Orthoester-based strategy for efficient synthesis of the virulent antigenic-1,2-linked oligomannans of Candida albicans

Article abstract of DOI:10.1055/s-2003-40350

Orthobenzoates of glucose and mannose provide donor and acceptor partners to produce a disaccharide bearing a benzoyl group at the site where gluco to manno conversion is required, the inverted center being ready to function, iteratively, as the next acce

Full text of DOI:10.1055/s-2003-40350

LETTER
1319
Orthoester-Based Strategy for Efficient Synthesis of the Virulent Antigenic-
1,2-Linked Oligomannans of Candida albicans
Virulent
A
ntig
e
enic -1,2-L
l
inked
O
i
ligom
x
annans of
C
andida
M
albicans athew,a Mateusz Mach,a Kevin C. Hazen,b Bert Fraser-Reid*a
a
Natural Products and Glycotechnology Research Institute, Inc. (NPG), 4118 Swarthmore Road, Durham, NC 27707, USA
Fax +1(919)4936113; E-mail: Dglucose@aol.com
b
Departments of Pathology and Microbiology, University of Virginia Health System, Charlottesville, VA 22908, USA
Received 10 May 2003
Dedicated to the memory, inspiration and teachings of Ray Lemieux, our mentor or grandmentor. The work described herein is possible
because of his pioneering studies on ‘Replacement reactions at the anomeric center’, which led directly to the ready synthesis of glycosyl
orthoesters and their use as glycosyl donors.
1
ported detailed H NMR analysis of a pentamannan7a
Abstract: Orthobenzoates of glucose and mannose provide donor
which confirms the ‘compact configuration’ predicted by
and acceptor partners to produce a disaccharide bearing a benzoyl
Rees.4
group at the site where gluco to manno conversion is required, the
inverted center being ready to function, iteratively, as the next
acceptor for the gluco n-pentenyl orthobenzoate to extend the oligo-
mannan.
The iterative cycle is depicted in Scheme 2. The high
yields maintained throughout, have afforded all units, in-
cluding the 8-mer, in ca 200 mg amounts. The key starting
glycosyl orthoesters 1 and 2, are easily prepared10 from
Key words: n-pentenylorthoesters, b-oligomannosides, iterative
synthesis, structure verification
the corresponding glycoses in five simple, identical steps
using 4-pentenol and benzyl alcohol respectively. Treat-
ment of each with ytterbium (III) triflate17 or TBDMSOTf
The spectrum of diseases1 caused by Candida albicans
effects quantitative rearrangement to the n-pentenyl and
exceeds that of most other microorganisms, this virulence
being attributable to the pathogen’s ability to survive in
several anatomically distinct sites.2 The organism’s cell
wall phospholipomannan, first identified by Shibata and
co-workers as a b-1,2-linked oligomannan,3 is the seat of
its activity. Rees’ 1971 prediction that such oligomannans
would display a-helicity,4 is fraught with structural and
biological implications,5 fulfilled by observations of
Poulain6 and Bundle7 on the correlation of periodic bio-
logical activity with chain length. Our long-standing inter-
est in glycosylphosphatidylinositols (GPIs),8 was
captured by Poulain’s observation that C. albicans trig-
gers ‘intense signaling and secretory responses’ similar to
those that are induced by GPI-related glycolipids.9
benzyl glycosides 3 and 4 respectively, and debenzoyla-
tion of the latter gives the starting acceptor 5, which con-
stitutes the reducing end unit of the prospective
oligomannan
Coupling of 5 with NPOE 1 mediated by TBDMSOTf/
NIS gave the glucoside 6 in 92% yield, which carries a
single benzoyl group. The exclusive use of benzyl protect-
ing groups elsewhere provided a convenient window
for monitoring the upcoming transformations leading to 7
1
by H and 13C NMR spectroscopy (Scheme 1). Thus,
employing the Garegg strategy,14,15 compound 6 was
deesterified, oxidized and reduced (with L-selectride as
pioneered by Danishefsky18) leading to the dimannan 7 in
excellent yield.
In this paper, we describe an n-pentenyl orthoester
(NPOE)-based approach that is simple and totally itera-
tive, using identical methodologies for synthetic as well as
structure verification protocols. It should be noted that al-
though the orthoester 1 and 2-O-benzoyl NPG 3 are func-
tionally equivalent, the former (a) usually gives much
higher yields of trans-coupled products,10,11 and (b) reacts
infinitely more quickly as measured by our procedure for
comparing relative reactivity of glycosyl donors.12
The iterative protocols,19 for both synthesis and structure
analysis remained constant up to the 8-mer (i.e. 15, n = 6),
Scheme 2, without any need to tamper with protecting
groups. As the synthesis progressed the acceptor, generat-
ed in the previous step, became more and more precious,
and so a generous excess of the easily prepared donor 1
was applied. Normally ketone 14b, obtained by Swern ox-
idation,20 was not isolated but was reduced directly with
L-selectride.21
The above biological concerns have induced much activi-
ty towards the synthetically defiant13 b-1,2-mannoside
motif. Sinay and co-workers14 employed the classical
Garegg protocol15 to prepare a tetramer. Elegant proce-
dures developed by Bundle7 and Crich16 have afforded
structures in the M4-M8 range, and recently Bundle re-
The critical use of NPOE 1 as donor gave only a trace, if
any, of cis-coupled product thereby simplifying purifica-
tion and monitoring of subsequent transformations. Thus
the 5.6 ppm region of the 1H NMR spectra of the coupling
products, exemplified for 7–10 in Figure 1A characteristi-
cally shows a triplet ca 5.4 ppm which is consistent with
H2 of the non-reducing end (gluco) entity. Upon the de-
benzoylation to 14a, the 5–6 ppm window is no longer
useful; however, 13C NMR provides excellent resort. Thus
as shown in Figure 1C, C2 occurs ca 105 ppm in the gluco
Synlett 2003, No. 9, Print: 11 07 2003.
Art Id.1437-2096,E;2003,0,09,1319,1322,ftx,en;S05403ST.pdf.
© Georg Thieme Verlag Stuttgart · New York

1320
F. Mathew et al.
LETTER
Scheme 1
BnO
BnO
BnO
O
yields
8 n=1 86%
9 n=2 92%
10 n=3 85%
11 n=4 85%
12 n=5 86%
13 n=6 83%
O
NIS
OBz
TBDMSOTf
1
7
n=1,2,3,4,5,6
+
BnO
BnO
BnO
O
O
OBn
O
8-13
BnO
BnO
15a
OBn
BnO
BnO
BnO
NaOMe
quant.
NIS
TBDMSOTf
O
O
O
X
n=1,2,3,4,5,6
Ph
O
BnO
BnO
BnO
O
O
1
14
X
Y
OR
R'O
R'O
R'O
Y
Swern
oxidation
O
a. H
b. --O--
OH
O
n=1,2,3,4,5,6
R'O
R'O
R'O
O
14b
O
OR'
O
L selectride
R'O
R'O
15
OR'
a. R=H;R'=Bn
b. R=Ac;R'=Bn
i
16
(n=4)
R=R'=H
(i) = HCOOH,Pd/C,MeOH, r.t. 18 h (quant.)
Scheme 2
Synlett 2003, No. 9, 1319–1322 ISSN 1234-567-89 © Thieme Stuttgart · New York

LETTER
Virulent Antigenic -1,2-Linked Oligomannans of Candida albicans
1321
References
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Figure 1 1H NMR and 13C NMR ‘windows’ for monitoring trans-
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Chem. Soc. 1985, 111, 8540. (b) Roberts, C.; Madsen, R.;
Fraser-Reid, B. J. Am. Chem. Soc. 1995, 117, 1546.
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Chem. Soc. 1995, 117, 10387. (e) Arasappan, A.; Fraser-
Reid, B. J. Org. Chem. 1996, 61, 2401.
moiety (13a, n = 0 and 1), but ca 100 ppm in the manno
(14a, n = 0 and 1). The stereoselectivity of the L-selectride
reduction,18 could also be easily checked by studying
1
the H NMR spectrum of the acetylated products 15b
(n = 0–3) which showed the H2 proton of the newly intro-
duced manno units to be shifted downfield to ca 5.6 ppm,
Figure 1B.
(9) Trinel, P.-A.; Plancke, Y.; Gerold, P.; Jouault, T.; Delplace,
F.; Schwarz, R. T.; Streckers, G.; Poulain, D. J. Biol. Chem.
1999, 274, 30520.
(10) Mach, M.; Schlueter, U.; Mathew, F.; Fraser-Reid, B.;
Hazen, K. C. Tetrahedron 2002, 58, 7345.
(11) Kochetkov, N. K.; Khorlin, A. Y.; Boschkov, A. F.
Interestingly the anomeric proton of the gluco-moiety of
7 is ca 4.55 ppm for n = 0 (not shown in Fig 1A) and
moves downfield to 5.59 for 8 (n = 1) and 5.76 for both 9
(n = 2) and 10 (n = 3). The spectroscopic data for the 8-
mer (15a, n = 6) fully support the assigned structure.22
Tetrahedron 1967, 23, 693.
(12) Wilson, B. G.; Fraser-Reid, B. J. Org. Chem. 1995, 60, 317.
(13) Paulsen, H. Angew. Chem. Int. Ed. Engl. 1982, 21, 155;
Angew Chem., 1982, 94, 184.
Debenzylation is most conveniently carried out by trans-
fer hydrogenation using formic acid. For example 15a
(n = 4) gave 16 quantitatively. Detailed analyses of
chemical shifts in protected and deprotected samples of
oligomers will be reported in due course.
(14) Fradin, C.; Jovanlt, T.; Mallet, A.; Mallet, J. M.; Camas, D.;
Sinay, P.; Poulain, D. J. J. Leukocyte Biol. 1996, 60, 81.
(15) Boren, H. B.; Ekborg, G.; Iklind, K.; Garegg, P. J.; Pilotti,
A.; Swan, C. G. Acta Chem. Scand. 1973, 27, 2639.
(16) (a) Crich, D.; Sun, S. J. Am. Chem. Soc. 1997, 119, 11217.
(b) Crich, D.; Li, H.; Yao, Q.; Wink, D. J.; Sommer, R. D.;
Rheingold, A. L. J. Am. Chem. Soc. 2001, 123, 5826.
(17) Jayaprakash, K. N.; Radhakrishnan, K. V.; Schlueter, U.;
Fraser-Reid, B. Tetrahedron Lett. 2002, 43, 6953.
(18) Wang, Z.-G.; Zhang, X.; Visser, M.; Live, D.; Zatorsky, A.
M.; Iserloh, U.; Lloyd, K. O.; Danishefsky, S. J. Angew.
Chem. Int. Ed. 2001, 40, 1728.
(19) Typical procedure for preparation of 9 by reaction of 15a
with 1: The acceptor, trisaccharide 15a (n = 1, 1.40 g, 1
mmol) and the donor (1, 4.36 g, 7 mmol) were separately
rotovaporised twice with dry toluene, and then dried for 8 h
under vacuum (0.5 mm Hg). To a solution of the acceptor in
15 mL of dry CH2Cl2 were added freshly activated molecular
sieves (4 g, 3 A beads, 8–12 mesh), NIS re-crystallized from
hot CH2Cl2 and cold hexane and dried under vacuum (0.5
mm Hg, over night, 1.35 g, 6 mmol) followed by
TBDMSOTf (0.1 mL, 0.4 mmol) at 10 ºC (ice bath). To this
solution was added a solution of NPOE in CH2Cl2 drop-wise
over 10 min, and then the ice bath was removed. After 10
In summary, once compounds 1 and 5 are in hand, one cy-
cle of the protocol shown in Scheme 2 can be accom-
plished in 2–3 days, allowing for rigorous characterization
of each intermediate. The data in Scheme 2 show that ex-
cellent yields are maintained as the array is lengthened.
Accordingly, the 9–16-mers of b-1,2-oligomannans,
which occur in the cell wall of C. albicans should be ob-
tainable by this iterative procedure.
Acknowledgement
We are grateful to the Public Health Service (RO1 AI43997) for
support.
Synlett 2003, No. 9, 1319–1322 ISSN 1234-567-89 © Thieme Stuttgart · New York

1322
F. Mathew et al.
LETTER
min TLC (7:3, hexane–ethyl acetate) showed that all of the
acceptor had been consumed. The mixture was diluted with
CH2Cl2 (200 mL), and washed with 10% aqueous Na2S2O3,
saturated aqueous NaHCO3 and brine. The dried (Na2SO4)
material was concentrated and flash chromatography (7:1&
ndash;4:1, hexane–ethyl acetate) afforded 1.78 g (92%) of 9
(n = 2). Rf = 0.5 (7:3 hexane–ethyl acetate).
(22) For 15a n = 6: 1H NMR (300 MHz, CDCl3): d = 7.45–6.77
(m, 125 H), 5.56–5.52 (broad 6 H), 5.19 (s, 1 H), 4.98 (s, 1
H), 4.91–4.22 (m, 51 H), 4.12–3.56 (m, 49 H). 13C NMR
(300 MHz, CDCl3): d = 139.4, 139.2, 139.1, 139.0, 138.9,
138.6, 138.4, 138.3, 138.1, 138.0, 137.9, 137.0, 128.7,
128.7, 128.5, 128.4, 128.3, 128.1, 128.0, 127.9, 127.9,
127.8, 127.8, 127.7, 127.6, 127.5, 127.1, 127.1, [8 anomeric
carbons comes at (100.9, 100.8, 100.5, 100.3, 100.1, 99.9,
98.7, 95.7 (a anomeric carbon)]. 83.3, 82.0, 81.9, 81.8, 81.7,
79.8, 78.1, 75.7, 75.5, 75.3, 75.1, 74.9, 74.7, 74.4, 74.1, 74.0,
73.6, 73.5, 73.2, 73.1, 73.0, 72.8, 72.7, 71.3, 70.3, 70.2, 70.1,
70.0, 69.9, 69.7, 69.6, 69.5, 69.4, 68.5, 67.6. FAB-MS (m/z)
3696 (M + Cs).
(20) Mancuso, A. J.; Huang, S. L.; Swern, D. J. Org. Chem. 1978,
43, 2480.
(21) Nitz, M.; Purse, B. W.; Bundle, D. R. Org. Lett. 2000, 2,
2939.
Synlett 2003, No. 9, 1319–1322 ISSN 1234-567-89 © Thieme Stuttgart · New York