Polymerization of mannosyl tricyclic orthoesters for the synthesis of α(1-6) mannopyranan-the backbone of lipomannan

Article abstract of DOI:10.1016/j.bmc.2010.04.014

Tuberculosis (TB) remains a major health problem worldwide. Understanding the interactions between the surface components of Mycobacterium tuberculosis (Mtb), the main causative agent of TB, with host immune response will be critical for developments of e

Full text of DOI:10.1016/j.bmc.2010.04.014

Bioorganic & Medicinal Chemistry 18 (2010) 3726–3734
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry
journal homepage: www.elsevier.com/locate/bmc
Polymerization of mannosyl tricyclic orthoesters for the synthesis of
mannopyranan—the backbone of lipomannan
a(1–6)
Chanokpon Yongyat a, Somsak Ruchirawat b,c, Siwarutt Boonyarattanakalin a,
*
a Department of Bio-Chemical Engineering, Sirindhorn International Institute of Technology, Thammasat University, Pathum Thani 12121, Thailand
b Laboratory of Medicinal Chemistry, Chulabhorn Research Institute, and Program in Chemical Biology, Chulabhorn Graduate Institute,
Vipavadee-Rangsit Highway, Bangkok 10210, Thailand
c The Center of Excellence on Environmental Health, Toxicology and Management of Chemicals, Vipavadee-Rangsit Highway, Bangkok 10210, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history:
Tuberculosis (TB) remains a major health problem worldwide. Understanding the interactions between
the surface components of Mycobacterium tuberculosis (Mtb), the main causative agent of TB, with host
immune response will be critical for developments of effective treatments and prevention of TB. Chem-
ically defined mimics of the bacterial envelope components serve as important tools for biological studies
of the bacterial interactions with mammalian hosts. We report here a rapid synthetic approach utilizing
mannosyl tricyclic orthoesters as monomers for regio- and stereo-controlled polymerizations to generate
Received 1 February 2010
Revised 1 April 2010
Accepted 6 April 2010
Available online 18 April 2010
Keywords:
Tuberculosis
TB
Mannopyranan
Lipomannan
Glycosyl orthoesters
Glycosylation
Glycosidic bonds
Carbohydrates
Polymerization
a
(1–6) mannopyranan—the backbone of lipomannan. The polymerizations generated multiple glycosidic
bonds in a single chemical transformation in regio- and stereo-selective manners. TMSOTf is the opti-
mum catalyst to promote the selective and high yielding polymerization when compared with other
Lewis acids. In addition, the monomers 3,4-O-benzyl-b-D-mannopyranose 1,2,6-orthobenzoate (1) and
3,4-O-benzyl-b- -mannopyranose 1,2,6-orthopivalate (2) can be synthesized in multiple-gram scale
D
and in a rapid fashion. Characterizations by GPC and NMR indicate the identity of
with DPn (degree of polymerization) = 20.
a(1–6) mannopyranan
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
and mannan capped lipoarabinomannan (ManLAM) are important
interspersed molecules that comprise the biologically and physio-
logically unique envelope of Mtb.1,11,14–16 The pathogenic bacteria
have sophisticatedly evolved to modulate host immune systems
and survive in mammalian hosts. LM is one of the macromolecules
that are implicated during the infectious, virulent, and survival
events of Mtb in host mammalian cells.17,18
After numerous years of being relatively dormant, infectious
tuberculosis (TB) has re-emerged as a life-threatening global
health problem.5,6 TB treatment requires long term usage of antibi-
otics which often is not affordable by patients in third world coun-
tries.7 Occurrences of multidrug resistant tuberculosis (MDR-TB)
and extensive drug resistant tuberculosis (XD-TB), resulting from
inadequate therapy, contribute to more epidemic concerns.8,9
Although, vaccination by BCG, the only available vaccine against
TB, creates protection in newborns, BCG fails to defend against
reactivation of pulmonary TB in adults.6
One third of the world population is infected with TB. This
chronic disease has continuously created major social, economic,
and medical problems for humans, especially in the third world
countries.10–12 Treatment of TB is complicated by the exceptionally
complex envelope of Mycobacterium tuberculosis (Mtb), the major
causative agent of tuberculosis (TB).13
LMs are composed of a(1–6) mannopyranan as the backbone of
the molecule.19 The reducing end of LM is equipped with phospha-
tidylinositol mannoside (Fig. 1). The mannoside backbone is fur-
ther branched at numerous C-2 positions with a single unit of
a-
mannopyranose. The mannopyranan consists of approximately
20–25 mannose residues.1–3
Lipomannan (LM) along with other glycolipids including phos-
phatidylinositol mannosides (PIMs), lipoarabinomannan (LAM),
Figure 1. 1–4Structural feature of lipomannan (LM): LM is the extended structure of
PIM by having
mannoside backbone is mannosylated at numerous C-2 positions with a single unit
of -mannopyranose. Approximately, 20–25 mannose residues make up the
mannopyranan.
a(1–6) mannopyranan elongated at the non reducing end of PIM. The
* Corresponding author. Tel.: +66 2 986 9009x2305; fax: +66 2 986 9009x2301.
E-mail address: siwarutt@siit.tu.ac.th (S. Boonyarattanakalin).
a
0968-0896/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmc.2010.04.014

C. Yongyat et al. / Bioorg. Med. Chem. 18 (2010) 3726–3734
3727
Understanding of LM interactions with the host immune system
will lead to effective treatments and prevention of TB. Chemically
defined LM structure will facilitate its biological studies. However,
many of these studies are impeded by the limited amount of nat-
urally occurring oligosaccharides that are also often difficult to iso-
late in their pure state. Chemical syntheses offer potential methods
to supply well-defined structures for detailed immunological stud-
ies.20–22 Nevertheless, its macromolecular nature makes the syn-
thesis of LM impossible to achieve by traditional stepwise
syntheses.
Apart from traditional stepwise synthetic methods, manual and
automated solid phase syntheses are employed, as more favorable
methods for the synthesis of large oligosaccharides, because these
protocols take much shorter time for chemical processing.23,24
Nonetheless, solid phase syntheses are still limited by the large
size of synthetic targets such as LM or LAM.
regio- and stereo-selectivity of the resulting polymer could be
achieved. The pattern of protecting groups on the hydroxyls of
building blocks heavily influences both the polymerization effi-
ciency and the selectivity of the resulting polymers. The regioselec-
tivity was created by the preferential attacks of a Lewis acid at
different oxygen atoms bonding with orthoester carbon. The stere-
oselectivity mainly took place by the unique geometry of the
orthoester intermediate that favors the incoming nucleophile of
the monomer in one direction during propagation steps. After care-
ful consideration, we decided to take advantage of the unique
geometry of the mannosyl 1,2,6-tricyclic orthoester (Fig. 2). The
protecting groups on mannose were manipulated to temporarily
mask C-2 hydroxyl and to be suitable for tricyclic formation. The
orthogonal protecting group on C-2 allows further mannosylation
on the LM backbone to generate branching of
units.
a-single mannose
The nature of the numerous repeating units directs us toward
utilization of polymeric reactions. In spite of many biological roles
of LM, rarely are there accounts for a one-step synthesis of selec-
tive protected poly- or oligo-mannosides. To serve this purpose,
we have designed and synthesized tricyclic orthoesters of mannose
to be used as monomers for the ring-opening polymerization to-
For a rapid synthesis of oligo- and poly-mannosides, tricyclic
orthoester building blocks of mannose are suitable structural fea-
tures for ring-opening polymerizations. The building blocks—3,4-
O-benzyl-b-
D-mannopyranose 1,2,6-orthobenzoate (1) and 3,4-O-
benzyl-b- -mannopyranose 1,2,6-orthopivalate (2) were synthe-
D
sized and utilized as monomers in ring-opening polymerizations.
The monomers 1 and 2 contain a highly strained tricyclic structure
which is readily susceptible for ring-opening polymerizations upon
activation with Lewis acids.
ward D-mannopyranan. Controlled polymerizations of a monosac-
charide monomer can create several glycosidic bonds in a single
chemical reaction, and give desired products in significant
amounts. We report here the application of controlled polymeriza-
The C-3 and C-4 hydroxyl groups of both building block mono-
mers 1 and 2 were protected with an electron donating group, O-
benzyl ether. The 1-, 2- and 6-O positions are masked in the form
of tricyclic orthobenzoate and orthopivalate in building blocks 1
and 2, respectively. During the ring-opening polymerization, the
3-O benzyl group has a considerable effect on chemical activity be-
cause the benzyl group is an electron-donating group. The electron
donating group is responsible for stabilizing the dioxolenium inter-
mediate, resulting in the formation of a glycosidic bond via SN2 at-
tack of the next monomer, after the activation by Lewis acid
initiators. A possible mechanism of cationic ring-opening polymer-
izations of mannosyl 1,2,6-tricyclic orthoester, that results in re-
gio- and stereo-controlled products, is summarized in Scheme 1.
The 4-O benzyl group plays an important role in the regioregu-
larity of the resulting polymer because it has a high electron-
donating property. The electron donating group contributes to
the high electron density of C-6 oxygen of the monomers. It has
been shown that when treated with BF3ꢀEt2O and allyl alcohol in
CH2Cl2, the C–O bond between the orthoester carbon and the C-6
oxygen atom is cleaved.4 The fact that the C-6 oxygen atom at-
taches to the C-6 methylene carbon, also makes the C-6 oxygen
atom less sterically hindered and facilitates the binding to a Lewis
acid, when compared with C-2 and C-1 oxygen atoms. Therefore,
we envision that during the initiation step of polymerization of
monomers 1 and 2 (Scheme 1), the Lewis acid will preferably coor-
dinate with the C-6 oxygen atom. The Lewis acid will weaken the
bond between the C-6 oxygen atom and the orthoester carbon.
Consequently, the C–O orthoester carbon–oxygen bonds will be
selectively cleaved at the C-6 oxygen atom. The cleavage may be
assisted by the delocalization of the lone pair electrons from both
C-1 and C-2 oxygen atoms. In a similar system to monomers 1 and
2, Kamitakahara et al. proposed that in the initiation step of cat-
tions of
D-mannopyranose tricyclic orthoesters for the synthesis of
a(1–6) mannopyranan—the backbone of LM and LAM.
2. Results and discussion
2.1. Design and synthesis of monosaccharide monomers
There are several reports on the synthesis of polysaccharides
by ring-opening polymerizations of various building blocks, for
example, 3-O-benzyl-b-
3,6-di-O-benzyl-
-glucopyranose 1,2,4-orthopivalate,26,27 3-O-
benzyl-6-O-pivaloyl-
-glucopyranose 1,2,4-orthopivalate,27 3-
O-benzyl-6-deoxy-
-glucopyranose 1,2,4-orthopivalate,28 and
3-O-benzyl-
-xylopyranose 1,2,4-orthopivalate.29 Hori and
Nakatsubo reported that the ring-opening polymerization of 3-
O-benzyl-b- -arabinofuranose 1,2,5-orthopivalate by using
BF3ꢀEt2O as catalyst gave the stereoregular polysaccharide
(1?5)-
-arabinofuranan with DPn = 91.25 The benzyl group at
L
-arabinofuranose 1,2,5-orthopivalate,25
a-D
a-D
a-D
a
-D
L
a
a
-L
the 3-O position and the pivaloyl group at the 2-O position are
necessary for stereoregularity and regioregularity in the synthesis
of arabinofuranan.25 Moreover, stereoregular polysaccharides of
glucose were synthesized by ring-opening polymerization of
3,6-di-O-benzyl-
benzyl-6-O-pivaloyl-
lar to the synthesis of arabinofuranan, the 3-O-benzyl group
and 2-O-pivaloyl group of glucopyranose play a significant role
in the stereospecificity and regiospecificity of the resulting poly-
mer.30,31 The resulting polymer contained only (1?4)-glycosidic
bonds, but not (1?2)-bonds.31
In addition to the substituents at 2-O and 3-O positions, the pro-
tecting group (R) of the –CH2OR at the C6-position affected the ste-
reo- and regio-regularity of the resulting polymer due to the
electronic effect of the protecting group.28 Therefore, the alkyl
group at the orthoester carbon should be an electron-donating or
a slightly withdrawing group such as pivaloyl group. Moreover,
types of initiators and temperatures also affect the regioregularity
of resulting polymers.25–27
a-
D-glucopyranose 1,2,4-orthopivalate or 3-O-
a-D
-glucopyranose 1,2,4-orthopivalate. Simi-
ionic ring-opening polymerization of
a-D-glucopyranose 1,2,4-
orthopivalate derivatives, the resulting dioxolenium ion is the
more likely intermediate.27
During the propagation step of polymerization (Scheme 1), the
next unit of monomer can approach the intermediate 4 by attack-
ing at the C-1 anomeric carbon. The most nucleophilic and most
accessible nucleophile is the C-6 oxygen atom on the incoming
monomer. Because of the unique geometry of the intermediate 4,
the next monomer will favorably attack at the bottom face of the
In brief, the previous studies of cationic ring-opening polymer-
izations of glycosyl tricyclic orthoesters suggested that the

3728
C. Yongyat et al. / Bioorg. Med. Chem. 18 (2010) 3726–3734
R
O
O
O
O
BnO
OBn
1
2
: R = C6H5
: R = C(CH3)3
Figure 2. The structures of tricyclic orthoester mannoside building blocks 1 and 2.
1.) Initiation
Lewis Acid
(LA)
R
R
O
LA
O
R
R
O
O
O
LAO
LAO
O
O
O
O
O
O
O
O
O
BnO
BnO
BnO
BnO
BnO
OBn
BnO
OBn
5
4
3
Selective Activation
on C-6OH
2.) Propagation
Incoming Monomer
R
O
O
O
R
OBn
OBn
O
O
O
O
R
LAO
LAO
O
O
BnO
BnO
O
O
O
R
BnO
BnO
LAO
BnO
BnO
O
Top face is not accessible.
O
4
O
R
O
R
R
O
O
O
BnO
BnO
O
OBn
OBn
O
O
O
O
O
R
R
O
BnO
BnO
n
O
O
OBn
OBn
O
O
O
O
O
Bottom face is accessible
for incoming monomer.
6
BnO
BnO
7
3.) Termination
O
O
O
O
R
LAO
BnO
BnO
O
R
LAO
BnO
BnO
O
O
O
O
O
R
O
BnO
BnO
O
-H +
O
R
R
O
BnO
BnO
n
O
O
O
n
O
O
BnO
BnO
O
H
O
R
O
(Nucleophile)
O
BnO
BnO
8
H
9
OH
Scheme 1. A possible mechanism of cationic ring-opening polymerizations of mannosyl 1,2,6-tricyclic orthoester that results in regio- and stereo-controlled products.
anomeric carbon. Consequently, the incoming nucleophilic oxygen
is likely to form an glycosidic bond with the anomeric carbon.
mers 1 and 2 was developed based on a previously published re-
port.32 We found that some chemical reagents used in the
previous report were not suitable for a high humidity climate in
a country like Thailand. We successfully develop a new synthetic
route that would not be affected by humid conditions. The overall
redesigned synthesis of monomers 1 and 2 is illustrated in
Scheme 2. We have developed a short synthetic route that requires
only six chemical transformations, in which only two column puri-
fications are necessary. Moreover, the same chemical conditions
can be applied to produce both building blocks 1 and 2 in multi-
ple-gram scales.
The synthetic routes for building blocks 1 and 2 are different
only in the first step of global protection of hydroxyl groups in
the native mannose. The native mannose sugar was globally pro-
tected with benzoate esters (Bz) and pivalate esters (Piv) for the
synthesis of building blocks 1 and 2, respectively.
a
Therefore, the stereospecificity in the polymannoside product is
achieved in concurrence with the reformation of the acyl protect-
ing group on the C-2 hydroxyl of the penultimate mannoside unit.
The acyl protecting group on the C-2 oxygen is orthogonal to the
permanent 3-O and 4-O benzyl ether protecting groups. This allows
for further selective mannosylations on the C-2 oxygen. The prop-
agation reactions will keep adding more mannose building blocks
on the growing chain of the polymer. And finally, the polymeriza-
tion will be terminated by attacks of nucleophiles other than the
monomer.
2.2. Synthesis of mannoside building blocks
With the ultimate goal of being able to prepare the building
blocks in large scale and a rapid fashion, we set out to develop a
synthetic route that is robust and scalable. The synthesis of mono-
We observed reactivity difference in the orthoester bicyclic for-
mation. The conversion yields from 12 to 14 and 13 to 15 were

C. Yongyat et al. / Bioorg. Med. Chem. 18 (2010) 3726–3734
3729
R
O
R1O
R1O
OR1
O
OR1
O
O
HO
HO
R1O
R1O
OH
O
R1O
R1O
a
b
c
O
O
R1O
R1O
HO
R1O
OH
OR1
Br
1
1
14: R1 = Bz, R = Ph
: R = Piv, R = -Bu
t
12
13
: R = Bz
: R = Piv
10
: R = Bz
D-mannose
1
11: R1 = Piv
15
1
d
R
O
R
O
R
O
O
O
O
O
HO
f
e
O
O
O
O
O
HO
HO
BnO
OBn
HO
OH
16: R = Ph
17: R = -Bu
18: R = Ph
19: R = -Bu
1: R = Ph
t
2
t
: R = -Bu
t
Scheme 2. Synthesis of building blocks 1 and 2. Reagents and conditions: (a) BzCl, pyridine, 0 °C to rt, 12 h or PivCl, pyridine, reflux, 24 h; (b) HBr/HOAc (33%), acetic
anhydride, 24 h (c) allyl alcohol, lutidine, 83% (three steps) for 14, 42% (three steps) for 15; (d) KOH, H2O, MeOH, THF, rt, 24 h; (e) 0.05 equiv CSA, CH2Cl2, 1 h; (f) BnBr, NaH,
DMF, 0 °C to rt, 12 h, 37% (three steps) for 1 and 56% (three steps) for 2.
monitored. We found that the conversion from 12 to 14 was signif-
icantly more effective than the conversion from 13 to 15. This dif-
ference highlights the influence of the substituent (R, Scheme 2) on
the orthoester carbocation stabilization. The presence of the phe-
nyl substituent on 12 resulted in the better conversion. Both phe-
nyl and t-butyl groups can donate electrons to stabilize the
carbocation. The phenyl group mainly donates electrons by delo-
calizing electrons from the aromatic ring to the benzylic carbocat-
ion. The t-butyl group inductively donates electrons to the
orthoester carbocation. In addition, the t-butyl substituent is con-
sidered to be bulkier; consequently, it is more difficult for the at-
tack of allyl alcohol. We also observed similar effects of the
substituent groups (R) during the polymerization reaction
(Scheme 1).
To globally remove the acyl protecting groups, which are ben-
zoyl and pivaloyl groups, we initially performed typical transeste-
rification reactions which relied on the basic sodium methoxide,
generated in situ from the reaction between solid sodium and
methanol. Under the transesterification conditions, benzoyl groups
were partially removed only at elevated temperatures. The
amounts of NaOMe used, from catalytic (5%) to stoichiometric at
reflux temperature (THF/MeOH ratio of 1:1), failed to remove the
acyl protecting groups. The starting material was gradually decom-
posed by basic and heated conditions. Even though, we attempted
to perform the reactions under inert atmosphere, we suspected
that NaOMe was degraded under the highly humid conditions in
Thailand.
ionic ring-opening polymerization. We have screened several
Lewis acids, including diethyl etherated boron trifluoride
(BF3ꢀEt2O) and triphenylmethyl tetrafluoroborate (Ph3CBF4), that
were successfully used to trigger cationic ring-opening polymeri-
zation of several glycosyl tricyclic orthoesters, as previously re-
ported.25,26 In addition, trimethylsilyl trifluoromethanesulfonate
(TMSOTf), a typical promoter for glycosylation reactions, was also
introduced as an initiator. The concentration of the catalyst was
kept constant at 5 mol % (0.05 equiv).
The polymerizations were carried out at different temperatures
ranging from ꢁ40 °C to room temperature (rt, 27 °C) to see the tem-
perature effects on the selectivity and yield of the polymer prod-
ucts. The reactions were mainly done in dichloromethane which
has proven to be the best solvent for all polymerization conditions
carried out in previous studies.26 In order to observe and account
for all the products resulting from polymerizations, the polymer
crude products were characterized by gel permeation chromatogra-
phy (GPC), 1H and 13C in 1D and 2D NMR spectroscopy, and optical
rotation. The representative results of polymerizations under dif-
ferent reaction conditions are highlighted in Table 1.
The Lewis acid catalysts, the temperatures, and the substituents
(R, Scheme 2) on the orthoester carbon strongly influenced the
yields, and the regio- and stereo-chemistry of the products from
polymerizations. The majority of the polymerization reactions
initiated by Ph3CBF4 and BF3ꢀEt2O as catalyst resulted in decompo-
sitions of the monomers (Table 1). In addition, there were large
We then turned to hydrolysis reactions to remove the acyl pro-
tecting groups. Moisture in the laboratory atmosphere was irrele-
vant during hydrolysis setup because water is one of the
reagents used in this reaction. The acyl groups of the bicyclic
orthoesters 14 and 15 were successfully removed after treatment
with KOH in H2O at rt. Compounds 16 and 17 were obtained in al-
most quantitative yield. After organic solvent/aqueous extraction,
compounds 16 and 17 were used in the next step without further
purifications.
Table 1
Overall results of polymerization reactions carried out at different experimental
conditions
Condition No. Monomer Catalyst
Temp (°C) Observations
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
BF3ꢀEt2O rt
Oligosaccharides
Oligosaccharides
BF3ꢀEt2O
0
BF3ꢀEt2O ꢁ10
BF3ꢀEt2O ꢁ40
Decomposed monomer
Oligosaccharides
Decomposed monomer
Oligosaccharides
Ph3CBF4
TMSOTf
TMSOTf
TMSOTf
rt
rt
The overall synthesis yields of building blocks 1 and 2 from D-
0
Oligosaccharides
mannose were 31% and 24%, respectively. The 1H and 13C NMR
spectra of compound 1 synthesized by the developed route are
similar to the previously published data.32 The monomer 2 was
characterized extensively by 1D and 2D NMR and the anomeric
doublet resonance of 2 was at 5.67 ppm (J = 5.8 Hz).
ꢁ40
Polysaccharides
Oligosaccharides
Oligosaccharides
BF3ꢀEt2O rt
BF3ꢀEt2O
0
BF3ꢀEt2O ꢁ10
Oligosaccharides
Oligosaccharides
Decomposed monomer
Decomposed monomer
Decomposed monomer
Oligosaccharides
BF3ꢀEt2O ꢁ40
Ph3CBF4
Ph3CBF4
Ph3CBF4
TMSOTf
TMSOTf
TMSOTf
rt
ꢁ10
ꢁ40
rt
2.3. Polymerization
0
Oligosaccharides
The monosaccharide monomers 1 and 2 were treated with a Le-
wis acid type initiator under inert atmosphere to set off the cat-
ꢁ40
Oligosaccharides

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C. Yongyat et al. / Bioorg. Med. Chem. 18 (2010) 3726–3734
portions of the monomers left when treated with Ph3CBF4. In a few
cases, oligosaccharides were observed along with decomposed
monomers. The BF3ꢀEt2O catalyst, when compared with TMSOTf,
was less effective especially when the orthopivalate 2 was used
as a monomer. The 1H and 13C NMR of the oligosaccharides isolated
from polymerizations with BF3ꢀEt2O mostly showed mixtures of
several products which are not useful for our goal of synthesizing
the regio- and stereo-controlled poly- or oligo-mannosides. The
better polymerization yields were obtained when TMSOTf was
used as a catalyst.
In general, TMSOTf gave larger polymer products with the de-
sired chemical selectivity than the polymerizations initiated by
BF3ꢀEt2O. The decompositions of monomers by treatment with
TMSOTf were less than when BF3ꢀEt2O was used. More importantly,
we observed better regio- and stereo-controlled products as well
as narrower molecular weight distributions in the case of TMSOTf.
In order to account for this difference, we used GAUSSIAN03 to per-
form quantum chemistry studies using the density functional the-
ory method at the B3LYP/6-311+g(d,p) level. The binding energies
between the Lewis acid catalysts and the monomer 1 at either C-6
oxygen or C-2 oxygen atoms were assessed. The calculations
showed that BF3ꢀEt2O indiscriminately binds to the monomer at
either C-6 oxygen or C-2 oxygen atoms. In contrast, TMSOTf selec-
tively activates the monomer at the C-6 oxygen atom.
monomer and small oligosaccharides. This result illustrates the
important role of the 4-O benzyl group as an electron donating
group to promote the polymer chain growth, as part of our mono-
mer design. The overall polymerization results can be distilled into
the proposed mechanism shown in Scheme 1.
The optimal polymerization conditions are listed in condition
No. 8 in Table 1. The orthobenzoate monomer 1 was efficiently
converted into the most homogeneous polymer products upon
activation with TMSOTf at ꢁ40 °C in CH2Cl2. The yield of the poly-
mer products from this condition was almost quantitative (98%).
The GPC chromatogram of the products from this reaction is shown
in Figure 4 as a dominant and narrow distribution curve. These
polymer products were extensively characterized and confirmed
to possess the desired regio- and stereo-chemistry (described in
detail in the next section). The high conversion yield, and the
highly homogenous polymer product make condition No. 8 a prime
method to access the
a(1–6) mannopyranan, the backbone of LM,
in substantial quantity.
2.4. Polymer characterizations
The homogeneity of polymerization products were verified by
gel permeation chromatography (GPC), and 1H and 13C NMR
(Figs. 3 and 4). Among the products obtained from polymerization
conditions listed in Table 1, the polymer products obtained from
condition No. 8 were the most chemically defined structures with
precise regio- and stereo-regulation. The 1H (600 MHz), 13C
(150 MHz) NMR spectra and GPC chromatogram of the product
from condition No. 8 are shown in Figures 3 and 4. The NMR
spectra show only one major peak of anomeric proton at
4.88 ppm, and of anomeric carbon at 98.65 ppm, which indicate
the high regio- and stereo-regularity of the polymer products.
In similar fashion to the anomeric proton, 1H NMR shows distinct
peaks of the ring protons of the chemical shift from 5.8 to
3.2 ppm.
The regioselectivity of the 1?6 glycosidic bonds is confirmed by
1H NMR of C-2 proton at the lower field peak of 5.68 ppm. This C-2
proton resonance shows up as a distinct major peak. The integrated
area is equal to the anomeric peak, which indicates one proton
each. The benzoyl ester protecting group on the C-2 hydroxyl
group withdraws electrons from the C-2 carbon and thus, sends
the C-2 proton downfield. If the polymer were to consist of the
other possible linkage of 1?2 glycosidic bond, there would not
be a downfield peak with integration equal to one proton.
The initial reaction temperatures affected the molecular weight
distributions of the polymer products (Table 2). We found that the
molecular weight distributions were narrower if the polymeriza-
tion reactions were carried out at lower temperatures (ꢁ40 °C or
ꢁ10 °C). Higher temperature can also increase the probability of
attaining the undesired regioisomers. Therefore, for the purpose
of our synthesis of preparing chemically well-defined structures,
ꢁ40 °C was the optimal temperature.
Polymerizations of the monomer 1, with the phenyl substituent
(R, Scheme 2) on the orthoester carbon, gave more uniform poly-
mer products with the desired regio- and stereo-selectivity (Ta-
ble 2). We observed that most of the oligomers and polymers,
resulting from the polymerizations with the monomer 2, were
more heterogeneous. For example the major product of reaction
condition No. 17 (Table 2) was polymannosides with ꢂ16 repeat-
ing units, but the polydispersity of this product was 2.09, which
implied that the products contain polymers in many different
sizes. In a separate study (data not shown), if the protecting group
on C-4 oxygen of the monomer is an electron withdrawing group
such as benzoyl (OBz), the polymerization resulted in decomposed
Table 2
GPC analysis of polymerization products obtained from reaction conditions listed in Table 1
c
Condition No.a
All productsb
Major product
d
Mw
Mn
DPn
Mw/Mn
Mw
Mn
DPn
Mw/Mn
% Yield
1
2
3
4
5
6
7
8
9
10
11
12
16
17
18
2566
1282
270
2140
978
9878
6767
11,580
2371
723
1077
1573
7178
12,514
4849
1339
998
267
1612
198
2687
2756
5417
950
531
545
1090
2061
2125
2410
3.0
2.2
0.6
3.6
0.4
6.0
6.2
12.1
2.2
1.2
1.3
2.6
4.8
5.0
5.7
1.92
1.28
1.01
1.33
4.94
3.68
2.46
2.14
2.50
1.36
1.98
1.44
3.48
5.89
2.01
3378
1491
2278
1370
5.1
3.1
1.48
1.09
92
89
Not analyzed (decomposed monomer)
2249
Not analyzed (decomposed monomer)
1919
4.3
1.17
96
10,520
6973
11,845
3012
866
1383
1726
7771
13,984
5089
5102
3707
8809
1962
812
1160
1457
3932
6700
3643
11.4
8.3
19.7
4.6
1.9
2.7
3.4
9.2
15.7
8.5
2.06
1.88
1.34
1.54
1.07
1.19
1.18
1.98
2.09
1.40
94
97
98
76
74
72
93
92
89
95
a
b
c
Reaction conditions are listed in Table 1 by condition No.
The reaction mixtures were directly analyzed without extraction or precipitation.
Separated by GPC.
d
Mw = weight average molecular weight; Mn = number average molecular weight; DPn = degree of polymerization; Mw/Mn = polydispersity index.

C. Yongyat et al. / Bioorg. Med. Chem. 18 (2010) 3726–3734
3731
The stereoselectivity of the
a glycosidic bonds is confirmed by
the value of NMR J coupling constant between the anomeric carbon
and the anomeric proton (JC1,H1). The typical JC1,H1 values of
a man-
noside is 171 Hz and b mannoside is lower at 159 Hz.33 The C–H
NMR coupled spectra of the polymer products from condition No.
8 is shown as 1-D spectra in Figure 3C. The JC1,H1 of the polymers
is 170 Hz, which closely matches the typical JC1,H1 of
a glycosidic
bonds in mannosides. If the polymer were to consist of other pos-
Figure 4. GPC chromatogram of polymer products generated in condition No. 8 of
Table 1.
sible linkages of b glycosidic bonds, the JC1,H1 would be signifi-
cantly lower.
The GPC chromatogram (Fig. 4) of major products from poly-
merization condition No. 8 (Table 1) shows a narrow molecular
weight distribution in a single peak. The polydispersity index of
the polymer products is 1.34, which signifies a uniform molecular
weight distribution. Since the products are very similar in the
number of repeating units, they will be useful as the backbone
polysaccharides for further
a
mannosylation at the C-2 hydroxyl
(1–6) mannopyranan is the core
groups. The C-2 mannosylated
a
a
component of the LM, LAM, and ManLAM.
The optical activity of the polymer obtained from condition No.
8 is highly positive with an alpha rotation of +54.40° (c 0.1, 1 mg/
mL in CHCl3). The melting point range of this polymer is narrow,
from 100.3 °C to 106.8 °C. Both high optical activity and a narrow
range of melting point further support the chemical selectivity
found in the polymer products.
3. Conclusion
The building blocks 3,4-O-benzyl-b-
D
-mannopyranose 1,2,6-
orthobenzoate (1) and 3,4-O-benzyl-b-
D-mannopyranose 1,2,6-
orthopivalate (2) were synthesized. Compounds 1 and 2 were uti-
lized as monomers in ring-opening polymerizations, for rapid syn-
theses of
a(1–6) mannopyranan, the backbone of lipomannan
(LM). LM is among important interspersed molecules that compose
the unique envelope of Mtb.
The synthetic protocols of both building blocks were developed
to be robust and scalable to prepare the monomers in a large scale
and a rapid fashion. The polymerization reactions can be carried
out with a simple basic organic synthesis set up. Tricyclic orthoes-
ters 1 and 2 were successfully prepared in six steps with multiple-
gram scale and high yielding chemical reactions. The new synthetic
routes are not affected by the highly humid conditions in Thailand.
The same reaction conditions were applied to produce both build-
ing blocks 1 and 2.
The polymerizations of monomers 1 and 2 were optimized at dif-
ferent conditions. The regio- and stereo-selectivity along with the
conversion efficiency were optimal when TMSOTf was used as a cat-
alyst with building block 1 as a monomer. Characterizations by gel
permeation chromatography, 1D and 2D NMR spectroscopy, and
optical rotation indicate the identity of
a(1?6) glycosidic bonds
present in the polymer products. The (1–6) mannopyranan prod-
a
uct serves as the backbone of the synthesis of lipomannan, which
is part of the envelope of major causative agent of tuberculosis.
4. Experimental sections
4.1. General information
All chemicals used were reagent grade and used as supplied ex-
cept where noted. All reactions were performed in oven-dried
glassware under an inert atmosphere unless noted otherwise.
Figure 3. NMR spectra of polymer products generated in condition no. 8 of Table 1:
(A) 1H NMR, (B) 13C NMR, (C) 13C–1H coupled 13C NMR, and (D) HMQC 2D NMR.

3732
C. Yongyat et al. / Bioorg. Med. Chem. 18 (2010) 3726–3734
Dichloromethane (CH2Cl2) was dried over calcium hydride (CaH2)
prior to use. Lutidine was treated by potassium hydroxide (KOH)
and allyl alcohol was treated with potassium carbonate (K2CO3)
prior to use. The monomers used for polymerizations were dried
by using a kügelrohr apparatus (Büchi GKR-51). Analytical thin
layer chromatography (TLC) was performed on Merck Silica Gel
60 F254 plates (0.25 mm). Compounds were visualized by staining
with cerium sulfate-ammonium molybdate (CAM) solution or
phosphomolybdic acid (PMA) solution. Flash column chromatogra-
phy was carried out using forced flow of the indicated solvent on
Fluka Kieselgel 60 (230–400 mesh).
HCl. The residual solid was extracted by H2O/CH2Cl2 and 1 N HCl/
CH2Cl2. The combined organic layer was washed with saturated
NaHCO3 solution (3ꢃ), dried over Na2SO4(s), concentrated in vacuo
and placed under high vacuum for at least 1 h. Rf 0.62 (hexanes/
EtOAc = 4:1); ½a rDt
ꢄ
= +35.1909 (c 1.1 mg/ml, CHCl3); mp = 147.2–
151.8 °C; 1H NMR (200 MHz, CDCl3) d 1.08–1.34 (m, 45H), 3.78–
3.90 (m, 1H), 4.10–4.27 (m, 2H), 5.11–5.21 (m, 1H), 5.40–5.55
(m, 2H), 5.82 (d, J = 1.0 Hz, 1H); 13C NMR (50 MHz, CDCl3) d
26.49, 27.04, 38.78, 39.12, 61.67, 64.49, 68.20, 69.35, 71.17,
90.72, 175.53, 176.56, 176.78, 177.35, 177.99; HRMS-ESI (m/z):
[M+Na]+ calculated for C31H52O11Na, 623.3402; Found: 623.3411.
All new compounds were characterized by NMR spectroscopy
(1H, 13C NMR and 2D NMR for some key intermediates), high reso-
lution mass spectrometry (HRMS), optical rotation activity, and
melting point. NMR spectra were recorded on a Varian Gemini
2000 (200 MHz), Bruker AVANCE 400 (400 MHz), and Bruker
AVANCE 600 (600 MHz), in CDCl3 with chemical shift reference
to internal standards CDCl3 (7.26 ppm for 1H and 77.0 ppm for
13C). Splitting patterns are indicated as s, singlet; br s, broad sin-
glet; d, doublet; dd, doublet of doublet; m, multiplet for 1H NMR
data. NMR chemical shifts (d) are reported in ppm and coupling
constants (J) are reported in Hz. High resolution mass spectral
(HRMS) analyses were performed by the MS-service at CRI. Peaks
are reported as m/z. Optical rotations were measured at 26 °C using
a JASCO P-1020 polarimeter. Melting points were measured in cap-
illaries on a BÜCHI apparatus (Model BÜCHI 535). Average molec-
ular weights (MW and MN) of the polymer were analyzed by gel
permeation chromatography (GPC/Agilent) using HPLC grade chlo-
roform as an eluent. The eluent flow rate was kept constant at
0.5 ml/min. The temperature of the column was maintained at
40 °C while the detector was maintained at 35 °C. Calibration
curves were generated by using polystyrene standards (Shodex
standard) with molecular weights of 3.9 ꢃ 106, 6.29 ꢃ 105,
6.59 ꢃ 104, 9.68 ꢃ 103, and 1.30 ꢃ 103 g/mol. The samples were
dissolved and diluted with chloroform (0.5 mg/ml) and filtered be-
fore injection. The GPC analysis system was equipped with a uni-
versal styrene-divinylbenzene copolymer column (PLgel Mixed-C,
4.1.3. 3,4,6-tri-O-Benzoyl-a-D-mannopyranose 1,2-orthobenzo-
ate (14) and 3,4,6-tri-O-pivaloyl-a-D-mannopyranose 1,2-ortho-
pivalate (15)
Without further purifications, compounds 10 and 11 were trea-
ted with acetic anhydride (21 ml, 221 mmol, 8 equiv) and a solu-
tion of 33% HBr in AcOH (142 ml, 828 mmol, 30 equiv). The
reaction mixture was allowed to stir at rt for 24 h and extracted
with ice-cold water and CH2Cl2 (3ꢃ). The combined organic layer
was dried over Na2SO4(s) and concentrated in vacuo. The crude
product obtained from this step was further dried under high vac-
uum for at least 4 h, without purifications, and was treated with
2,6-lutidine (20 ml, 166 mmol, 6 equiv) and allyl alcohol (57 ml,
828 mmol, 30 equiv). The reaction mixture was stirred at rt for
20 h and concentrated in vacuo. The crude product was co-evapo-
rated with toluene (3ꢃ), dried under high vacuum, and washed
with water (3ꢃ). The combined organic layer was dried over Na2-
SO4(s), and concentrated in vacuo. The crude product was purified
by flash silica column chromatography (hexanes/EtOAc) to obtain
bicyclic orthoesters 14 (14 g, 83%, three steps) and 15 (7 g, 42%,
three steps) as foamy syrups. The side products obtained from this
reaction were intermediates of compounds 12 and 13. They each
have an OH group at the anomeric carbon. These compounds can
be converted to 12 and 13 via reaction condition b (Scheme 2).
NMR spectra for compound 14 are the same as reported in the lit-
erature.32 Characterizations of compound 15 are as the following:
300 ꢃ 7.5 mm, 5
LENT/RI-G1362A), an online degasser (AGILENT/G1322A), an auto-
sampler (AGILENT/G1329A), thermostatted column
lm), a differential refractometer detector (AGI-
Rf 0.62 (hexanes/EtOAc = 7:3);
½ ꢄ = ꢁ31.1048 (c 1.0 mg/ml,
a rDt
CHCl3); mp = 107.2–110.0 °C; 1H NMR (400 MHz, CDCl3) d 1.05–
1.20 (m, 36H), 3.61–3.66 (m, 1H), 3.91–4.10 (m, 3H), 4.20–4.26
(m, 1H), 4.75 (dd, J = 5.2, 3.0 Hz, 1H), 4.95–5.0 (m, 1H), 5.06–5.12
(m, 1H), 5.21–5.28 (m, 1H), 5.44–5.52 (m, 2H), 5.77–5.88 (m,
1H); 13C NMR (80 MHz, CDCl3) d 25.68, 26.93, 27.02, 27.09,
38.70, 38.76, 38.86, 39.46, 61.66, 63.91, 64.38, 70.52, 71.77,
75.69, 97.40, 115.49, 127.84, 134.37, 176.16, 177.89, 178.22;
HRMS-ESI (m/z): [M+Na]+ calculated for C29H48O10Na, 579.3140;
Found: 579.3140.
a
compartment (AGILENT/G1316A) and a quaternary pump (AGI-
LENT/G1311A).
4.1.1. 1,2,3,4,6-Penta-O-benzoyl-D-mannose (10)
The first step is global protection of hydroxyl groups in the na-
tive mannose. First, the native mannose sugar (5 g, 28 mmol) was
globally protected with benzoate esters (Bz). Benzyl chloride
(42 ml, 331 mmol, 12 equiv) was added dropwise to the sugar
starting material suspended in pyridine (101 ml, 1.24 mol,
45 equiv) at 4 °C. The reaction mixture was allowed to stir for
12 h at rt. The reaction mixture was filtered to remove suspended
solid. The filtrate was concentrated in vacuo and extracted by
EtOAc and 1 N HCl. The residual solid was extracted by H2O/CH2Cl2
and 1 N HCl/CH2Cl2. The combined organic layer was washed with
saturated NaHCO3 solution (3ꢃ), dried over Na2SO4(s), concen-
trated in vacuo and placed under high vacuum for at least 1 h.
NMR spectra for compound 10 are the same as reported in the
literature.32
4.1.4. a-D-Mannopyranose 1,2,6-orthoesters (18 and 19)
To bicyclic orthoesters 14 and 15, THF (339 ml for 14, 154 mL
for 15), MeOH (678 ml for 14, 308 mL for 15) and H2O (28 ml for
14, 13 mL for 15) in the ratio of 12:24:1 were added at rt. KOH
(5 g, 89 mmol for 14, 3 g, 53 mmol for 15, 4 equiv) in H2O solution
was prepared and added to the reaction mixture. After stirring for
24 h at rt, the resulting mixture was extracted with CH2Cl2 and
water in a separatory funnel (3ꢃ), dried over Na2SO4(s), concen-
trated in vacuo, and dried under high vacuum. Compounds 16
and 17 were obtained in quantitative yield without further purifi-
cations. To obtain tricyclic orthoesters 18 and 19, CSA (266 mg,
1.2 mmol for 16, 135 mg, 0.6 mmol for 17, 0.05 equiv) was added
to a solution of compounds 16 and 17 in CH2Cl2 (687 ml for 16,
348 mL for 17). The reaction mixture was stirred at rt for 1–2 h
and quenched by the addition of triethylamine (0.24 ml, 1.7 mmol
for 16, 0.12 mL, 0.9 mmol for 17, 0.075 equiv). The crude product
was concentrated in vacuo and placed under high vacuum for at
least 1 h to give compounds 18 and 19 as white solids. NMR
4.1.2. 1,2,3,4,6-Penta-O-pivaloyl-D-mannose (11)
To a solution of native mannose sugar (5 g, 28 mmol) in pyri-
dine (101 ml, 1.24 mol, 45 equiv), pivaloyl chloride (41 ml,
331 mmol, 12 equiv) was added dropwise at 4 °C. The reaction
mixture was allowed to stir and refluxed for 24 h to yield 11. The
reaction mixture was filtered to remove suspended solid. The fil-
trate was concentrated in vacuo and extracted by EtOAc and 1 N

C. Yongyat et al. / Bioorg. Med. Chem. 18 (2010) 3726–3734
3733
spectra for compound 18 are the same as reported in the litera-
ture.32 Characterizations of compound 19 are as the following: Rf
(50 MHz, CDCl3) d 65.91, 68.58, 71.10, 71.46, 73.89, 75.17, 78.35,
98.69 (br s), 128.41, 130.17, 137.70, 138.64, 165.68.
0.55 (CH2Cl2/MeOH = 9:1); ½a Drt
ꢄ
= +5.2588 (c 0.85 mg/ml, CHCl3);
mp = 131.0–131.6 °C; 1H NMR (200 MHz, CDCl3) d 1.001 (s, 9H),
3.67 (dd, J = 8.0, 2.2 Hz, 1H), 3.85–4.10 (m, 4H), 4.36 (dd, J = 6.0,
2.2 Hz, 1H), 5.76 (d, J = 5.8 Hz, 1H); 13C NMR (50 MHz, CDCl3) d
24.72, 27.18, 38.26, 69.80, 70.53, 70.59, 72.02, 76.42, 80.18,
99.67; HRMS-ESI (m/z): [M+Na]+ calculated for C11H18O6Na,
269.0996; Found: 269.0996.
4.2.3. Product from condition 8 in Table 1
½
a rDt = +54.40 (c 0.1, 1 mg/ml, CHCl3); mp = 100.3–106.8 °C; 1H
ꢄ
NMR (600 MHz, CDCl3) d 3.23–4.00 (m, 5H), 4.10–4.45 (m, 2H),
4.53–4.76 (m, 2H), 4.88 (s, 1H), 5.68 (br s, 1H), 6.87–7.15 (m,
10H), 7.20–7.50 (m, 3H), 7.88–8.05 (m, 2H); 13C NMR (150 MHz,
CDCl3) d 65.86, 68.53, 71.06, 71.30, 71.39, 73.86, 75.09, 75.24,
78.27, 98.65, 127.17, 127.22, 127.40, 127.44, 127.47, 127.52,
127.65, 127.69, 127.74, 127.83, 127.90, 128.08, 128.80, 128.15,
128.21, 128.23, 128.26, 128.31, 128.32, 128.38, 128.40, 128.45,
128.48, 128.56, 128.59, 128.62, 128.65, 128.72, 129.89, 129.94,
129.97, 130.06, 130.09, 130.13, 133.36, 137.64, 138.57, 165.63.
4.1.5. 3,4-O-Benzyl-a-D-mannopyranose 1,2,6-orthoesters (1
and 2)
To a solution of tricyclic orthoesters 18 and 19 in DMF (183 ml
for 18, 93 mL for 19), BnBr (14 ml, 115 mmol for 18, 7 mL, 58 mmol
for 19, 5 equiv) and NaH (8 g, 115 mmol for 18, 4 g, 58 mmol for 19,
5 equiv) were added at 0 °C. The reaction mixture was gradually
warmed up to rt during 12 h. The reaction mixture was extracted
with Et2O and water (3ꢃ), washed with brine, and dried over anhy-
drous Na2SO4(s). The combined organic layer was concentrated in
vacuo, and purified by flash silica column chromatography (hex-
anes/EtOAc) to obtain compounds 1 (3.8 g, 37%, three steps) and
2 (2.8 g, 56%, three steps) as white solids. NMR spectra for com-
pound 1 are the same as reported in the literature.32 Characteriza-
tions of compound 1 are as the following: Rf 0.55 (hexanes/
4.2.4. Product from condition 12 in Table 1
½
a rDt = +24.10 (c 1.0, 10 mg/ml, CHCl3); mp = 56.8–68.1 °C; 1H
ꢄ
NMR (600 MHz, CDCl3) d 1.12–1.28 (s, 9H), 3.28–4.12 (m, 5H),
4.30–5.02 (m, 5H), 5.31–5.52 (m, 1H), 7.10–7.40 (s, 10H); 13C
NMR (50 MHz, CDCl3) d 27.33, 39.05, 62.18, 65.73, 67.92, 69.86,
70.95, 71.50, 71.90, 73.81, 74.20, 75.17, 78.18, 78.79, 52.60, 98.06
(br s), 128.35, 138.10, 177.50, 177.74.
4.2.5. Product from condition 16 in Table 1
EtOAc = 4:1);
½
a rDt
ꢄ
= ꢁ1.4133 (c 1.5 mg/ml, CHCl3); mp = 83.2–
½
a rDt = +40.19 (c 1.0, 10 mg/ml, CHCl3); mp = 63.3–80.4 °C; 1H
ꢄ
84.0 °C; 1H NMR (400 MHz, CDCl3) d 1.02–1.05 (m, 9H), 3.64–
3.71 (m, 2H), 3.92–3.98 (m, 1H), 4.06–4.11 (m, 2H), 4.36–4.40
(m, 1H), 4.62 (d, J = 11.6 Hz, 1H), 4.76 (d, J = 11.6 Hz, 1H), 4.823
(s, 2H), 5.67 (d, J = 5.8 Hz, 1H) 7.26–7.44 (m, 10H); 13C NMR
(80 MHz, CDCl3) d 24.67, 38.17, 70.12, 72.22, 72.91, 74.28, 77.52,
79.18, 99.31, 126.76, 127.70, 127.76, 127.90, 128.37, 128.43,
138.21; HRMS-ESI (m/z): [M+Na]+ calculated for C25H30O6Na,
449.1935; Found: 449.1940.
NMR (600 MHz, CDCl3) d 1.08–1.33 (m, 9H), 3.15–4.13 (m, 5H),
4.18–5.05 (m, 6H), 5.32–5.58 (br s, 1H), 6.95–7.41 (m, 10H); 13C
NMR (50 MHz, CDCl3) d 27.22, 38.91, 65.53, 67.44, 70.60, 71.26,
73.60, 74.85, 78.97, 83.16, 98.25 (br s), 128.18, 137.83, 138.50,
177.14.
Acknowledgments
This research was supported by Thammasat University Re-
search Fund and Thailand Research Fund. Ms. Moragot Sungsilp
has contributed to this project, as an undergraduate senior project
at Sirindhorn International Institute of Technology, Thammasat
University. C. Yongyat and M. Sungsilp thank the National Science
and Technology Development Agency (NSTDA) for the undergrad-
uate scholarships (SIIT-NSTDA: S1Y48/F-002) from the Young Sci-
entist and Technologist Program (YSTP). We thank Chulabhorn
Research Institute (CRI) for chemical reagents and equipment.
We thank Mr. Paul Vincent Neilson for editing this manuscript. A
patent has been filed for the chemical procedures and compounds
reported in this paper.
4.2. Polymerizations
A monomer was dried in Kügelrohr under high vacuum at 90 °C
for 24 h. Dichloromethane was distilled from CaH2. The solvent
was transferred under inert atmosphere. To a solution of monomer
in the solvent at the desired temperature (0 °C by ice-water bath,
ꢁ10 °C by ice-acetone bath, and ꢁ40 °C by dry ice-acetonitrile
bath),
a
catalytic amount of initiator (BF3ꢀEt2O, Ph3CBF4, or
TMSOTf) was added and kept at the desired temperature for 2 h.
The reaction mixture was kept at nearly the desired temperature
in the refrigerator or freezer for 12 h. The reaction mixture was di-
luted with CH2Cl2, washed with saturated aqueous NaHCO3 (3ꢃ),
dried over anhydrous Na2SO4(s), and concentrated in vacuo. The
crude product was dissolved in a minimum amount of CH2Cl2
and added pentane until there was a white precipitate. The precip-
itate was allowed to settle overnight. After the mother liquor was
removed, the precipitated polymer was dried under high vacuum
to give a white powder.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmc.2010.04.014.
References and notes
4.2.1. Product from condition 6 in Table 1
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5. Sohail, M. J. Mol. Genet. Med. 2006, 2, 87.
½
a rDt = +56.78 (c 0.1, 1 mg/ml, CHCl3); mp = 105.0–111.8 °C; 1H
ꢄ
NMR (200 MHz, CDCl3) d 3.30–4.22 (m, 6H), 4.24–4.67 (m, 3H),
4.70–5.20 (m, 3H), 5.73–5.88 (m, 1H), 6.83–7.67 (m, 17H), 7.92–
8.30 (m, 2H); 13C NMR (50 MHz, CDCl3) d 68.56, 71.07, 71.41,
73.87, 75.11, 78.29, 98.65 (br s), 127.16, 127.42, 127.72, 127.93,
128.223, 128.40, 128.74, 129.93, 130.05, 133.37, 137.62, 138.55,
165.58.
6. Andersen, P. Nat. Rev. Microbiol. 2007, 5, 484.
7. Tiruviluamala, P.; Reichman, L. B. Annu. Rev. Public Health 2002, 23, 403.
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