Intramolecular aglycon delivery for (1 → 2)-β-mannosylation: Towards the synthesis of phospholipomannan of Candida albicans

Article abstract of DOI:10.1016/j.tetlet.2014.03.099

A high yielding method for 1,2-cis-β-D-mannosylation by intra-molecular aglycon delivery (IAD) through p-methoxy benzyl ether/acetal exchange and phenylsulfoxide donor is reported, along with its application in iterative assembly of antigenic (1 → 2)-β-pe

Full text of DOI:10.1016/j.tetlet.2014.03.099

Tetrahedron Letters 55 (2014) 2945–2947
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Intramolecular aglycon delivery for (1 ? 2)-b-mannosylation:
towards the synthesis of phospholipomannan of Candida albicans
Veeranjaneyulu Gannedi a,b, Asif Ali b, Parvinder Pal Singh b, Ram A. Vishwakarma a,b,
⇑
a Academy of Scientific and Innovative Research, Indian Institute of Integrative Medicine, Jammu 180001,India
b Medicinal Chemistry Division, CSIR-Indian Institute of Integrative Medicine, Jammu 180 001, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A high yielding method for 1,2-cis-b-D-mannosylation by intra-molecular aglycon delivery (IAD) through
p-methoxy benzyl ether/acetal exchange and phenylsulfoxide donor is reported, along with its applica-
tion in iterative assembly of antigenic (1 ? 2)-b-pentamannoside domain of phospholipomannan
(PLM) of fungal pathogen Candida albicans.
Received 20 December 2013
Revised 19 March 2014
Accepted 20 March 2014
Available online 28 March 2014
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
1,2-cis-b-D-mannosylation
Intra-molecular aglycon delivery
Phenylsulfoxide donor
Phospholipo mannan
Candida albicans
Candida albicans, the most widespread human pathogen respon-
sible for cutaneous and systemic fungal infections, synthesize a un-
ique sequence of (1 ? 2)-b-oligomannans on its cell surface that
act as adhesin, induce production of pro-inflammatory cytokines
and antibodies. One of the key (1 ? 2)-b-mannan expressed by C.
albicans is the membrane-anchored glucosphingolipid (GSL),
named phospholipomannan1 (PLM, Fig. 1) where oligomeric
human cells (host) reveals remarkable biomimetic features
(Fig. 1). These include: (a) (1 ? 2)-b-mannan in PLM in place of
(1 ? 2)-
a
-mannan motif in GPI, (b) (1 ? 2)-a-mannose linked to
myo-inositol in place of (1 ? 6)-
a
glucosamine-inositol motif in
GPI; and c) presence of phytoceramide in PLM and glycerolipid in
GPI. Perhaps, these remarkable bio-mimetic features represent
mechanisms of molecular and evolutionary adaptations between
the species. In continuation to our long-term interest in chemistry
(1 ? 2)-b-mannan domain is linked through a unique a-1,2-man-
nosyl anomeric phosphodiester moiety to a mannose-inositol-
phosphoceramide (MIPC) glycolipid anchor, which is embedded
in the cell wall of the C. albicans. The PLM of Candida albicans is
highly immunogenic due to its (1 ? 2)-b-mannan structural motif,
which is absent in human host. When C. albicans infects and inter-
acts with the macrophages (the first-line-of-defence cells of the
human host), a large amount of soluble PLM fragments (largely
(1 ? 2)-b-mannans) are rapidly shed by the pathogen resulting
OH
HO
HO
HO
H
N
O
HO P
O
O
Protein
O
O
O
HO
HO
HO
OH
O
O
O
HO
HO
HO
HO
HO
O
OH
O
O
O
HO
HO
HO
HO
HO
HO
NH2
O
O
O
P
O
in severe pro-inflammatory response2 (TNF-
a production and re-
O
OH
O
O
O
HO
HO
HO
lease) from host cells.3 Several biochemical4 and immunological
studies5 have shown that PLM motif provides a scaffold for anchor-
ing of various virulence factors to the cell surface of the pathogen, a
mechanism quite similar to that used by the mammalian cells for
anchoring various cell-surface proteins and glycans through the
glycosylphosphatidylinositol6 (GPI) anchor. A comparison of the
structures of PLM of C. albicans (pathogen) and GPI anchor of
HO
OH
O
O
O
O
P
HO
C15H31
O
HO
O
O
H2N
HO
OH
O
O
OH
O
C23H47
HO
OH
O
P
O
O
HO
O
C17H35
HO
O
O
O
HO
O
HN
O
OH
HO
OH
OH
P
O
C21H43
O
C14H29
O
HO
Glycosylphosphatidylinositol (GPI)
Phospholipomannan (PLM) of C. albicans
⇑
Corresponding author. Tel.: +91 191 2569111; fax: +91 191 2569333.
E-mail address: ram@iiim.res.in (R.A. Vishwakarma).
Figure 1. Phospholipomannan of C. albicans and GPI anchor of H. sapiens.
http://dx.doi.org/10.1016/j.tetlet.2014.03.099
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

2946
V. Gannedi et al. / Tetrahedron Letters 55 (2014) 2945–2947
and biology of GPI anchors,7 we initiated the synthesis of PLM of C.
albicans, which present substantial opportunities.
OMe
OAc
O
O
BnO
BnO
O
Ref 15
BnO
BnO
BnO
BnO
O
i
Mannose
2
3
SPh
Arguably, one of the most challenging problems in carbohy-
drate chemistry is the construction of thermodynamically unstable
(1 ? 2)-b-mannoside bond, ubiquitous in several pathogenic
microorganisms. First synthesis of b-oligomannosides was re-
ported by Hindsgaul,8a and Stork8b,c followed by Ogawa and Ito9
using intramolecular aglycon delivery (IAD) and temporary tether-
ing concept. Later on, a number of other leading groups designed
(1 ? 2)-b-glycosidations via intermolecular glycosylation using
orthogonal functional groups.10
Here, we report the first application of intramolecular glycosyl-
ation approach by linkage of the accepting atom to the donor via a
bifunctional group (intramolecular aglycon delivery) for the syn-
thesis of (1 ? 2)-b-mannosides. Previously, Crich11 and Seeber-
ger12 reported the synthesis of (1 ? 2)-b, (1 ? 3)-b and (1 ? 4)-
b-mannosides using 4,6-O-benzylidene protected mannosyl-1-O-
sulfoxides or thiomannosides respectively through intermolecular
glycosylation. In our approach, we used mannosyl-1-O-sulfoxide
donor first time in PMB mediated IAD approach which because of
good leaving nature gave high yield of (1 ? 2)-b-mannosides even
without the 4,6-benzylidene group.
The intramolecular aglycon delivery method (IAD) has been
successfully used for the synthesis of (1 ? 3)-b and (1 ? 4)-b-
mannosides, but the most challenging (1 ? 2)-b-mannosides have
not been successful by IAD method so far. Keeping in view the
challenge, we designed a new strategy for the synthesis of
(1 ? 2)-b-mannosides via IAD approach where we envisioned that
a p-methoxybenzyl group at C-2 position of mannosyl residue will
participate in both ether/acetal exchange as well as in intra-molec-
ular aglycon delivery for glycosylation. To test the hypothesis, we
first synthesized and coupled 2-O-p-methoxybenzyl-3,4,6-tri-O-
benzylmannosyl phenyl sulfoxide 6 as donor with suitably pro-
ii
OH
O
BnO
BnO
BnO
OPMB
O
OPMB
O
BnO
BnO
BnO
BnO
BnO
BnO
iii
iv
4
6
5
SPh
SOPh
SPh
Scheme 2. Synthesis of mannoside fragments 4 and 6. Reagents and conditions: (i)
BF3Et2O, PhSH; (ii) NaOMe, MeOH; 90% over 2 steps; (iii) PMBCl, NaH, DMF; (iv) m-
CPBA, DCM, 85%.
a correlation with H-10 indicating that the two protons H-1 & H-10
were situated in a same face and assigned as
linkage (Fig. 2 of ESI).
Now we extended our IAD method for the synthesis of final
product 1, the (1 ? 2)-b-pentamannoside of PLM. The intermedi-
ates were prepared as shown in Schemes 2–5. For this, we started
with mannose and converted it into an orthoester donor 2
(Scheme 2) using reported method.16 The orthoester was success-
fully converted to an acceptor 1017 as well as mannosyl TCA donor
12 (Scheme 3).18
a-proton with the b-
Having the new IAD method and the key intermediates in hand,
the (1 ? 2)-b-mannopyranoside 8 was now successfully converted
into the corresponding sulfoxide donor 14 (Scheme 4).19 The syn-
thesis of the second key trisaccharide intermediate 16 was
achieved by the coupling of mannopyranosyl donor 12 with the
acceptor 10 (Scheme 5), which on further deacetylation by using
sodium methoxide provided the alcohol 15.20 The
a-stereochemis-
try of the resultant glycoside 15 was established by NMR analysis.
To construct the required trisaccharide 16, the disaccharide 15
was coupled with donor 6 using our intra-molecular aglycon deliv-
2
tected
a-phenyl thiopyranosides 4 as acceptor in the presence of
iii
i
DDQ (1.4 equiv), under anhydrous conditions which afforded the
mixed acetals 7 quite smoothly.9,13,14 Further the mixed acetal
was used for glycosylation by performing reaction with triflic
anhydride (Tf2O, 0.95 equiv) in the presence of 2,6-di-tert-butyl-
4-methylpyridine (DTBMP, 3.0 equiv) at ꢀ78 °C which gave exclu-
sively the desired (1 ? 2)-b-mannopyranoside 8 with 84% yield as
revealed by spectroscopic studies (Scheme 1). The starting materi-
als 4 and 6 were used in this glycosylation, which in turn were syn-
thesized from corresponding orthoester donor of mannose 2 as
shown in Scheme 2.15,16
OAc
O
OH
O
OAc
O
OAc
O
BnO
BnO
BnO
BnO
BnO
BnO
BnO
BnO
BnO
BnO
BnO
BnO
ii
iv
9
10
11
12
O
CCl3
OH
OAllyl
OAllyl
NH
Scheme 3. Synthesis of mannoside fragments 10 and 11. Reagents and conditions:
(i) BF3Et2O, AllOH; (ii) NaOMe, MeOH; 90% over 2 steps; (iii) p-TSA, DCE; (iv) Tri
chloro acetonitrile, K2CO3, DCM, 90% over 2 steps.
The stereochemistry of compound 8 was determined by 1H
NMR and further confirmed by 2D NMR spectral data analysis.
OPMB
O
OPMB
O
BnO
BnO
BnO
BnO
BnO
BnO
OH
O
BnO
BnO
BnO
i
O
O
ii
The a- and b-configurations of 8 were assigned by the appearance
O
BnO
BnO
BnO
BnO
BnO
BnO
BnO
BnO
BnO
of anomeric protons as doublets at d 5.60, 4.58 with coupling con-
stant values of J1,2 1.2 and 6.4 Hz, respectively. The HSQC and
HMBC correlation of anomeric proton at d 5.60 (d, J = 1.2 Hz) with
O
O
O
SOPh
SPh
SPh
13
14
8
13C (d 85.2) indicated the
a-orientation whereas the other anomer-
ic proton at d 4.58 (d, J = 6.4 Hz) with 13C (d 96.6) confirmed the b-
orientation. The relative configuration of 8 was authenticated by
NOESY correlation as being compatible with computer modelling
in which the close contact of atoms in space calculated were con-
sistent with NOESY correlation. In NOESY correlation H-1 exhibited
Scheme 4. Synthesis of b-dimannoside donor 14. Reagents and conditions: (i)
PMBCl, NaH, DMF; (ii) m-CPBA, DCM, 96%, over 2 steps.
OH
O
BnO
BnO
BnO
O
OH
O
BnO
BnO
BnO
10
1iii
BnO
BnO
BnO
BnO
BnO
BnO
iii) and iv)
O
i) and ii)
OMe
BnO
1ii
O
+
OBn
O
BnO
O
BnO
BnO
BnO
BnO
BnO
BnO
2 steps, 95%
2 steps, 86%
OAllyl
O
O
O
12
OH
O
O
BnO
DTBMP, Tf2O,
-78 oC-rt, 84%
O
DDQ, DCM,
BnO
BnO
O
1i
SOPh
OH
6
0o
C
OMe
PhOS
BnO
BnO
BnO
OAllyl
O
BnO
BnO
BnO
O
IAD
15
O
16
BnO
BnO
BnO
O
SPh
SPh
4
8
Scheme 5. Synthesis of b-trimannoside acceptor 16. Reagents and conditions: (i)
TMSOTf, DCM; (ii) NaOMe, MeOH; (iii) 6, DDQ, 1.5 equiv; (iv) DTBMP, Tf2O, DCM,
ꢀ78 °C to rt.
SPh
7
Scheme 1. Formation of (1 ? 2) b-mannosylation by IAD.

V. Gannedi et al. / Tetrahedron Letters 55 (2014) 2945–2947
2947
OH
O
BnO
BnO
BnO
pentamannoside domain of phospholipomannan (PLM) of Candida
albicans.
O
BnO
BnO
BnO
O
O
Acknowledgments
16
+
BnO
BnO
BnO
i)
O
O
18%
BnO
BnO
BnO
BnO
BnO
BnO
The financial support from a CSIR research Grant (BSC0108) is
gratefully acknowledged. V.G. is thankful to CSIR (India) for finan-
cial assistance. We wish to acknowledge S. Aravinda and Deepika
Singh for NMR spectral data.
14
O
O
O
OAllyl
19
Supplementary data
Scheme 6. Synthesis of b-pentamannoside by 2+3 coupling. Reagents and condi-
tions: (i) DDQ, (1.5 equiv); (ii) DTBMP, Tf2O, DCM, ꢀ78 °C to rt.
Supplementary data (spectral data for all compounds 1–20)
associated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.tetlet.2014.03.099.
OH
O
HO
HO
HO
OR
O
BnO
BnO
BnO
O
O
OH
O
BnO
References and notes
HO
HO
HO
BnO
BnO
BnO
BnO
O
O
O
BnO
O
O
BnO
BnO
BnO
O
HO
HO
HO
BnO
BnO
BnO
1. Trinel, P. A.; Maes, E.; Zanetta, J. P.; Delplace, F.; Coddeville, B.; Jouault, T.;
Strecker, G.; Poulain, D. J. Biol. Chem. 2002, 277, 37260.
2. Jouault, T.; Bernigaud, A.; Lepage, G.; Trinel, P.; Poulain, D. Immunology 1994,
83, 268.
3. Jouault, T.; Fradin, C.; Trinel, P. A.; Bernigaud, A.; Poulain, D. J. Infect. Dis. 1998,
178, 792.
4. Fradin, F.; Slomianny, M. C.; Mille, C.; Masset, A.; Robert, R.; Sendid, B.; Ernst, J.
F.; Michalski, J. C.; Poulain, D. Infect. Immun. 2008, 76, 4509.
O
O
O
O
O
BnO
i),ii)
i),ii)
75%
iv)
94%
O
HO
BnO
BnO
BnO
BnO
16
O
BnO
O
HO
84%
HO
BnO
O
O
O
BnO
HO
BnO
BnO
BnO
O
O
BnO
O
HO
HO
OAllyl
O
19 OAllyl
R = H
: R = Bn
18b
1
19:
20
iii)
5. (a) Murciano, C.; Moyes, D. L.; Runglall, M.; Islam, A.; Mille, C.; Fradin, C.;
Poulain, D.; Gow, N. A. R.; Naglik, J. R. Infect. Immun. 2011, 79, 4902; (b)
Johnson, M. A.; Bundle, D. R. Chem. Soc. Rev. 2013, 42, 4327.
Scheme 7. Iterative synthesis of b-pentamannoside 1. Reagents and conditions: (i)
6, DDQ, (1.5 equiv); (ii) DTBMP, Tf2O, DCM, ꢀ78 °C to rt; (iii) BnBr, NaH, DMF; (iv)
20, Pd/C 10%, THF, MeOH, H2, 94%.
6. (a) Ferguson, M. A. J.; Homans, S. W.; Dwek, R. A.; Rademacher, T. W. Science
1988, 239, 753; (b) Homans, S. W.; Ferguson, M. A. J.; Dwek, R. A.; Rademacher,
T. W.; Anand, R.; Williams, A. F. Nature 1988, 333, 269; (c) McConville, M. J.;
Feguson, M. A. J. Biochem. J. 1993, 294, 305; (d) Ruhela, D.; Banerjee, P.;
Vishwakarma, R. A. Curr. Sci. 2012, 102, 194; (e) Tsai, Y. H.; Liu, X.; Seeberger, P.
H. Angew. Chem., Int. Ed. 2012, 51, 11438. and references cited therein.
7. (a) Ali, A.; Gowda, D. C.; Vishwakarma, R. A. Chem. Commun. 2005, 519; (b)
Vishwakarma, R. A.; Menon, A. K. Chem. Commun. 2005, 453; (c) Vishwakarma,
R. A.; Vehring, S.; Mehta, A.; Sinha, A.; Pomorski, T.; Herrmann, A.; Menon, A. K.
Org. Biomol. Chem. 2005, 3, 1275; (d) Ali, A.; Vishwakarma, R. A. Tetrahedron
2010, 66, 4357; (e) Ruhela, D.; Vishwakarma, R. A. J. Org. Chem. 2003, 68, 4446;
(f) Chawla, M.; Vishwakarma, R. A. J. Lipid Res. 2003, 44, 594; (g) Saikam, V.;
Raghupathy, R.; Yadav, M.; Gannedi, V.; Singh, P. P.; Qazi, N. A.; Sawant, S. D.;
Vishwakarma, R. A. Tetrahedron Lett. 2011, 52, 4277.
ery (IAD) condition and the desired product obtained in 86% yield.
The stereochemistry of 16 having a- and b-anomeric linkages was
determined by HSQC NMR. Bundle et al. also reported the synthesis
of compound 16 but by a different synthetic strategy.21
To further confirm the stereochemistry at the anomeric position
of trisaccharide 16, disaccharide 15 was then converted to the tri-
saccharides 17 and 18 by 1,2-a-glycosylation with 12 (Scheme 5b,
of ESI) and then compared with trisaccharide 16 (HSQC given in
ESI).
8. (a) Barresi, F.; Hindsgaul, O. J. Am. Chem. Soc. 1991, 113, 9376; (b) Stork, G.; Kim,
G. J. Am. Chem. Soc. 1992, 114, 1087; (c) Stork, G.; La Clair, J. L. J. Am. Chem. Soc.
1996, 118, 247.
9. (a) Dan, A.; Ito, Y.; Ogawa, T. J. Org. Chem. 1995, 60, 4680; (b) Ishiwata, A.; Lee,
Y. J.; Ito, Y. Org. Biomol. Chem. 2010, 8, 3596; (c) Ishiwata, A.; Sakurai, A.;
Nishimiya, Y.; Tsuda, S.; Ito, Y. J. Am. Chem. Soc. 2011, 133, 19524.
10. (a) Code´e, J. D. C.; Hossain, L. H.; Seeberger, P. H. Org. Lett. 2005, 7, 3251; (b)
Baek, J. Y.; Choi, T. J.; Jeon, H. B.; Kim, K. S. Angew. Chem., Int. Ed. 2006, 45, 7436;
(c) Crich, D.; Sun, S. J. Org. Chem. 1997, 62, 1198; (d) El Ashry, E. S. H.; Rashed,
N.; Ibrahim, E. S. I. Curr. Org. Synth. 2005, 2, 175.
11. (a) Crich, D.; Li, W.; Li, H. J. Am. Chem. Soc. 2004, 126, 15081; (b) Crich, D.; Li, H.;
Yao, Q.; Wink, D. J.; Sommer, R. D.; Rheingold, A. L. J. Am. Chem. Soc. 2001, 123,
5826; (c) Crich, D. Acc. Chem. Res. 2010, 43, 1144; (d) Huang, M.; Garrett, G. E.;
Birlirakis, N.; Bohe, L.; Pratt, D. A.; Crich, D. Nat. Chem. 2012, 4, 663.
12. Jeroen, D. C.; Krock, C. L.; Castagner, B.; Seeberger, P. H. Chem. Eur. J. 2008, 14,
3987.
Finally, the key synthetic step to construct (1 ? 2)-b-pentam-
annoside via 2+3 glycosylation with the mannosyl donor 14 and
the acceptor trimannoside 16 was tried, which gave the desired
pentamannoside 19 in only low 18% yield; not satisfactory for
the total synthesis of PLM (Scheme 6).
Therefore, we modified our approach to construct the (1 ? 2)-
b-pentamannoside 19 by an iterative synthetic strategy, wherein
the trimannoside 16 was first converted into tetra-mannoside
18b and subsequently into pentamannoside 19 using the donor 6
as shown in Scheme 7. The (1 ? 2)-b-pentamannoside 19 was fur-
ther converted into perbenzylated pentamannoside 20. The stereo-
chemistry of (1 ? 2)-b-pentamannoside 20 was confirmed by NMR
and HSQC experiment. Finally, controlled global hydrogenolysis
with Pd/C afforded the target (1 ? 2)-b-pentamanan 1 in 94% iso-
lated yield.
13. Ito, Y.; Ogawa, T. J. Am. Chem. Soc. 1997, 119, 5562.
14. Ito, Y.; Ohnishi, Y.; Ogawa; Nakahara, Y. Synlett 1998, 1102.
15. Franks, N. E.; Montgomery, R. Carbohydr. Res. 1968, 6, 286.
16. Zhang, Y. M.; Mallet, J. M.; Sinay, P. Carbohydr. Res. 1992, 236, 73.
17. Liu, X.; Stocker, B. L.; Seeberger, P. H. J. Am. Chem. Soc. 2006, 128, 3638.
18. Mayor, G. T.; Schmidt, R. R. Eur. J. Org. Chem. 1998, 1153.
19. Ito, Y.; Ogawa, T. Angew. Chem., Int. Ed. 1994, 33, 1765.
In summary, we have designed a high yielding method for
(1 ? 2)-b-mannosylation employing IAD via PMB ether/acetal
intermediate and also achieved the first synthesis of (1 ? 2)-b-
20. Grathwohl, M.; Schmidt, R. R. Synthesis 2001, 15, 2263.
21. Wu, X. Y.; Bundle, D. R. J. Org. Chem. 2005, 70, 7381.