4 Å), toluene, rt (a 85%; b 83%; c 81%).
SCHEME 4 a
a Mukaiyama glycosidation: 10 mol % of AgClO4-SnCl4, Et2O, 0 °C.
M-b and glycosyl acceptor M-c, and the donor and the acceptor
were coupled under the modified Mukaiyama protocol, to
furnish M-d. Importantly, except for n, M-d was identical to
M-a and, therefore, it was not necessary to adjust the protecting
groups for the next cycle. In the sMGP series, however, the
results summarized in Table 1 suggested that the optimal
protecting groups for the donor and the acceptor are benzyl and
benzoate groups, respectively, to achieve both optimal R/â-
stereoselectivity and chemical yield (cf. G-b and G-c). There-
fore, some additional steps would be required to transform the
glycosidation product G-d to the glycosyl donor and acceptor
for the next cycle (cf. G-d vs G-b and G-c). To avoid this
potentially cumbersome adjustment of protecting groups after
each cycle, we became interested in the possibility of synthesiz-
ing the glycosyl donor G-b and acceptor G-c from R-, â-, and
γ-cyclodextrins (CDs).
CDs are naturally occurring cyclic oligosaccharides composed
of R-(1f4)-linked D-glucose. R-, â-, and γ-CDs contain 6-, 7-,
and 8-units of R-(1f4)-linked D-glucose, respectively. Assum-
ing that the C6 hydroxyl groups of CDs can be selectively
methylated (cf. R-, â-, and γ-CDs f I) and also that one
R-(1f4) glycosidic bond of CDs can be cleaved preferentially
over the R-(1f4) glycosidic bonds present in the cleaved
products11,12 (cf. I f II), we anticipated that the 6-, 7-, and
8-mers of G-b (n ) 6, 7, 8) and G-c (n ) 6, 7, 8) should be
synthesized from R-, â-, and γ-CDs (Scheme 2). With this
analysis, we began the experimental work on selective func-
tionalization of CDs.
a Reagents and conditions: (a) 2% H2SO4 in Ac2O, rt (a 71%; b 66%;
c 55%); (b) (1) BnNH2, THF, rt (a 96%; b 93%; c 92%); (2) Ag2O, MS (4
Å), allyl bromide, DMF, rt (a 90%; b 97%; c 93%); (c) HBF4, CH2Cl2-
MeOH (3:1), 40 °C (a 54%; b 44%; c 52%).
Our first task was to develop a reliable and scalable method
for selective O-methylation of the C6 hydroxyl groups, which
was achieved in four steps in excellent overall yields (Scheme
3). The selective C6 O-silylation was reported in the literature.11
Under the specified conditions, no scrambling of the silyl or
benzoate groups was observed during benzoylation, desilylation,
and O-methylation.12,13
Our second task was to develop a reliable and scalable method
to selectively cleave the R-glycosidic bond of CDs. As the
cleaved product still contains six, seven, or eight glycosidic
bonds, the proposed transformation could be problematic.
Nonetheless, we hoped that the desired, first cleavage could be
faster than the undesired, following cleavages because of the
ring constraints present in protected CDs. Some examples known
in the literature support the proposed step. In particular, the
results reported by Kuzuhara were informative, as they observed
a selective cleavage of one glycosidic bond on peracylated CDs
but observed a complex product formation of permethylated CDs
on acetolysis.14 These experiments may suggest that the presence
of an electron-withdrawing protecting group is the key to achieve
the desired selective hydrolysis. Indeed, the controlled acetolysis
(11) (a) Fugedi, P. Carbohydr. Res. 1989, 192, 366. (b) Pregel, M. J.;
Buncel, E. Can. J. Chem. 1991, 69, 130. (c) Takeo, K.; Uemura, K.; Mitoh,
M. J. Carbohydr. Chem. 1998, 7, 293.
(12) 4a was previously prepared by methylation (BF3/CH2N2) of 3a: see
ref 11a.
(13) 3a was previously prepared by controlled hydrolysis (iPrOK/iPrOH-
benzene) of perbenzoylated R-CD. See: (a) Boger, J.; Corcoran, R. J.; Lehn,
J. M. HelV. Chim. Acta 1978, 61, 2190. (b) Uccello-Barretta, G.; Cuzzola,
A.; Balzana, F.; Menicagli, R.; Iuliano, A.; Salvadori, P. J. Org. Chem.
1997, 62, 827.
J. Org. Chem, Vol. 72, No. 6, 2007 1943

Meppen et al.
TABLE 2. Screening of the Selective Cleavage of â-CD Derivative 4b
Lewis acid
(v/v %)
entry
R1
R2
X
Y
conversionb
1
2
3
4
5
6
7
8
9
none
none
none
HBF4 (2%)
BF3‚Et2O (2%)
BF3‚Et2O (2%)
BF3‚Et2O (4%)
BF3‚Et2O (12%)
BF3‚Et2O (16%)
CH3
CF3
CF3
CF3
CF3
Ac
-
-
H
H
H
H
H
H
Ac
-
ca. 50%
n.a.c
(MeO)CH2
(MeO)CH2
(MeO)CH2
(MeO)CH2
(MeO)CH2
(MeO)CH2
(MeO)CH2
-
n.a.c
(MeO)CH2
(MeO)CH2
noned
(MeO)Ac
(MeO)Ac
(MeO)Ac
(MeO)Ac
(MeO)Ac
(MeO)Ac
< 5%
ca. 20%
ca. 25%
ca. 30%
ca. 90%
ca. 97%
noned
noned
noned
a
b
The ratio of R1CO2H to (R2CO)2O was 1:1 where the acid anhydride was used. Conversion was estimated from 1H NMR of the crude reaction
c
d
mixture. No desired product was found. The acid anhydride was omitted.
TABLE 3. Substrate Scope of the Selective Cleavage Reaction of CD Derivatives
crystalline
isolated
entry
n
R
conversion
precipitation
purification
yield (%)
1
2
3
4
6
7
7
8
Me
Me
Et
ca. 85%a
ca. 97%b
ca. 97%b
ca. 97%b
no
chromatography
recrystallization
recrystallization
recrystallization
61
86
88
88
yes
yes
yes
Me
a
b
1
Conversion was based on recovered starting material. The reaction mixture contains unidentified side products. Conversion was estimated from H
NMR of the crude reaction mixture.
of 4a-c yielded the desired products as an approximately 6∼7:1
mixture of anomeric R- and â-acetates. The relative rate for
the first cleavage was in the order of 4a (R-CD series) > 4b
(â-CD series) > 4c (γ-CD series), and importantly, the first
cleavage rate was significantly faster than the following cleav-
ages even for 4c. However, the material throughput was best
achieved by quenching the acetolysis at approximately 60-70%
of completion and recycling the recovered starting materials.
The next stage of synthesis was to adjust the protecting group
at C1 of the reducing end and at C4 of the nonreducing end.
On treatment with benzylamine, the C1 acetate at the reducing
end of 5a-c was selectively hydrolyzed to yield the desired
products. The resultant anomers were allylated with Ag2O/allyl
bromide/DMF, furnishing the R-allylglucosides as the major
products (R/â ratio ) ca. 7:1). We then ran into difficulty when
selectively hydrolyzing the acetate at the nonreducing end.
Selective hydrolysis of an acetate over a benzoate should not
usually present a problem. However, as 6a-c contained a large
number of benzoates (12, 14, and 16 benzoates for a, b, and c,
respectively), it became more challenging to achieve this
seemingly simple selective hydrolysis cleanly in an overall
sense. Thus far, HBF4 in MeOH/CH2Cl2 was found to give the
best result. Even under this condition, it was necessary to quench
the reaction at around 50% conversion to avoid the overhy-
drolysis (Scheme 4).
An obvious solution to this problem was to replace the C4
acetyl group with a more reactive acyl group such as a
trifluoroacetyl or methoxyacetyl group; the reactivity difference
between CF3CO- or MeOCH2CO- and PhCO- should be
more pronounced than the reactivity difference between MeCO-
and PhCO-.15 Thus, we attempted to cleave the CD 4b under
a variety of conditions (Table 2). After considerable trial-and-
error efforts, we finally found that BF3‚Et2O-promoted cleavage
of 4b in the presence of methoxyacetic acid provides an ultimate
solution.
There are several appealing aspects for this cleavage reaction.
First, as the reaction progressed, the cleaved product crashed
out as a crystalline precipitate, recrystallization of which
furnished the desired product (isolated as a 10:1 mixture of the
R/â-anomers) in an excellent yield.16 Second, this reaction was
routinely run in a 5∼10 g scale without any technical difficulty.
Third and most importantly, the isolated product was shown to
(15) Green, T. W.; Wuts, P. G. ProtectiVe Groups in Organic Synthesis,
3rd ed.; John Wiley & Sons: New York, 1999; pp 160-166 and references
cited therein.
(16) The products were isolated as a mixture of R/â-anomers at the
reducing ends with ca. 10:1 ratios favoring R-anomers. The homogeneity
of products was assessed by 1H NMR and MS (MALDI-TOF) and was
further confirmed by transforming them to the known acceptors 9a-c.
(14) For cleavage of peracylated cyclodextrins, see: (a) Sakari, N.; Wang,
L.-X.; Kuzuhara, H. J. Chem. Soc., Chem. Commun. 1991, 289. (b) Sakari,
N.; Wang, L.-X.; Kuzuhara, H. J. Chem. Soc., Perkin Trans. 1 1995, 437.
(c) Sakari, N.; Matsui, K.; Kuzuhara, H. Carbohydr. Res. 1995, 266, 263.
(d) Sakairi, N.; Kuzuhara, H. Carbohydr. Res. 1996, 280, 139.
1944 J. Org. Chem., Vol. 72, No. 6, 2007

Synthetic 6-O-Methylglucose-Containing Polysaccharides
SCHEME 5 a
TABLE 4. Selected Examples for Screening the Catalyst Loading
for the Glycosidation
a Reagents and conditions: (a) R-CD 4a: BF3‚Et2O (12% v/v),
MeOCH2CO2H-CH2Cl2 (12% v/v; [C] ) 0.01 M, 40 °C, 1 h, 82% yield
(recrystallization)). â-CD 4b and γ-CD 4c: see Table 3.
SCHEME 6 a
equiv of
Lewis acid
temp
(°C)
time
(h)
yield
(%)a
12
9
9/12
R/âb
m ) 6
n ) 6
1.0
1.0
1.0
1.0
1.0
2.0
2.0
2.0
0.2
1.0
0.2
1.0
1.0
2.0
2.0
3.0
0
-30
0
-30
-30
-30
-30
-30
6
24
6
24
24
24
24
24
60
60
38
59
54
61
41
51
6:1
6:1
5:1
5:1
5:1
5:1
6:1
5:1
6
6
7
7
8
8
6
6
7
7
8
8
14
14
a Reagents and conditions: (a) (1) TMSOTf, Et3N, CH2Cl2 (a 96%; b
93%; c 95%); (2) ethanolamine, CH2Cl2 (a 92%; b 86%; c 86%); (3) allyl
bromide, Ag2O, MS (4 Å), DMF, rt (a 92%; b 86%; c 91%); (b) (i) H3O+;
(ii) NaOMe, MeOH-CH2Cl2 (1:1), rt; (iii) NaH, BnBr, DMF, rt (a 77%;
b 87%; c 71%); (c) (1) (Ph3P)3RhCl, Dabco, EtOH/toluene/H2O (6:3:1),
90 °C; (2) acetone-1 N HCl (9:1), 90 °C (a 82%; b 76%; c 77%); (d)
EDCI, DMAP, 2-(2-methoxyethoxy)acetic acid, CH2Cl2, 0 °C (a 96%; b
96%; c 91%).
a
Combined yield of R- and â-anomers. bStereoselectivity estimated by
1H NMR.
silylation of the C4 hydroxyl group at the nonreducing end,
selective removal of the methoxyacetate group at C1 of the
reducing end, and allylation of the resultant hemiacetal (Scheme
6). As described in the previous series, allylation of the resultant
anomers in DMF furnished R-allylated products with a stereo-
selectivity of ca. 7:1. Interestingly, the stereoselectivity (R:â
ratio ) ca. 7:1) was reversed in allylation in dichloroethane
(R:â ratio ) ca. 1:6). Although we later found that the
stereochemistry at the reducing end of the polysaccharides made
no noticeable difference in chemical behaviors,1 we pursued
the homogeneous material to eliminate any concerns possibly
related to the stereochemistry. Separation of the mixtures was
effectively achieved by Biotage flash chromatography, to afford
R-allyl acceptors with purities of >98% in gram quantities.
The glycosyl acceptors 9a-c thus obtained served as the
starting materials for preparation of the glycosyl donors 12a-
c, which was uneventfully accomplished (Scheme 6). For the
reason discussed earlier, the newer Mukaiyama ester was chosen
for the activator of glycosyl donors.
The glycosidation was first attempted in the â-CD series, i.e.,
9b (m ) 7) + 12b (n ) 7) f 13b (m + n ) 14) in Table 4.
However, we soon realized that, because of poor solubility of
the acceptor 9 in Et2O, the glycosidation conditions optimized
for model substrates (Table 1) could not be applied to this case.
This difficulty was overcome by using a 1:2 Et2O-CH2Cl2
mixture as the solvent, but the R/â-selectivity diminished from
16:1 (Et2O) down to 6:1 (1:2 Et2O-CH2Cl2). Fortunately, the
lowered selectivity of glycosidation did not present a technical
problem because the desired R-isomer was least polar on silica
gel TLC and readily isolable in a pure form by silica gel
chromatography.
have the free hydroxyl group at C4 of the nonreducing end.
Thus, unlike in the case of 6b f 7b (Scheme 4), it was
unnecessary to face the problem associated with the selective
hydrolysis.
We applied the BF3‚Et2O/MeOCH2CO2H conditions to the
remaining R- and γ-CD substrates. The behavior of γ-CD
substrate 4c was found to be virtually identical to that of â-CD
substrate 4b (Table 3). However, the R-CD substrate 4a behaved
differently; in the R-CD series, the cleaved product did not
precipitate out from the reaction mixture. Apparently, this
difference was largely due to the solubility of products in the
reaction medium. We should note that the precipitation seemed
to have two additional beneficial effects: (1) the precipitation
appeared to drive the cleavage reaction to completion and (2)
the precipitation appeared to protect the products from further
cleavage.
We conducted optimization work on the cleavage reaction
in the R-CD series and found that the reaction rate and selectivity
were greatly influenced by solvents. Among several solvents
tested, CH2Cl2 was found to be the most effective cosolvent
for acceleration of the cleavage reaction. Under the optimized
conditions, the cleavage reaction proceeded smoothly to near
completion and the pure product was isolated in a good yield
by recrystallization (Scheme 5).
As pointed out, the linear oligomers 8a-c have a free
hydroxyl group at C4 of the nonreducing end, which facilitated
the functional group manipulation required for preparation of
the glycosyl acceptors and donors. The glycosyl acceptors 9a-c
were prepared straightforwardly from 8a-c in three steps:
As mentioned earlier, there was one major difference
recognized for the Mukaiyama glycosidation in the sMGP series
J. Org. Chem, Vol. 72, No. 6, 2007 1945

Meppen et al.
SCHEME 7 a
SCHEME 8 a
a Reagents and conditions: (a) iodoethane, Ag2O, MS (4 Å), toluene,
75%; (b) follow the steps in Table 3; (c) follow step a in Scheme 6 (77%
overall yield); (d) follow steps b-d in Scheme 6 (69% overall yield); (e)
(1) 200 mol % of [SnCl3ClO4], CH2Cl2-Et2O (2:1), -30 °C, 50%; (2)
follow step b in Scheme 7 (54% overall yield). For the details, see
Supporting Information.
a Reagents and conditions: (a) 100 mol % of [SnCl3ClO4], CH2Cl2-
Et2O (2:1), -30 °C (a 49%; b 50%; c 51%); (b) (i) H2, Pd(OH)2 on C,
THF, rt; (ii) NaOMe, CH2Cl2-MeOH (3:1), rt (a 86%; b 66%; c 68%).
SCHEME 9 a
from that in the sMMP series. Yet, we recognized another major
difference: a catalytic amount (10 mol %) of the Lewis acid
[SnCl3ClO4] was sufficient to achieve a good conversion in the
sMMP series, whereas a higher loading of the catalysis seemed
to be required to obtain a satisfactory result in the sMGP series.
We speculated that the lower reactivity of the glucosyl donor
in the sMGP series in comparison to that in the sMMP series
accounts for the differences and conducted an optimization
study. In particular, we tested higher loadings of the Mukaiyama
acid. As seen from the representative examples listed in Table
4, with an increase in the size of substrates, one or more
equivalents of the Mukaiyama acid was required to achieve a
good conversion with satisfactory reproducibility. Unlike in the
sMMP series, the truncation/scrambling17,18 was negligible in
the sMGP series. In this series, we noticed byproduct formation
from the donors19 but found that this byproduct formation was
significantly suppressed at -30 °C. With these modifications,
we were able to achieve the glycosidation with satisfactory
reproducibility.
a Reagents and conditions: (a) (1) (i) NaOMe, MeOH-CH2Cl2 (1:1),
rt; (ii) NaH, BnBr, DMF, rt (a 70%; b 72%); (2) (i) follow steps c and d
in Scheme 6 (a 57%, b 57%); (b) 300 mol % of [SnCl3ClO4], CH2Cl2-
Et2O (2:1), -30 °C (a 44%; b 43%); (c) follow step b in Scheme 7 (14e
58%; 14f 64%).
The glycosidation products were subjected to a two-step
procedure of deprotection, and the sMGP 12-, 14-, and 16-mers
were isolated by reverse-phase column chromatography on C18
silica gel (Scheme 7). To assess the purity of sMGP, extensive
spectroscopic studies were conducted, thereby demonstrating
that no detectable amount of â-anomer was contaminated at
the newly introduced anomeric center.20
The successful synthesis of sMGP encouraged us to explore
the flexibility of the strategy for other closely related polysac-
charides. First, to address the hydrophobic effect of polysac-
charides for the complexation event with fatty acids, we pursued
the synthesis of sMGP analogues composed of 6-O-ethyl-D-
glucose 14d. Gratifyingly, the same synthetic scheme as that
for the sMGP synthesis, except for the ethylation step of the
â-CD derivative 3b, yielded the polysaccharide 14-mer, 14d
(Scheme 8).21,22
(17) Following the same sequences as shown in Schemes 6 and 7,
â-anomer enriched 9a-c were converted to the â-anomer enriched sMGP
14a-c. For details, see Supporting Information.
(18) The profile of Mukaiyama glycosidation in the glucose series was
different from that in the mannose series. However, considering the total
number of glycosidic bonds present in the product as well as the starting
materials, we were concerned with the possibility that the “truncated/
scrambled” products might be contaminated in the products in the gluco
series as well. To address this issue, the glycosidation was purposely run
for a prolonged time at 0 °C, and the product was subjected to mass
spectrometry and size-exclusion chromatography, thereby showing that the
product mixture thus obtained was indeed contaminated by a small amount
of “truncated” oligomers. Interestingly, these oligomers were formed by
cleavage of glycosidic bonds exclusively at the hexoses bearing benzyl
protection groups, suggesting that electron-withdrawing groups on C2
hydroxyl groups destabilize carbocation formation and suppress the trunca-
tion. When the reaction was carried out at -30 °C, the process forming the
truncated oligomers was completely suppressed.
Second, to test the size effect of sMGPs in the fatty acid
biosynthesis catalyzed by FAS I, we wished to have a broad
(20) An extensive study on the 1H NMR spectrum was carried out. The
absence of the doublet peak (J ) 8.0 Hz) at 4.48 ppm demonstrated no
contamination with the â-anomer at the newly introduced anomeric center.
Similarly, the absence of the doublet (J ) 8.0 Hz) at 4.45 ppm demonstrated
no contamination with the â-propyl anomer at the reducing end.
(21) For details, see Supporting Information.
(22) Structure assignment in the sEMG 14-mer series was made on the
basis of the 1H NMR analysis in comparison with the corresponding sMGP
14-mer series.
(19) On the basis of the 1H NMR analysis, the major byproduct appeared
to be a benzyl glycoside of the donors.
1946 J. Org. Chem., Vol. 72, No. 6, 2007

Synthetic 6-O-Methylglucose-Containing Polysaccharides
and was washed with 1 N HCl, aqueous NaHCO3, and brine. After
concentration, the product was recrystallized from EtOH (23.5 g,
95%).
range of sMGPs. With use of the reported synthesis, we were
able to synthesize sMGPs ranging from 6-mer to 20-mer.23 The
case of sMGP 18-mer and 20-mer summarized in Scheme 9
highlights the flexibility and usefulness of the reported synthesis.
C. Transformation of 2b,c to 3b,c. For the â- and γ-CDs series,
the following procedure of benzoylation was used. The 6-O-TBS-
â-CD 2b (16 g, 8.3 mmol) in pyr (500 mL) was cooled to 0 °C to
which was added BzCl (54 mL, 464 mmol) slowly. The mixture
was stirred at 50 °C for 5 days. The progress of the reaction was
monitored by NMR. The mixture was concentrated by evaporation
of pyr (400 mL), to which was added aqueous NaHCO3 (100 mL)
at 0 °C, and was stirred for 10 min. From the mixture, white solid
was precipitated out by the addition of ice-water (300 mL) which
was filtered and washed with cold water (300 mL). The solid was
taken with CH2Cl2 (300 mL) and was washed with 1 N HCl,
aqueous NaHCO3, and brine. After concentration, the product was
recrystallized from EtOH (26 g, 93%).
For the desilylation, the following procedure was used for all
series. The 6-O-TBS-2,3-di-O-benzoyl-R-CD thus obtained (1.0 g,
0.34 mmol) was dissolved in CH3CN-CH2Cl2 (20 mL, 3:1) in a
plastic reactor equipped with a stirring bar. To this mixture was
added 48% aqueous HF (2.0 mL, 28 mmol) slowly at room
temperature, and it was stirred for 2 h. The reaction mixture was
diluted with CH2Cl2 (25 mL) and was poured into aqueous NaHCO3
(100 mL). The aqueous phase was further extracted with CH2Cl2
(15 mL × 2). The combined organic phase was washed with brine
and dried over Na2SO4. The product was purified by flash
chromatography (eluent: CH2Cl2/MeOH) to give a white solid, 3a
(0.63 g, 83%).
Conclusion
An effective synthetic route to sMGPs from CDs was
developed. The synthetic route proved to be flexible and general,
to furnish a series of sMGPs ranging from 6-mer to 20-mer. A
practical and scalable method was discovered selectively to
cleave the CD derivatives and furnish the linear precursor to
the glycosyl donors and acceptors. The Mukaiyama glycosida-
tion was adopted to couple the glycosyl donors with the glycosyl
acceptors. Unlike in the sMMP series, the amount of the
Mukaiyama acid required in the sMGP series increased with
an increase of substrate size, so for large oligomers, more than
one equivalent of the acid was required.
sMGPs thus obtained provided us, for the first time, with an
opportunity to study the chemical and biological properties of
synthetic MGPs. To our delight, the preliminary experiments
demonstrated that synthetic and natural MGPs exhibit identical,
or at least very similar, properties.24 With this encouraging result,
we felt it critically important to address the scalability of sMGP
synthesis to ensure the supply of the materials. Recognizing a
difficulty in the scalability of glycosidation, we initiated a study
on the second generation of sMGP synthesis, resulting in an
efficient and scalable synthesis of this class of polysaccharides.25
For the spectroscopic data for 3a-c as well as the dibenzoates
2a-c, see Supporting Information.
D. Transformation of 3a to 4a. To a solution of 2,3-di-O-
benzoyl-R-CD 3a (890 mg, 0.40 mmol) in toluene-CH2Cl2 (40
mL, 3:1) were added MeI (1.2 mL, 19 mmol), Ag2O (1.7 g, 7.2
mmol), NaHCO3 (0.20 g, 2.4 mmol), and crushed 4 Å molecular
sieves (3.3 g, dried). The resulting slurry was sonicated for 12 h
and stirred for 8 h, after which an additional portion of Ag2O (0.6
g) and 4 Å molecular sieves (1 g) were added. Sonication was
resumed for a further 8 h. The slurry was filtered over Celite, rinsing
with CH2Cl2 and CH2Cl2-EtOAc (2:1) successively. The filtrate
was reduced in vacuo and purified by flash chromatography on
silica gel (eluent: CH2Cl2/EtOAc) to give the product as a white
solid (790 mg, 85%).
Experimental Section
Synthesis Summarized in Scheme 3. A. Transformation of
1a to 2a. To a solution of R-CD (1a, 10.0 g, 10.3 mmol), dried at
90 °C for 12 h under reduced pressure, in dry pyr (200 mL) was
added TBSCl (10.2 g, 67.8 mmol) in dry pyr (100 mL) dropwise
at 0 °C. The mixture was stirred at room temperature, and the
reaction was monitored by TLC every 12 h. Additional TBSCl (up
to 0.2 × 6 equiv) was added, if the reaction was incomplete. The
excess of pyr was evaporated under reduced pressure, and from
the residue, white solid was precipitated out by the addition of ice-
water (300 mL). The solid was filtered and washed with cold water
(500 mL). The solid was taken with CH2Cl2 (300 mL) and was
washed with 1 N HCl, aqueous NaHCO3, and brine. After drying
over Na2SO4 and concentration, the product was recrystallized from
EtOH (14.1 g, 83%).
With use of the same procedure, â- and γ-CDs (3b and 3c) were
converted to the corresponding 4b and 4c, respectively.
1
E. Spectroscopic Data for 4a: H NMR δ 3.56 (s, 3 × 6 H),
3.85 (d, J ) 10.0 Hz, 1 × 6 H), 4.16 (dd, J ) 11, 4.0 Hz, 1 × 6
H), 4.19 (dd, J ) 8.5, 8.5 Hz, 1 × 6 H), 4.50 (dd, J ) 9.5, 3.5 Hz,
1 H), 5.04 (dd, J ) 11.0, 4.0 Hz, 1 × 6 H), 5.44 (d, J ) 3.5 Hz,
1 × 6 H), 6.16 (t, J ) 8.5 Hz, 1 H), 6.83 (t, J ) 8.0 Hz, 2 × 6 H),
6.90 (t, J ) 7.5 Hz, 2 × 6 H), 7.11 (t, J ) 8.0 Hz, 1 × 6 H), 7.15
(t, J ) 8.0 Hz, 1 × 6 H); 13C NMR (100 MHz) δ 59.5, 71.4, 71.9,
72.2, 72.4, 78.2, 98.1, 127.7, 127.8, 128.3, 129.7, 130.0, 132.2,
132.6, 164.6, 166.3; MS (MALDI-TOF) calcd for (C126H120O42-
With use of the same procedure, â- and γ-CDs (1b and 1c) were
converted to the corresponding 2b and 2c, respectively. For the
spectroscopic data of 2a-c, see Supporting Information.
B. Transformation of 2a to 3a. For the R-CD series, the
following procedure of benzoylation was used. The 6-O-TBS-R-
CD 2a (14.0 g, 8.45 mmol) in pyr (400 mL) was cooled to 0 °C to
which was added freshly prepared BzOTf (51.3 mL, 310 mmol)
slowly. The mixture was warmed gradually to 40 °C and was stirred
for 48 h. The progress of the reaction was monitored by NMR.
The mixture was concentrated down to ca. 300 mL of pyr, to which
was added aqueous NaHCO3 (10 mL) at 0 °C, and was stirred for
10 min. From the mixture, white solid was precipitated out by the
addition of ice-water (30 mL) which was filtered and washed with
cold water (30 mL). The solid was taken with CH2Cl2 (300 mL)
Na+) 2327.71, found 2327.9; [R]25 +94.8 (c 0.26, CHCl3).
D
1
F. Spectroscopic Data for 4b: H NMR δ 3.55 (s, 3 × 7 H),
3.81 (d, J ) 11.0 Hz, 1 × 7 H), 4.18 (dd, J ) 9.5 Hz, 1 × 7 H),
4.22 (dd, J ) 11.0, 3.5 Hz, 1 × 7 H), 4.44 (m, 1 × 7 H), 5.04 (dd,
J ) 11.0, 3.5 Hz, 1 × 7 H), 5.50 (d, J ) 3.5 Hz, 1 × 7 H), 5.95
(dd, J ) 9.5, 9.5 Hz, 1 × 7 H), 6.96 (m, 4 × 7 H), 7.22 (m, 4 ×
7 H), 7.46 (m, 4 × 7 H); 13C NMR (100 MHz) δ 59.5, 71.2, 71.6,
72.0, 76.3, 97.1, 127.8, 128.0, 128.7, 129.8, 129.95, 130.02, 132.4,
132.6, 164.7, 166.3; MS (MALDI-TOF) calcd for (C147H140O49-
Na+) 2711.84, found 2711.8; [R]25 +70.5 (c 0.33, CHCl3).
D
G. Spectroscopic Data for 4c: 1H NMR δ 3.52 (s, 3 × 8 H),
3.78 (d, J ) 10.0 Hz, 1 × 8 H), 4.10 (m, 1 × 8 H), 4.27 (dd, J )
9.5, 9.5 Hz, 1 × 8 H), 4.32 (m, 1 × 8 H), 5.07 (dd, J ) 10.5, 3.5
Hz, 1 × 8 H), 5.54 (d, J ) 3.5 Hz, 1 × 8 H), 5.90 (dd, J ) 8.5,
8.5 Hz, 1 × 8 H), 6.99 (t, J ) 8.0 Hz, 2 × 8 H), 7.07 (t, J ) 8.0
Hz, 2 × 8 H), 7.17 (t, J ) 7.15 Hz, 1 × 8 H), 7.30 (d, J ) 7.5 Hz,
(23) With use of the reported synthetic route, sMGP 6-, 7-, 8-, 10-, 12-,
14-, 16-, 18-, and 20-mers were synthesized. For details, see Supporting
Information.
(24) (a) Cheon, H. S.; Wang, Y.; Ma, J.; Kishi, Y. ChemBioChem., in
press. (b) Papaioannou, N.; Cheon, H.-S.; Kishi, Y., in preparation.
(25) Cheon, H.-S.; Kishi, Y., in preparation.
J. Org. Chem, Vol. 72, No. 6, 2007 1947

Meppen et al.
1 × 8 H), 7.56 (m, 4 × 8 H); 13C NMR (100 MHz) δ 59.6, 70.9,
71.3, 71.8, 72.1, 74.3, 96.2, 128.0, 128.1, 128.7, 129.7, 129.9, 130.0,
132.6, 132.8, 164.9, 166.1; MS (MALDI-TOF) calcd for (C168H160O56-
Μ, φ ) 4.0 cm) and eluted by EtOAc/CHCI3 with a linear gradient
from 4% to 20% (1.2 L), to furnish the R-anomer (1.0 g, R/â ) ca.
100:1) and the â-enriched anomeric mixture (0.34 g, R/â ) ca.
1:2). Spectroscopic data for 9a (R/â ) ca. 100:1): 1H NMR (500
MHz) characteristic peaks for R-anomer, TMS-H δ -0.02 (s, 9
H); six methoxy-H 3.30 (s, 3 H), 3.33 (s, 3 H), 3.35 (s, 3 H), 3.44
(s, 3 × 2 H), 3.56 (s, 3 H); the reducing end anomeric-H 5.26 (d,
J ) 3.4 Hz, 1 H); MS (MALDI-TOF) calcd for (C132H134O43SiNa+)
Na+) 3095.96, found 3096.1; [R]25 +105 (c 0.26, CHCl3).
D
Synthesis Summarized in Scheme 5. A.Transformation of 4a
to 8a. To a solution of 4a (5.20 g, 2.24 mmol) in CH2Cl2 (210
mL) at room temperature were added methoxyacetic acid (33 mL)
and BF3‚OEt2 (33 mL), and the mixture was warmed to 40 °C and
stirred for 1 h. The mixture was diluted with CH2Cl2 and poured
into aqueous NaHCO3 (500 mL). The organic phase was separated,
washed with brine, dried over Na2SO4, and concentrated. Recrys-
tallization from MeOH (100 mL) afforded the ring-opened product
8a as a white solid (4.50 g, 82%, R/â ) ca. 10): 1H NMR (500
MHz) characteristic peaks for R-anomer, seven methoxy-H δ 3.30
(s, 3 H), 3.35 (s, 3 H), 3.37 (s, 3 H), 3.40 (s, 3 H), 3.45 (s, 3 H),
3.47 (s, 3 H), 3.55 (s, 3 H); the reducing end anomeric-H 6.57 (d,
J ) 3.4 Hz, 1 H); MS (MALDI-TOF) calcd for (C129H126O45Na+)
2457.80, found 2457.9; [R]25 +68.8 (c 0.28, CHCl3).
D
C. Transformation of 8b,c to 9b,c. Using the same procedure,
8b and c were converted to 9b and c, respectively. The R-enriched
9b and 9c (R/â ) ca. 100:1) were obtained by using the same
chromatographic method.
D. Spectroscopic Data for 9b (R-Anomer Enriched): TMS-H
δ 0.00 (s, 9 H); seven methoxy-H 3.30 (s, 3 × 3 H), 3.31 (s, 3 H),
3.34 (s, 3 H), 3.44 (s, 3 × 2 H), 3.56 (s, 3 H); the reducing end
anomeric-H 5.26 (d, J ) 3.4 Hz, 1 H); MS (MALDI-TOF) calcd
2417.74, found 2418.0; [R]25 +78.1 (c 0.43, CHCl3).
for (C153H154O50SiNa+) 2841.92, found 2881.9; [R]25 +76.4 (c
D
D
0.38, CHCl3).
B. Transformation of 4b to 8b. Into a 1 L flask containing 4b
(6.00 g, 2.20 mmol) and a stirring bar was added a mixture of
methoxyacetic acid (250 mL) and BF3‚OEt2 (47 mL) at room
temperature. After approximately 2 h of stirring, the starting material
dissolved completely, followed by the appearance of a white
precipitate after about 5 h, and was stirred further for 18 h. Then,
cold water (300 mL) was added to the reaction mixture. The white
precipitate was filtered and washed with water (3 × 100 mL). The
solid was taken up with CH2Cl2 (200 mL) and washed with aqueous
NaHCO3 (2 × 100 mL) and brine (100 mL). After drying over
Na2SO4 and concentration, recrystallization from EtOAc/hexanes
gave 8b as a white solid (5.44 g, 88%, R/â ) ca. 3): 1H NMR
(500 MHz) characteristic peaks for R-anomer, eight methoxy-H δ
3.30 (s, 3 H), 3.32 (s, 3 H), 3.35 (s, 3 H), 3.37 (s, 3 H), 3.40 (s, 3
H), 3.45 (s, 3 H), 3.47 (s, 3 H), 3.55 (s, 3 H); the reducing end
anomeric-H 6.57 (d, J ) 3.4 Hz, 1 H); MS (MALDI-TOF) calcd
for (C150H146O52Na+) 2801.87, found 2801.9; [R]25D +75.6 (c 0.31,
CHCl3).
E. Spectroscopic Data for 9c (R-Anomer Enriched): 1H NMR
characteristic peaks for R-anomeric-OH product, 4-OTMS-H δ
-0.02 (s, 9 H); eight methoxy-H 3.29 (s, 3 H), 3.30 (s, 3 × 3 H),
3.34 (s, 3 H), 3.44 (s, 3 × 2 H), 3.56 (s, 3 H); the reducing end
anomeric-H 5.25 (d, J ) 3.4 Hz, 1 H), (d, J ) 8.0 Hz, 1 H); MS
(MALDI-TOF) calcd for (C153H154O50SiNa+) 3226.04, found
3226.0; [R]25 +78.0 (c 0.45, CHCl3).
D
F. Transformation of 9a-c to 10a-c. The following procedure
was applied for all series. To 9a (1.65 g, 0.677 mmol) was added
a mixture of 1 N HCl/acetone (40 mL, 1:9), and the mixture was
stirred at room temperature for 1 h. Then the mixture was reduced
in vacuo, and the residue was taken with CH2Cl2 (50 mL) and
washed with NaHCO3 and brine successively. After drying over
Na2SO4, solvents of the organic phase were exchanged with CH2-
Cl2-CH3OH (30-15 mL).
To the resultant residue was added NaOMe (400 mg, 7.40 mmol).
The mixture was stirred at room temperature for 22 h and then
evaporated to dryness.
C. Transformation of 4c to 8c. Following the procedure given
for 4b f 8b, 4c (3.80 g, 12.5 mmol) was converted to 8c (3.46 g,
88%, R/â ) ca. 2): 1H NMR (500 MHz) characteristic peaks for
R-anomer δ 3.30 (s, 3 H), 3.32 (s, 3 H), 3.33 (s, 3 H), 3.34 (s, 3
H), 3.35 (s, 3 H), 3.41 (s, 3 H), 3.47 (s, 3 H), 3.49 (s, 3 H), 3.57
(s, 3 H); the reducing end anomeric-H 6.57 (d, J ) 3.4 Hz, 1 H);
MS (MALDI-TOF) calcd for (C171H166O59Na+) 3185.99, found
To the resultant residue were added THF (40 mL), DMF (10
mL), NaH (95%, 602 mg, 24.0 mmol), and TBAI (250 mg, 0.677
mmol). The heterogeneous mixture was stirred at room temperature
for 30 min before benzyl bromide (3.13 mL, 26.4 mmol) was added.
The mixture was stirred at room temperature for 21 h. After
quenching by 5 mL of methanol at 0 °C, the mixture was taken
with CH2Cl2 (100 mL) and rinsed with 1 N HCl, aqueous NaHCO3,
and brine. Flash chromatography on silica gel (eluent: benzene/
EtOAc) afforded 10a (1.20 g, 77%): 1H NMR (500 MHz)
characteristic peaks for R-anomer, six methoxy-H δ 3.29 (s, 3 H),
3.31 (s, 3 H), 3.32 (s, 3 H), 3.33 (s, 3 H), 3.37 (s, 3 H), 3.42 (s, 3
H); allyl-CH2 5.22 (dd, J ) 11.0, 1.5 Hz, 1 H), 5.36 (dd, J ) 17.0,
1.5 Hz, 1 H); six a-anomeric-H 5.60 (d, J ) 4.0 Hz, 1 H), 5.63 (d,
J ) 3.5 Hz, 1 H), 5.68 (m, 4 H); MS (MALDI-TOF) calcd for
3185.9; [R]25 +77.2 (c 0.33, CHCl3).
D
Synthesis Summarized in Scheme 6. A. Transformation of
8a to 9a. To a solution of 8a (4.50 g, 1.89 mmol) and Et3N (1.7
mL, 11.4 mmol) in CH2Cl2 (100 mL) at 0 °C was added TMSOTf
(1.05 mL, 5.67 mmol) slowly. The solution was gradually warmed
to room temperature and was stirred for 1 h. Workup with CH2-
Cl2/aqueous NaHCO3 was done. The crude product was recrystal-
lized from EtOAc/hexanes to give the TMS-ether of 8a as a white
solid (4.40 g, 96%).
(C136H160O34Na+) 2308.05, found 2307.9; [R]25 +72.0 (c 0.23,
D
CHCl3).
To a solution of the TMS-ether of 8a (4.28 g, 1.73 mmol) in
CH2Cl2 (60 mL) was added ethanolamine (1.04 mL, 17.3 mmol)
at room temperature, and the mixture was stirred for 5 h. The
reaction mixture was diluted with CH2Cl2 (50 mL), poured into
water (100 mL), and partitioned. The organic phase was washed
with brine and dried over Na2SO4. Flash chromatography (eluent:
CH2Cl2/EtOAc) yielded the glycoside (3.85 g, 92%, R/â ) ca. 2).
B. 9a (R-Anomer Enriched). To a solution of the glycoside
thus obtained (2.00 g, 0.842 mmol) in DMF (60 mL) were added
Ag2O (1.56 g, 6.47 mmol), crushed 4 Å MS (activated, 2.0 g), allyl
bromide (0.73 mL, 8.42 mmol), and NaHCO3 (0.42 g, 5.05 mmol).
The dark suspension was stirred for 18 h. The slurry was filtered
over Celite, rinsing with CH2Cl2 and CH2Cl2-EtOAc (2:1) suc-
cessively. The filtrate was reduced in vacuo and purified by flash
chromatography on silica gel (eluent: CH2Cl2/EtOAC) to give the
product 9a as a white solid (2.03 g, 92%, R/â ) ca. 7). The
anomeric mixture (1.5 g) was loaded on a Biotage column (40 +
1
G. Spectroscopic Data for 10b: H NMR (500 MHz) charac-
teristic peaks for R-anomer, seven methoxy-H δ 3.29 (s, 3 H), 3.30
(s, 3 H), 3.32 (s, 3 H), 3.33 (s, 3 H), 3.37 (s, 3 H), 3.39 (s, 3 H),
3.42 (s, 3 H); allyl-CH2 5.22 (dd, J ) 11.0, 1.5 Hz, 1 H), 5.36 (dd,
J ) 17.0, 1.5 Hz, 1 H); seven R-anomeric-H 5.60 (d, J ) 4.0 Hz,
1 H), 5.63 (d, J ) 3.5 Hz, 1 H), 5.68 (m, 5 H); MS (MALDI-TOF)
calcd for (C157H180O36Na+) 2664.21, found 2664.3; [R]25 +75.3
D
(c 0.15, CHCl3).
H. Spectroscopic Data for 10c: 1H NMR (500 MHz) charac-
teristic peaks for R-anomer, eight methoxy-H δ 3.29 (s, 3 H), 3.30
(s, 3 H), 3.316 (s, 3 H), 3.322 (s, 3H), 3.36 (s, 3 H), 3.37 (s, 3 H),
3.39 (s, 3 H), 3.42 (s, 3 H); allyl-CH2 5.21 (dd, J ) 11.0, 1.5 Hz,
1 H), 5.36 (dd, J ) 17.0, 1.5 Hz, 1 H); eight a-anomeric-H 5.59
(d, J ) 3.5 Hz, 1 H), 5.64 (d, J ) 3.5 Hz, 1 H), 5.67 (m, 6 H); MS
(MALDI-TOF) calcd for (C178H204O41Na+) 3020.38, found 3020.1;
[R]25 +68.3 (c 0.43, CHCl3).
D
1948 J. Org. Chem., Vol. 72, No. 6, 2007

Synthetic 6-O-Methylglucose-Containing Polysaccharides
I. Transformation of 10a-c to 11a-c. The following procedure
was applied for all series. To a solution of 10a (0.80 g, 0.35 mmol)
in EtOH-toluene (6-3 mL) were added (Ph3P)3RhCl (16 mg, 0.018
mmol) and DABCO (5.9 mg, 0.053 mmol). The mixture was heated
at 80 °C for 20 h. After the removal of solvents, the residue was
stirred with 1 N HCl-acetone (1:9, 20 mL) at 80 °C for 6 h. The
solvents were removed in vacuo, and the residue was taken up with
CH2Cl2 and washed with aqueous NaHCO3 and brine. Flash
chromatography on silica gel (eluent: CH2Cl2/EtOAc) afforded 11a
(0.65 g, 82%, R/â ) ca. 2:1): 1H NMR (500 MHz) characteristic
peaks for R-anomer, the reducing end C1 OH δ 2.94 (d, J ) 2.5
Hz, 1 H); six methoxy-H 3.29 (s, 3 H), 3.30 (s, 3 H), 3.32 (s, 3 H),
3.33 (s, 3 H), 3.37 (s, 3 H), 3.38 (s, 3 H); six R-anomeric-H 5.23
(d, J ) 3.0 Hz, 1 H), 5.59 (d, J ) 3.5 Hz, 1 H), 5.62 (d, J ) 3.5
Hz, 1 H), 5.67 (m, 3 H); MS (MALDI-TOF) calcd for (C133H152O31-
drous AgClO4, see refs 26a,b.b All solvents were freshly distilled
prior to use (CH2Cl2 from CaH2; Et2O from LiAlH4) and transferred
via cannula into the solvent flasks. All flasks were flame dried just
before use. Syringes, septa, needles, AgClO4 (placed in a pear-
shaped flask equipped with a stirrer bar, septum, and needle),
aluminum foil, stoppers, and plastic bags were dried over P2O5/
Drierite in a desiccator in vacuo overnight. The starting materials
were also dried over P2O5 under reduced pressure overnight. After
all the reagents, solvents, and the desiccator were placed in a glove
bag, the atmosphere was exchanged three times with nitrogen
(introduced through Drierite plug).
B. Glycosidation to Form 13a. To AgClO4 (52 mg, 0.25 mmol)
in a 10 mL pear-shaped flask equipped with a stirring bar was added
Et2O (5 mL). This suspension was stirred for 20 min, and the flask
was wrapped with aluminum foil to avoid light. To the AgClO4
suspension was added SnCl4 (30 mL, 0.25 mmol) slowly, and the
mixture was stirred for an additional 20 min. The suspension was
then left to stand for 5 min to allow AgCl precipitate to settle down.
To the reaction flask, which was equipped with a stirring bar and
sealed with a septum, was added the supernatant of the Mukaiyama
acid solution (0.36 mL, 0.018 mmol). The starting materials (R-
anomer enriched 9a (45 mg, 0.018 mmol) and 12a (44 mg, 0.018
mmol)) were dissolved in CH2Cl2 (0.36 mL). The solution was
transferred into a 1 mL plastic syringe whose needle exit was fixed
on the septum of the catalyst solution, and they were carefully
placed in two plastic bags to isolate them from the outer atmosphere.
They were cooled to -30 °C in a cryobath. After allowing for
thermal equilibration, the starting material solution was added
slowly over 10 min and the solution was stirred for 24 h. The
reaction mixture was quenched with aqueous NaHCO3 and extracted
with CH2Cl2 (2 mL), dried over Na2SO4, and reduced in vacuo.
The crude mixture was stirred in 1 N HCl/acetone (10 mL, 1:9)
for 1 h at room temperature to desilylate the unreacted 9a (20 mg,
43% recovered). The product (R/â selectivity of glycosidation )
ca. 5.5:1 by the 1H NMR spectrum) was purified by two successive
flash chromatographies on silica gel. The first chromatography
(hexanes/THF) was used to remove the byproduct, and the second
chromatography (CH2Cl2/EtOAc) was used to isolate the desired
R-glycosidation product (isolated R-isomer: 43 mg; 48%): 1H NMR
(500 MHz) characteristic peaks for the product with R-allyl terminal,
twelve methoxy-H δ 3.284 (s, 3 H), 3.297 (s, 3 H), 3.300 (s, 3 H),
3.311 (s, 3 H), 3.318 (s, 3 H), 3.328 (s, 3 H), 3.346 (s, 3 × 2 H),
3.358 (s, 3 H), 3.402 (s, 3 H), 3.477 (s, 3 H), 3.64 (s, 3 H); MS
(MALDI-TOF) calcd for (C262H276O73Na+) 4612.78, found 4612.3;
Na+) 2268.02, found 2268.4; [R]25 +57.5 (c 0.40, CHCl3).
D
J. Spectroscopic Data for 11b (R/â ) ca. 2:1): 1H NMR (500
MHz) characteristic peaks for R-anomer, the reducing end C1 OH
δ 2.94 (d, J ) 2.5 Hz, 1 H); seven methoxy-H 3.29 (s, 3 H), 3.30
(s, 3 H), 3.32 (s, 3 H), 3.33 (s, 3 H), 3.37 (s, 3 H), 3.39 (s, 3 H),
3.42 (s, 3 H); seven R-anomeric-H 5.24 (m, 1 H), 5.60 (d, J ) 3.5
Hz, 1 H), 5.64 (m, 5 H); MS (MALDI-TOF) calcd for (C154H176O36-
Na+) 2624.18, found 2624.2; [R]25 +79.1 (c 0.18, CHCl3).
D
K. Spectroscopic Data for 11c (R/â ) ca. 2:1): 1H NMR (500
MHz) characteristic peaks for R-anomer, the reducing end C1 OH
δ 2.94 (d, J ) 2.5 Hz, 1 H); eight methoxy-H 3.29 (s, 3 H), 3.30
(s, 3 H), 3.32 (s, 3 H), 3.33 (s, 3 H), 3.37 (s, 3 H), 3.39 (s, 3 H),
3.42 (s, 3 H); eight R-anomeric-H 5.23 (t, J ) 3.5 Hz, 1 H), 5.60
(d, J ) 3.5 Hz, 1 H), 5.63 (d, J ) 3.5 Hz, 1 H), 5.67 (m, 5 H); MS
(MALDI-TOF) calcd for (C175H200O41Na+) 2980.35, found 2980.2;
[R]25 +79.3 (c 0.19, CHCl3).
D
L. Transformation of 11a-c to 12a-c. The following proce-
dure was applied for all series. To a solution of 11a (0.50 g, 0.22
mmol), 2-(2-methoxyethoxy)acetic acid (38.4 mL, 0.33 mmol), and
DMAP (13.6 mg, 0.11 mmol) in CH2Cl2 (15 mL) at 0 °C was added
EDCI (85 mg, 0.446 mmol) in portions. The cooling bath was
removed after 10 min, and the mixture was stirred at room
temperature for 2 h. The mixture was diluted with CH2Cl2 (20 mL)
and washed with aqueous NaHCO3 and brine. Flash chromatog-
raphy on silica gel (eluent: CH2Cl2/EtOAc) afforded 12a as a
colorless solid (0.50 g, 96%, R/â ) ca. 2): 1H NMR (500 MHz)
characteristic peaks for R-anomer, seven methoxy-H δ 3.30 (s, 3
H), 3.31 (s, 3 H), 3.32 (s, 3 H), 3.34 (s, 3 H), 3.38 (s, 3 × 2H),
3.41 (s, 3 H); six R-anomeric-H 5.61 (d, J ) 3.5 Hz, 1 H), 5.63 (d,
J ) 3.5 Hz, 1 H), 5.66 (d, J ) 3.5 Hz, 1 H), 5.68 (d, J ) 3.5 Hz,
1 H), 5.74 (d, J ) 3.5 Hz, 1 H), 6.46 (d, J ) 3.5 Hz, 1 H); MS
(MALDI-TOF) calcd for (C138H160O34Na+) 2384.07, found 2384.2;
[R]25 +72.9 (c 0.48, CHCl3).
D
C. Glycosidation to Form 13b. With 9b (R-anomer enriched,
52 mg, 0.0184 mmol), 12b (50 mg, 0.0184 mmol), and the
Mukaiyama acid (0.0184 mmol), the glycosidation was conducted
under the essentially same conditions (R/â selectivity ) ca. 5:1;
R-isomer isolated: 48 mg, 50%): 1H NMR (500 MHz) characteristic
peaks for the product with R-allyl terminal, fourteen methoxy-H δ
3.289 (s, 3 H), 3.301 (s, 3 H), 3.306 (s, 3 × 2 H), 3.316 (s, 3 H),
3.321 (s, 3 H), 3.327(s, 3 H), 3.358 (s, 3 × 3H), 3.364 (s, 3 H),
3.408 (s, 3 H), 3.483 (s, 3 H), 3.647 (s, 3 H); MS (MALDI-TOF)
[R]25 +80.1 (c 0.23, CHCl3).
D
M. Spectroscopic Data for 12b (R/â ) ca. 2:1): 1H NMR (500
MHz) characteristic peaks for R-anomer, eight methoxy-H δ 3.29
(s, 3 H), 3.30 (s, 3 H), 3.31 (s, 3 H), 3.32 (s, 3 H), 3.36 (s, 3 H),
3.377 (s, 3 H), 3.38 (s, 3 × 2 H), 3.41 (s, 3 H); seven R-anomeric-H
5.59 (d, J ) 3.5 Hz, 1 H), 5.63 (m, 4 H), 5.74 (d, J ) 3.5 Hz, 1
H), 6.45 (d, J ) 3.5 Hz, 1 H); MS (MALDI-TOF) calcd for
calcd for (C304H320O85Na+) 5353.06, found 5353.1; [R]25 +76.3
D
(c 0.66, CHCl3).
(C159H184O39Na+) 2740.23, found 2740.0; [R]25 +81.2 (c 0.26,
D
D. Glycosidation to Form 13c. With 9c (R-anomer enriched,
32 mg, 0.010 mmol), 12c (31 mg, 0.010 mmol), and the Mukaiyama
acid (0.010 mmol), the glycosidation was conducted under the
essentially same conditions (R/â selectivity ) ca. 5:1; R-isomer
isolated: 28 mg, 46%): 1H NMR (500 MHz) characteristic peaks
for the product with R-allyl terminal, sixteen methoxy-H: δ 3.288
(s, 3 H), 3.300-3.325 (7s, 3 × 7 H), 3.360 (s, 3 × 5 H), 3.407 (s,
CHCl3).
N. Spectroscopic Data for 12c (R/â ) ca. 2:1): 1H NMR (500
MHz) characteristic peaks for R-anomer, nine methoxy-H δ 3.29
(s, 3 H), 3.30 (s, 3 H), 3.318 (s, 3 H), 3.322 (s, 3 H), 3.36 (s, 3 H),
3.37 (s, 3 H), 3.38 (s, 3 × 2 H), 3.41 (s, 3 H); eight a-anomeric-H
5.59 (d, J ) 3.5 Hz, 1 H), 5.64 (d, J ) 3.5 Hz, 1 H), 5.66 (m, 4
H), 5.75 (d, J ) 3.5 Hz, 1 H), 6.45 (d, J ) 3.5 Hz, 1 H); MS
(MALDI-TOF) calcd for (C180H208O44Na+) 3096.39, found 3095.8;
(26) (a) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals, 3rd ed.; Pergamon: New York, NY, 1988. (b) Encyclopedia of
Reagents for Organic Synthesis; Paquette, L. A., Ed.; John Wiley & Son:
New York, NY, 1995.
[R]25 +83.2 (c 0.25, CHCl3).
D
Glycosidations Summarized in Scheme 7. A. General Note
on Glycosidation Reactions. Precautions: for handling of anhy-
J. Org. Chem, Vol. 72, No. 6, 2007 1949

Meppen et al.
3 H), 3.481 (s, 3 H), 3.646 (s, 3 H); MS (MALDI-TOF) calcd for
14b (10.2 mg (68%)): 1H NMR (D2O, 500 MHz) δ 0.89 (t, J )
7.5 Hz, 3 H), 1.60 (m, 2 H), 3.37 (s, 3 × 13 H), 3.38 (s, 3 H), 3.42
(t, J ) 9.5 Hz, 1 H), 3.54-3.94 (m, 85 H), 4.89 (d, J ) 4.0 Hz, 1
H), 5.36 (d, J ) 4.0 Hz, 1 H), 5.42 (m, 12 H); MS (MALDI-TOF)
(C346H364O97Na+) 6093.34, found 6093.6; [R]25 +83.0 (c 0.32,
D
CHCl3).
Glycosidations with â-anomer enriched acceptors 9a-c were also
carried out under the essentially same conditions. For details, see
Supporting Information.
calcd for (C101H176O71Na+) 2548.01, found 2548.2; [R]25 +203
D
(c 0.29, H2O).
Deprotection Summarized in Scheme 7. A. R-14a. To a
solution of 13a (R-allyl anomer, 20 mg, 4.3 mmol) in THF-MeOH
(3 mL, 2:1) was added 10% Pd(OH)2 on carbon (10 mg). The
mixture was equipped with a H2 balloon and stirred for 24 h. The
reaction mixture was passed through a sintered-glass filter with
thorough rinsing with CH2Cl2-MeOH (30 mL, 2:1). The filtrate
was concentrated with an evaporator, and the residue was taken
up with CH2Cl2 (2.0 mL). To the mixture was added 0.1 M NaOMe
in MeOH (1.0 mL, 0.1 mmol). The mixture was stirred at room
temperature for 12 h and neutralized by adding 1 N HCl (0.1 mL,
0.1 mmol). After evaporation of solvents, the residue was subjected
to reverse-phase C18-column chromatography (eluent: H2O/MeOH
3:1 to 1:1) to furnish sMGP 12-mer 14a (8.1 mg, 86%). The product
was further purified by HPLC on a reverse-phase C18-column with
an RI detector (eluent: MeOH/H2O): 1H NMR (D2O, 500 MHz)
δ 0.90 (t, J ) 7.5 Hz, 3 H), 1.61 (m, 2 H), 3.38 (s, 3 × 11 H), 3.39
(s, 3 H), 3.43 (t, J ) 9.5 Hz, 1 H), 3.44-3.95 (m, 73 H), 4.88 (d,
J ) 4.0 Hz, 1 H), 5.37 (d, J ) 4.0 Hz, 1 H), 5.42 (m, 10 H); MS
(MALDI-TOF) calcd for (C87H152O61Na+) 2195.87, found 2196.0;
C. R-14c. The deprotection was performed under the same
conditions, where 13c (R-anomer enriched, 62 mg, 10 mmol) gave
14c (19.5 mg (68%)): 1H NMR (D2O, 500 MHz) δ 0.89 (t, J ) 7.5
Hz, 3 H), 1.61 (m, 2 H), 3.37 (s, 3 × 15 H), 3.38 (s, 3 H), 3.43 (t,
J ) 9.5 Hz, 1 H), 3.54-3.94 (m, 97 H), 4.89 (d, J ) 4.0 Hz, 1 H),
5.36 (d, J ) 4.0 Hz, 1 H), 5.42 (m, 14 H); MS (MALDI-TOF)
calcd for (C115H200O81Na+) 2900.14, found 2900.0; [R]25 +205
D
(c 0.28, H2O).
Using the same protection procedure, â-enriched sMGP 12-, 14-,
and 16-mers were prepared. For details, see Supporting Information.
For details for the syntheses summarized in Schemes 8 and 9,
see Supporting Information.
Acknowledgment. Financial support from the National
Institutes of Health (NS 12108) and Eisai Research Institute is
gratefully acknowledged.
Supporting Information Available: Synthetic procedures and
spectroscopic data. This material is available free of charge via
the Internet at http://pubs.acs.org.
[R]25 +213 (c 0.28, H2O).
D
B. R-14b. The deprotection was performed under the same
conditions, where 13b (R-anomer enriched, 20 mg, 7.5 mmol) gave
JO061990V
1950 J. Org. Chem., Vol. 72, No. 6, 2007