Total synthesis of phosphatidylinositol dimannoside: A cell-envelope component of mycobacterium tuberculosis

Article abstract of DOI:10.1002/chem.200802189

Synthesis of phosphatidylinositol mannosides (PIM₂), a cell-envelope component of mycobacterium tuberculosis, toward the preparation of higher PIMs, lipoarabinomannan (LM), and lipomannan(LAM), by employing the myo-inositol-derived meso-4,6-dio

Full text of DOI:10.1002/chem.200802189

COMMUNICATION
DOI: 10.1002/chem.200802189
Total Synthesis of Phosphatidylinositol Dimannoside: A Cell-Envelope
Component of Mycobacterium ACHTNUTRGNEtNUG uberculosis
Pratap S. Patil[a] and Shang-Cheng Hung*[a, b]
Dedicated to Professor Chun-Chen Liao on the occasion of his 65th birthday
Mycobacterium tuberculosis, a gram-positive bacteria, has
priate protecting groups are, however, obtained by multi-
step routes from d-glucose through Ferrier rearrangement[10]
or by resolution from myo-inositol by using chiral auxiliaries
in a 1:1 ratio.[11] To tackle these problems, we report herein
a straightforward synthesis of PIM2 1, a basic molecule
toward the preparation of higher PIMs, LM, and LAM, em-
ploying direct 6-O-mannosylation of the myo-inositol-de-
rived meso-4,6-diol as a key step.
The synthetic challenges of PIM2 1 include the efficient
generation of chiral myo-inositol-derived compounds, the
highly regioselective distinction of C-1, C-2, and C-6 hydrox-
yl groups from the others, and the installation of two mann-
a strong ability to infect mammalian cells and survive in
host tissues for a long-term period. Among the two billion
people currently infected, 5–10% of individuals are estimat-
ed to develop the active disease, tuberculosis (TB).[1] TB has
become a major public health problem due to the emer-
gence of drug-resistant strains and the combination of TB
and HIV infection.[2] The development of new anti-TB drugs
or vaccines is indeed an urgent issue.[3]
The mycobacterial cell envelope possesses three structural
components: plasma membrane, wall, and outer layer or
capsule.[4] The plasma membrane contains a class of phos-
pholipids (Figure 1), including the di- (R, R1), tri- (R, R1,
R2), and tetra-O-acylated (R, R1, R2, R3) phosphatidylinosi-
tol mannosides (PIMs) and lipoarabinomannan (LAM) con-
sisting of a lipomannan (LM) core. PIMs and LAM play an
important role in the integrity and survival of the pathogen
by associating with many immunomodulatory events occur-
ring in the progression of disease, including suppression of
immunity, neutralization of potentially cytotoxic O2 free
radicals, induction of cytokines, phagocytosis of the organ-
ism by binding with the mannose receptor, and growth of
the organism in the host macrophage.[5] Due to their struc-
tural complexity and critical roles in TB studies, some
AHCTUNGTREGoNNUN syl units and a lipid chain at the desired positions. Our ret-
rosynthetic plan for PIM2 1 is illustrated in Scheme 1. Cou-
pling of the 1-alcohol 2 with the H-phosphonate 3[8] fol-
lowed by removal of the permanent protecting groups (PG)
[6]
groups have reported the preparation of PIM2 and its ana-
logues,[7] PIM6,[8] as well as LM components.[9] In these strat-
egies, the optically pure myo-inositol derivatives with appro-
[a] P. S. Patil, S.-C. Hung
Department of Chemistry, National Tsing Hua University
101, Section 2, Kuang-Fu Road, Hsinchu 300 (Taiwan)
Fax : (+886)3-5711082
E-mail: hung@mx.nthu.edu.tw
[b] S.-C. Hung
Genomics Research Center
Academia Sinica, 128, Section 2, Academia Road
Taipei 115 (Taiwan)
Figure 1. Structures of phosphatidylinositol mannosides (PIMs), lipo-
mannan, and lipoarabinomannan (R, R1: palmitic acid, stearic acid or tu-
berculostearic acid; R2, R3: H, palmitic acid or tuberculostearic acid).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200802189.
Chem. Eur. J. 2009, 15, 1091 – 1094
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N-benzoyloxy benzotriazole (BzOBT) afforded the corre-
sponding 4,6-diol 10 in an improved yield (72%, lit. [13]
60%). Table 1 outlines the conditions and results for the re-
gioselective and stereoselective coupling reactions of the
meso-4,6-diol 10 with
a variety of d-mannose-derived
donors 11–15 in THF. Four possible diastereoisomers 16–19
were separated by column chromatography on silica gel. In
entries 1–6, BF3·OEt2-catalyzed coupling of 9 with imidate
11[14] was tested at different temperatures, and the best yield
of the desired 6-O-a-mannosylated 4-OH 16 (64%) was ob-
tained when the reaction was initially carried out at ꢀ788C
and then gradually warmed up to ꢀ208C. Under these con-
ditions, the other regioisomer 18 was isolated in 12% yield
(Table 1, entry 6). Changing the promoter to TMSOTf
(Table 1, entry 7) and AgOTf (Table 1, entry 8) furnished 16
in 46 and 20% yield, respectively. In the case of glycosyl
phosphate 12 (Table 1, entry 9), only its hydrolyzed deriva-
tive 15 was found, without generating any of the expected
products. When thioglycoside 13 (Table 1, entry 10), glycosyl
fluoride 14 (Table 1, entry 11), and lactol 15 (Table 1,
entry 12) were used as donors, the alcohol 16 was obtained
in 42, 21, and 22% yield, respectively.
Table 1. Regioselective and stereoselective coupling of the myo-inositol-
derived 4,6-diol 10 with various d-mannopyranosyl donors.
Scheme 1. Retrosynthetic design of phosphatidylinositol dimannoside
(PIM2). PG: permanent protecting group; PG1: temporary protecting
group; LG: leaving group.
would give the target molecule 1. Conceptually, alcohol 2
could be obtained via regioselective protection of the
1,3,4,5-tetraol 4 at the O-3, O-4, and O-5 positions since the
C-1 hydroxy group would be adjacent to both d-mannosyl
rings and the axial pyranosyl ring at O-2 would orient
toward the same b-face. These steric effects could cause a
more hindered environment for the C-1 hydroxy group than
other three hydroxyls. Tetraol 4 would be yielded by hydro-
Entry Donor Promoter
T [8C]
Yield [%]
16 17 18 19
ACHTUNGTRENNUNGlyACHTUNGTRENNUNGsis of the orthoformate 5 under mild acidic conditions. A
1
2
3
4
5
6
7
8
11
11
11
11
11
11
11
11
12
13
14
15
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
TMSOTf
AgOTf
0
51
42
45
56
47
64
46
20
0
0
4
9
5
5
0
0
0
0
0
8
19
23
18 14
21 10
14
12
15
10
0
16
19
0
0
8
temporary protecting group (PG1) would be needed to mask
the O-2 position of the inositol unit in 6 that could be con-
secutively cleaved and d-mannosylated to furnish the 4-alco-
hol 5. The preparation of 6 in an optically pure form would
be carried out via diastereoselective 6-O-glycosidation of
the 2,3,4,6-tetra-O-protected d-mannosyl donor 7 with the
2-O-protected myo-inositol-derived 4,6-diol 8. Conceptually,
the chiral sugar 7 could be used for the desymmetrization of
the meso-compound 8, which is different from the resolution
method employing a racemic mixture of d-glucosamine de-
rivatives.[12] This direct coupling could afford the desired di-
ꢀ20
ꢀ30
ꢀ40
ꢀ60
6
0
0
0
0
0
7
0
ꢀ78!ꢀ20
ꢀ78!ꢀ20
ꢀ78!ꢀ20
ꢀ78!ꢀ20
ꢀ78!ꢀ20
9
TMSOTf
TESOTf, NIS
10
11
12
42
Cp2HfCl2, AgOTf ꢀ78!ꢀ20[a] 21
DMS, Tf2O
0!RT
22 18
[a] A mixed solvent of THF/CH2Cl2 1:5 was used.
ACHTUNGTRENNUNGsacACHTUNGTRENNUNGcharide moiety and offer an opportunity for the synthesis
of PIM2 in an efficient manner.
To address our approach, the benzoyl group was selected
as a temporary protecting group in the study. Regioselective
benzoylation at the C-2 equatorial hydroxy group of com-
mercially available myo-inositol 1,3,5-orthoformate 9 with
The structural determination of four diastereoisomers 16–
19 was a challenging task since no single crystal for X-ray
diffraction analysis could be obtained. A chemical correla-
tion method, as illustrated in Scheme 2, was utilized as the
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Chem. Eur. J. 2009, 15, 1091 – 1094

Phosphatidylinositol Dimannoside
COMMUNICATION
Scheme 3. a) BF3·OEt2, 11, CH2Cl2, ꢀ608C, 3 h, 87%; b) p-TSA, CH2Cl2/
MeOH 1:1, RT, 20 h, 99%; c) TMSCl, Et3N, CH2Cl2, 08C!RT, 36 h,
quant; d) TMSOTf, PhCHO, Et3SiH, 3 ꢁ MS, CH2Cl2, ꢀ40!ꢀ108C,
28 h, 80%; e) 3, PivCl, pyridine, RT, 4 h, then I2, pyridine/H2O 50:1, RT,
3 h, 82%; f) 10% Pd/C, 60 psi H2, EtOAc/THF/1-PrOH/H2O 2:1:1:1, RT,
36 h, 52%. p-TSA: p-toluenesulfonic acid monohydrate; TMSCl: chloro-
trimethylsilane; PivCl: pivaloyl chloride.
Scheme 2. a) (S)-2-Acetyloxy-2-phenylacetyl chloride, pyridine/CH2Cl2
1:1, 08C!RT, 10 h, 20: 15%, 21: 39%; b) 1) TMSOTf, 11, 3 ꢁ MS,
CH2Cl2/CH3CN 3:1, ꢀ78!ꢀ208C, 3 h; 2) NaOMe, MeOH, RT, 5 h; 22:
56%, 23: 13%, 24: 45%, 25: 9% in two steps; c) NaOMe, MeOH, RT,
5 h, 22–25: quant. TMSOTf: trimethylsilyl trifluoromethanesulfonate;
MS: molecular sieves.
solution. Diacylation of myo-inositol 1,3,5-orthoformate 9
with (S)-2-acetyloxy-2-phenylacetyl chloride, generated
from the corresponding carboxylic acid, led to the 6-alcohol
20 and 4-alcohol 21 in 15 and 39% yield, respectively. The
isolation and spectral characterization of both compounds
have been reported.[15] TMSOTf-catalyzed coupling of the
imidate donor 11 with the 6-alcohol 20 followed by deacyl-
benzylated to give the desired 1-alcohol 29[17] in 80% yield.
The excellent regioselectivity is perhaps induced by the
steric hindrance of both O-2- and O-6-mannosyl rings pre-
venting the nucleophilic attack of O-1 to benz
ACHTUNGTRENNaUNG ldehyde.
Coupling of compound 29 with the H-phosphon
AHCTUNGTRENNUNG
using a combination of pivaloyl chloride, iodine, and pyri-
dine furnished the product 30[18] (82%), which was subjected
to hydrogenolysis to give the target molecule 1[18] in 52%
yield. The 1H and 13C NMR spectra of compounds 29,[17]
30,[18] and 1[18] are identical to the literature reports (see
Supporting Information).
In summary, we have developed an efficient and conve-
AHCTUNTGREGNUNnN ient route to synthesize PIM2 1 from commercially avail-
able myo-inositol 1,3,5-orthoformate 9 in nine steps in 13%
overall yield. The meso-diol 10 can be d-mannosylated at
the O-6 position to yield the corresponding chiral disaccha-
ACHTUNGTRENNUNGation with sodium methoxide furnished the a-d-mannosylat-
ed 2,4-diol 22 and its b-isomer 23 in 56 and 13% yield, re-
spectively. Similar conditions were applied to the 4-alcohol
21, and the a-d-mannosylated 2,6-diol 24 (45%) and its b-
isomer 25 (9%) were individually obtained. Debenzoylation
of compounds 16–19 with sodium methoxide gave the identi-
cal diols 22–25 in quantitative yields, respectively.
With the synthon 22 in hand, the total synthesis of PIM2 1
was further investigated (Scheme 3). Since the equatorial C-
2 hydroxy group in 22 is more reactive than the axial one at
C-4, regioselective and stereoselective coupling of the d-
mannosyl donor 11 with 22 in the presence of BF3·OEt2 as
catalyst provided the desired 4-alcohol 26 (87%) as a single
diastereoisomer. Removal of the orthoformate protecting
group in 26 under mild acidic conditions afforded tetraol 27
in almost quantitative yield. The next challenge was the re-
gioselective protection of the hydroxy groups at the C-3, C-
4, and C-5 positions. Williamson etherification of 27 (NaH,
BnBr) led to a mixture of different O-benzylated isomers,
which was very difficult to purify and identify. An alterna-
tive approach via TMSOTf-catalyzed Et3SiH-reductive
etherification of the per-O-trimethylsilylated compound was
then pursued.[16] Silylation of 27 yielded the corresponding
tetra-O-TMS ether 28 (quant.), which was regioselectively
AHCTUNGTREGrNNUN ide 16 in high regioselectivity and stereoselectivity. The
stepwise sugar coupling described here allows the introduc-
tion of two different d-mannopyranosides at the O-2 and O-
6 positions for the synthesis of higher di-O-acylated and tri-
O-acylated PIMs. The Et3SiH-reductive benzylation using
TMSOTf as a catalyst enables the installation of two benzyl
groups at O-4 and O-5 by controlling the amount of benzal-
dehyde that can be applied to prepare the tetra-O-acylated
PIMs.
Experimental Section
Procedure for BF3·OEt2-activated regioselective and stereoselective cou-
pling of 10 with 11: A mixture of the 4,6-diol 10 (0.35 g, 1.2 mmol), man-
Chem. Eur. J. 2009, 15, 1091 – 1094
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1093

S.-C. Hung and P. S. Patil
nosyl trichloroacetimidate 12 (0.82 g, 1.2 mmol), and freshly dried 3 ꢁ
molecular sieves (1.2 g) was stirred in THF (25 mL) at room temperature
for 1 h under nitrogen. The reaction flask was cooled to ꢀ788C,
BF3·OEt2 (45 mL, 0.36 mmol) was added to the solution, and the mixture
was gradually warmed to ꢀ208C. After stirring for 1 h, a solution of the
imidate 11 (0.82 g, 1.2 mmol) in THF (2 mL) and BF3·OEt2 (45 mL,
0.36 mmol) were consecutively added to the reaction solution, and the
mixture was continuously stirred at the same temperature for 2 h. Tri-
[6] a) Y. Watanabe, T. Yamamoto, T. Okazaki, Tetrahedron 1997, 53,
903–918; b) C. J. J. Elie, R. Verduyn, C. E. Dreef, D. M. Brounts,
G. A. Vandermarel, J. H. van Boom, Tetrahedron 1990, 46, 8243–
8254.
[7] a) K. N. Jayaprakash, J. Lu, B. Fraser-Reid, Bioorg. Med. Chem.
Lett. 2004, 14, 3815–3819; b) A. Stadelmaier, M. B. Biskup, R. R.
Schmidt, Eur. J. Org. Chem. 2004, 3292–3303.
[8] X. Liu, B. L. Stocker, P. H. Seeberger, J. Am. Chem. Soc. 2006, 128,
3638–3648.
ACHTUNGTRENNUNGethylamine (0.2 mL) was added to quench the reaction, the whole mix-
ture was filtered through celite, and the filtrate was evaporated under re-
duced pressure. The residue was purified by flash column chromatogra-
phy (gradient EtOAc/Hex 1:3.25 to 1:2.5) to obtain the products 16
(0.62 g, 64%) and 18 (0.12 g, 12%). For the preparation of other com-
pounds, please see the detailed procedure in Supporting Information.
[9] a) K. N. Jayaprakash, J. Lu, B. Fraser-Reid, Angew. Chem. 2005, 117,
6044–6048; Angew. Chem. Int. Ed. 2005, 44, 5894–5898; b) B.
Fraser-Reid, J. Lu, K. N. Jayaprakash, C. Lopez, Tetrahedron: Asym-
metry 2006, 17, 2449–2463; c) A. Hçlemann, B. L. Stocker, P. H.
Seeberger, J. Org. Chem. 2006, 71, 8071–8088.
[10] S. L. Bender, R. J. Budhu, J. Am. Chem. Soc. 1991, 113, 9883–9885.
[11] A. Stadelmaier, R. R. Schmidt, Carbohydr. Res. 2003, 338, 2557–
2569.
[12] A. Ali, D. C. Gowda, R. A. Vishwakarma, Chem. Commun. 2005,
519–521.
Acknowledgements
[13] S. Ozaki, Y. Koga, L. Ling, Y. Watanabe, Y. Kimura, M. Hirata,
Bull. Chem. Soc. Jpn. 1994, 67, 1058–1063.
[14] R. R. Schmidt, W. Kinzy, Adv. Carbohydr. Chem. Biochem. 1994, 50,
21–123.
This work was supported by the National Science Council of Taiwan
(NSC 95-2113-M-007-028-MY3) and Academia Sinica (AS-95-TP-AB1).
[15] K. M. Sureshan, Y. Watanabe, Tetrahedron: Asymmetry 2004, 15,
1193–1198.
Keywords: glycosylation · phosphatidylinositol mannosides ·
total synthesis · tuberculosis
[16] a) C.-C. Wang, J.-C. Lee, S.-Y. Luo, H.-F. Fan, C.-L. Pai, W.-C. Yang,
L.-D. Lu, S.-C. Hung, Angew. Chem. 2002, 114, 2466–2468; Angew.
Chem. Int. Ed. 2002, 41, 2360–2362; b) J.-C. Lee, S.-W. Chang, C.-C.
Liao, F.-C. Chi, C.-S. Chen, Y.-S. Wen, C.-C. Wang, S. S. Kulkarni, R.
Puranik, Y.-H. Liu, S.-C. Hung, Chem. Eur. J. 2004, 10, 399–415;
c) C.-C. Wang, J.-C. Lee, S.-Y. Luo, S. S. Kulkarni, Y.-W. Huang, C.-
C. Lin, K.-L. Chang, S.-C. Hung, Nature 2007, 446, 896–899; d) C.-
C. Wang, S. S. Kulkarni, J.-C. Lee, S.-Y. Luo, S.-C. Hung, Nat.
Protoc. 2008, 3, 97–113.
[17] C. J. J. Elie, R. Verduyn, C. E. Dreef, G. A. Vandermarel, J. H. van
Boom, J. Carbohydr. Chem. 1992, 11, 715–739.
[18] G. D. Ainge, J. Hudson, D. S. Larsen, G. F. Painter, G. S. Gillc, J. L.
Harperb, Bioorg. Med. Chem. 2006, 14, 5632–5642.
[1] World Health Organization, Tuberculosis, Fact Sheet no 104, http://
www.who.int/mediacentre/factsheets/who104/en.
[2] B. R. Bloom, C. J. L. Murray, Science 1992, 257, 1055–1064.
[3] a) M. Joe, Y. Bai, R. C. Nacario, T. L. Lowary, J. Am. Chem. Soc.
2007, 129, 9885–9901; b) C. Rademacher, G. K. Shoemaker, H.-S.
Kim, R. B. Zheng, H. Taha, C. Liu, R. C. Nacario, D. C. Schriemer,
J. S. Klassen, T. Peters, T. L. Lowary, J. Am. Chem. Soc. 2007, 129,
10489–10502; c) K. S. Kim, B.-Y. Lee, S. H. Yoon, H. J. Jeon, J. Y.
Baek, K.-S. Jeong, Org. Lett. 2008, 10, 2373–2376.
[4] a) D. Chatterjee, K.-H. Khoo, Glycobiology 1998, 8, 113–120; b) J.
Nigou, M. Gilleron, G. Puzo, Biochimie 2003, 85, 153–166.
[5] a) A. L. K. Hestvik, Z. Hmama, Y. Av-Gay, FEMS Microbiol. Rev.
2005, 29, 1041–1050; b) P. J. Brennan, Tuberculosis 2003, 83, 91–97
and references therein.
Received: October 23, 2008
Published online: December 22, 2008
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Chem. Eur. J. 2009, 15, 1091 – 1094