Highly stereoselective synthesis of α-D-mannopyranosyl phosphosugars

Article abstract of DOI:10.1021/jo902254w

(Chemical Equation Presented) α-Mannopyranosyl phosphosugars are obtained in 61-90% yields from 4,6-O-benzylidene-protected mannosyl thioglycosides bearing ester functionality in the 3-O-position by coupling reactions with ammonium salts of phosphosugars

Full text of DOI:10.1021/jo902254w

pubs.acs.org/joc
synthesis of mannoheptoside containing oligosaccharides
Highly Stereoselective Synthesis of
r-D-Mannopyranosyl Phosphosugars
and their deoxy congenors,3 we have now directed our atten-
tion to the synthesis of this repeating unit and report here on
the accomplishment of an essential first step;the highly
stereocontrolled preparation of R-mannopyranosyl phospho-
sugars.
ꢀ
David Crich* and Sebastien Picard
Centre de Recherche de Gif, Institut de Chimie des Substances
Naturelles, CNRS, 1 avenue de la Terrasse, 91198 Gif-
sur-Yvette, France, and Department of Chemistry, Wayne
State University, 5101 Cass Avenue, Detroit, Michigan 48202
dcrich@icsn.cnrs-gif.fr
Received October 20, 2009
Glycosyl phosphosugars have previously been synthesized
by the phosphate diester and phosphite triester methods,4 as
well as by the H-phosphonate approach.5 More recently,
Boons et al. employed glycosyl phosphoramidites in an
approach to the synthesis of the proteophosphoglycans from
Leishmania.6 In our laboratory, we prepared β-mannopyran-
nosyl phosphoisoprenoids diastereoselectively by coupling of
a 2,3-di-O-benzyl-4,6-O-benzylidene-protected mannopyra-
nosyl sulfoxide with tetrabutylammonium phosphates.7 The
stereoselectivity observed in these reactions was highly sol-
vent-dependent, being highly β-selective in toluene, but giving
R,β-mixtures in dichloromethane. In view of the importance
of the acetals of the 4,6-O-benzylidene type in our general
strategy toward the Campylobacter jejuni RM1221 repeating
unit, it was necessary to adapt this chemistry so as to render it
highly R-selective, ideally, in the optimal solvent for this
chemistry;dichloromethane. To this end, we opted to inves-
tigate the influence of an ester in the 3-O-position, which we
had previously shown to result in very high R-selectivity in the
synthesis of a range of typical glycosides despite the presence
of the 4,6-O-benzylidene group.8,9
R-Mannopyranosyl phosphosugars are obtained in 61-90%
yields from 4,6-O-benzylidene-protected mannosyl thio-
glycosides bearing ester functionality in the 3-O-position by
coupling reactions with ammonium salts of phosphosugars
on activation with 1-benzenesulfinyl piperidine, 2,4,6-tri-tert-
butylpyrimidine, and trifluoromethanesulfonic anhydride.
Due to the presence of the disarming ester group, only the
formation of the R-isomer was observed.
The ammonium salts of phosphosugars required as
glycosyl acceptors were prepared using the phosphoramidite
strategy.10 Thus, a range of phosphosugars 6 were obtained
from known carbohydrates 2 (D-glucose, L-rhamnose, or
D-mannose derived) by coupling with benzyl 2-cyano-
ethyl (CE) N,N-diisopropylphosphoramidite 37 in the pre-
sence of benzimidazolium trifluoromethanesulfonate 411 as
Glycosyl phosphosugars are fragments of several glycopro-
teins. They are also structural blocks of poly(glycosyl
phosphates) which are present in the cell walls and capsules
of numerous bacteria. Such capsular polysaccharides are
composed of mono- or polysaccharide units linked with
phosphate diester bridges through hemiacetal and alcohol
hydroxyl group neighboring units.1 Recently, Vinogradov et
al. determined the structure of the capsular polysaccharide
from Campylobacter jejuni RM1221, the causative agent of
human gastroenteritis.2 This polysaccharide 1 has a regular
structure made up of a linear main chain of trisaccharide
repeating units, comprising two R- and one β-6-deoxy-D-
manno-hepto-pyranose residues punctuated by a phosphodi-
ester linkage. Continuing our interest in the stereocontrolled
(4) (a) Cawley, T. N.; Letters, R. Carbohydr. Res. 1971, 19, 373–382.
(b) Thiem, J.; Franzkowiak, M. J. Carbohydr. Chem. 1989, 8, 1–28.
(5) Nikolaev, A. V.; Ivanova, I. A.; Shibaev, V. N.; Kochetkov, N. K.
Carbohydr. Res. 1990, 204, 65–78.
(6) (a) Elsayed, G. A.; Boons, G.-J. Synlett 2003, 9, 1373–1375.
(b) Majumdar, D.; Elsayed, G. A.; Buskas, T.; Boons, G.-J. J. Org. Chem.
2005, 70, 1691–1697.
(7) (a) Crich, D.; Dudkin, V. Org. Lett. 2000, 2, 3941–3943. (b) Crich, D.;
Dudkin, V. J. Am. Chem. Soc. 2002, 124, 2263–2266.
(8) (a) Crich, D.; Cai, W.; Dai, Z. J. Org. Chem. 2000, 65, 1291–1297.
(b) Crich, D.; Yao, Q. J. Am. Chem. Soc. 2004, 126, 8232–8236.
(9) For a discussion of this effect, see: Crich, D.; Hu, T.; Cai, F. J. Org.
Chem. 2008, 73, 8942–8953.
(10) (a) Matteucci, M. D.; Caruthers, M. H. J. Am. Chem. Soc. 1981, 103,
3185–3191. (b) For a review, see: Beaucage, S.; Iyer, R. P. Tetrahedron 1992,
48, 2223–2311.
(1) (a) von Figura, K.; Hasilik, A. Annu. Rev. Biochem. 1986, 55, 167–194.
(b) Raetz, C. R. H.; Whitfield, C. Annu. Rev. Biochem. 2002, 71, 635–700.
(c) Nikolaev, A. V.; Botvinko, I. V.; Ross, A. J. Carbohydr. Res. 2007, 342,
297–334.
(2) Gilbert, M.; Mandrell, R. E.; Parker, C. T.; Li, J.; Vinogradov, E.
ChemBioChem 2007, 8, 625–631.
(3) (a) Crich, D.; Banerjee, A. Org. Lett. 2005, 7, 1395–1398. (b) Crich, D.;
Banerjee, A. J. Am. Chem. Soc. 2006, 128, 8078–8086. Crich, D.; Li, M.
J. Org. Chem. 2008, 73, 7003–7010.
(11) (a) Hayakawa, Y.; Kataoka, M.; Noyori, R. J. Org. Chem. 1996, 61,
7996–7997. (b) Sakakura, A.; Hayakawa, Y. Tetrahedron 2000, 56, 4427–
4435.
9576 J. Org. Chem. 2009, 74, 9576–9579
Published on Web 11/19/2009
DOI: 10.1021/jo902254w
r
2009 American Chemical Society

Crich and Picard
JOCNote
TABLE 1. Preparation of Acceptors 6
SCHEME 1. Preparation of Tetrabutylammonium Phospho-
sugars 6
SCHEME 2. Preparation of R-Mannopyranosyl Phosphosugars 8
and these were employed in the subsequent glycosylation
reactions without purification.12
The R-thiomannoside 7 was prepared from the phenyl
2-O-benzyl-4,6-O-benzylidene-1-thia-R-D-mannopyranoside13
by acetylation in 98% yield (Scheme 2).
Glycosylation of the acceptors 6 with donor 7, carried out
by means of the 1-benzenesulfinyl piperidine (BSP)/Tf2O
preactivation protocol14 in the presence of the hindered
base 2,4,6-tri-tert-butylpyrimidine (TTBP),15 was achieved
in dichloromethane at -78 °C (Scheme 2). These reac-
tions were rapid and, in line with our expectations, gave
promoter followed in situ by oxidation with m-CPBA
(Scheme 1). The diesters 5 were isolated in high yields after
this two-step sequence as approximately 1/1 mixtures of dia-
stereoisomers (Table 1). This mixture was confirmed by the 31P
NMR, which showed two signals. The 2-cyanoethyl protecting
group was removed by treatment of 5 with tetrabutylammo-
nium hydroxide in a CH2Cl2/water mixture (Scheme 1) to
give the ammonium phosphosugars 6 quantitatively (Table 1),
(13) Crich, D.; Li, W.; Li, H. J. Am. Chem. Soc. 2004, 126, 15081–15086.
(14) Crich, D.; Smith, M. J. Am. Chem. Soc. 2001, 123, 9015–9020.
(15) Crich, D.; Smith, M.; Yao, Q.; Picione, J. Synthesis 2001, 323–326.
(12) The ammonium salts 6 were not stable to silica gel chromatography.
J. Org. Chem. Vol. 74, No. 24, 2009 9577

Crich and Picard
JOCNote
TABLE 2. Glycosylation Reactions of 7
SCHEME 3. Deprotection with Sodium in Liquid Ammonia
compounds 8 were generally not stable in methanolic solu-
tion when they gave the corresponding methyl glycosides.18
Finally, we turned our attention to deprotection. After
several dead ends,19 we had recourse to treatment of com-
pounds 8 with sodium in liquid ammonia when cleavage of
all benzyl esters and ethers, benzylidene acetals, and acetate
esters was achieved cleanly in excellent yields (Scheme 3).
In summary, we have shown that R-mannopyranosyl
phosphosugars can be obtained in good yield and excellent
selectivity through the use of a donor carrying a stereodir-
ecting, but nonparticipating, 3-O-carboxylate ester. This
chemistry is currently being applied to the syntheses of
capsular polysaccharide repeating units from Campylobacter
jejuni RM1221, on which we will report in due course.
Experimental Section
General Procedure for the Formation of Phosphosugars 5. To a
solution of sugars 2 (1.0 equiv) and benzimidazolium trifluoro-
methanesulfonate 4 (1.5 equiv) in dry CH2Cl2 (10 mL/mmol of
2) was added at room temperature 3 (1.2 equiv). The reaction
mixture was stirred 2 h at room temperature before it was cooled
to -78 °C and m-CPBA (2 equiv) was added. The reaction
mixture was stirred 1 h at this temperature, warmed to room
temperature, and stirred overnight at room temperature before
it was quenched with saturated aqueous NaHCO3 and then
extracted several times with CH2Cl2. The combined organic
layer was washed with brine, dried (MgSO4), and concentrated
under reduced pressure. The crude product was purified by
chromatography on silica gel (EtOAc/heptane, 1:1) to afford
phosphosugars 5.
General Procedure for Cyanoethyl Ester Cleavage. To a solu-
tion of phosphosugars 5 (1.0 equiv) in a mixture of CH2Cl2/H2O
(2:1, 9 mL/mmol of 5) was added at room temperature tetra-
butylammonium hydroxide (1 M in water, 1.2 equiv). The
reaction mixture was stirred 2 h at room temperature before it
was diluted with CH2Cl2 and washed with saturated aqueous
NaHCO3. The organic layer was washed with brine, dried
(MgSO4), and concentrated under reduced pressure. The ob-
tained ammonium salt was used for the next step without
purification.
the R-anomers 8 with very high anomeric selectivity and
good to high yield following silica gel chromatography16
(Table 2). No significant selectivity was observed at the level
of the stereogenic phosphorus center, and compounds 8 were
therefore characterized as approximately 1/1 mixtures of
diastereoisomers.
Perhaps not surprisingly in view of the occasional use of
glycosyl phosphates as glycosyl donors,17 we observed that
General Procedure for Glycosylation. To a cooled (-78 °C)
solution of 7 (1.0 equiv), BSP (1.2 equiv), and TTBP (1.5 equiv)
in dry CH2Cl2 (20 mL/mmol of 7) was added Tf2O (1.2 equiv).
The resulting orange reaction mixture was stirred 30 min
(16) The selectivity was determined by 1H and 31P NMR analyses of crude
reaction mixtures and confirmed by the determination of the 1JCH coupling
constants of pure diastereoisomers. Bock, K.; Pedersen, C. J. Chem. Soc.,
Perkin Trans. 2 1974, 293–297.
(17) (a) Hashimoto, S. I.; Honda, T.; Ikegami, S. J. Chem. Soc., Chem
Commun. 1989, 11, 685–687. (b) Plante, O. J.; Palmacci, E. R.; Andrade, R.
B.; Seeberger, P. H. J. Am. Chem. Soc. 2001, 123, 9545–9554. (c) For a review,
see: Zhu, X.; Schmidt, R. R. Angew. Chem., Int. Ed. 2009, 48, 1900–1934.
(18) For this reason, ESIMS analyses of compounds 8 were conducted
with solutions in acetonitrile.
(19) With a view to stabilizing the glycosyl phosphate linkage before
removal of the benzylidene acetal, compounds 8 were first treated with NaI in
acetone to afford quantitatively the corresponding stable sodium phos-
phates. Unfortunately, hydrogenolyses of these compounds with a variety
of catalysts (Pd/C or Pd(OH)2/C) were either incomplete or resulted in
complex mixtures.
9578 J. Org. Chem. Vol. 74, No. 24, 2009

Crich and Picard
JOCNote
at -78 °C, and then a solution of ammonium phosphosugars 6
(1.5 equiv) in dry CH2Cl2 (1 mL/mmol) was added. The reaction
mixture was stirred 20 min at -78 °C, warmed slowly to room
temperature, and stirred 1 h at room temperature. The reaction
was quenched with saturated aqueous NaHCO3 and then
extracted twice with CH2Cl2. The combined organic layer was
washed with brine, dried (MgSO4), and concentrated under
reduced pressure. The crude product was purified by chro-
matography on silica gel (30% EtOAc in heptane) to afford
the R-D-mannopyranosyl phosphosugars 8.
7.16-7.35 (m, 18H), 7.42 (d, J = 6.7 Hz, 2H); 13C NMR (75
MHz, CDCl3) δ 13.7, 19.8, 24.0, 54.6, 58.7, 68.6, 70.0, 71.9, 73.2,
73.5, 74.4, 75.0, 75.6, 77.2, 100.1, 126.5, 126.9, 127.2, 127.6, 127.8,
127.9, 128.2, 138.7, 139.4, 139.7, 140.2; 31P NMR (202 MHz,
CDCl3) δ - 0.73; ESI-HRMS calcd for C35H38O9P [M -
n-Bu4Nþ]- 633.2253, found 633.2263.
Benzyl (3-O-acetyl-2-O-benzyl-4,6-O-benzylidene-r-D-man-
nopyranos-1-yl) (methyl 2,4,6-tri-O-benzyl-r-D-mannopyrano-
sid-3-yl)phosphate (8g): Mixture of diastereoisomers; colorless
oil, 67%;1H NMR (500 MHz, CDCl3) δ 2.03 (s, 3H), 2.04 (s, 3H),
3.36 (s, 6H), 3.70-3.87 (m, 10H), 4.41 (d, J = 11.7 Hz, 1H),
4.46-4.62 (m, 7H), 4.68-4.88 (m, 12H), 4.97-5.15 (m, 4H),
5.26-5.31 (m, 2H), 5.54 (s, 1H), 5.55 (s, 1H), 5.79-5.84 (m, 2H),
7.21-7.48 (m, 60H); 13C NMR (125 MHz, CDCl3) δ 21.3, 55.3,
55.4, 66.1, 66.3, 68.6, 69.4, 69.8, 69.9, 70.0, 70.3, 72.0, 72.1, 73.4,
73.7, 73.8, 74.2, 74.3, 74.3, 74.4, 74.5, 74.6, 75.1, 75.4, 75.8, 75.9,
General Procedure for Deprotection with Sodium and Liquid
Ammonia. The protected R-D-mannopyranosyl phosphosugars
8 were dissolved in dry THF (0.1 mol/L) and placed in a three-
necked flask equipped with a bubbler, an ammonia inlet, and a
coldfinger condenser. The system was cooled to -78 °C, flushed
with argon, and the condenser was filled with dry ice/acetone
before ammonia gas was passed through the system. To the
ensuing stirred solution was added Na as small spheres until the
solution retained a dark blue color, after which stirring was
continued for 0.5 h before the reaction was quenched with
saturated aqueous NH4Cl, warmed to room temperature, and
concentrated under reduced pressure. The residue was puri-
fied on Sephadex LH20 (eluent absolute EtOH) to afford the
R-D-mannopyranosyl phosphosugars 9.
76.5, 76.6, 76.7, 79.6, 79.8, 96.7 (1JCH = 178 Hz), 96.8 (1JCH
=
176 Hz), 99.3 (1JCH = 171 Hz), 99.4 (1JCH = 171 Hz), 102.2,
102.3, 126.6, 126.7, 127.4, 127.5, 127.6, 127.7, 127.8, 127.9, 128.0,
128.1, 128.2, 128.4, 128.5, 128.6, 129.0, 135.4, 135.5, 137.1, 137.2,
137.9, 138.0, 138.1, 138.2, 169.7, 169.9; 31P NMR (202 MHz,
CDCl3) δ -3.65, -3.47; ESI-HRMS (solvent MeCN) calcd for
C57H61O15PNa [M þ Na]þ 1039.3646, found 1039.3638.
Ammonium (methyl r-D-mannopyranosid-3-yl) (r-D-manno-
pyranos-1-yl)phosphate (9b): White powder, 89% from 8g;
Benzyl 2-Cyanoethyl(methyl 2,4,6-tri-O-benzyl-r-D-manno-
pyranosid-3-yl)phosphate (5g): Mixture of diastereoisomers;
[R]26 þ 55 (c 0.15, CH3OH); mp 180 °C; 1H NMR (500
D
1
colorless oil, 81%; H NMR (500 MHz, CDCl3) δ 2.26-2.50
MHz, CD3OD) δ 3.38 (s, 3H), 3.50-3.56 (m, 1H), 3.61 (t, J =
9.7 Hz, 1H), 3.70 (dd, J = 6.3 Hz, J = 11.5 Hz, 1H), 3.74-3.85
(m, 5H), 3.86-3.93 (m, 2H), 4.03-4.05 (m, 1H), 4.32 (td, J = 2.0
(m, 4H), 3.36 (s, 6H), 3.71-3.86 (m, 6H), 3.88-4.14 (m, 8H),
4.51-4.60 (m, 4H), 4.68-4.80 (m, 12H), 4.98-5.12 (m, 4H),
7.20-7.40 (m, 40H); 13C NMR (75 MHz, CDCl3) δ 19.1, 19.2,
19.3, 54.9, 61.6, 61.7, 61.8, 68.9, 69.6, 69.7, 69.8, 71.5, 73.1, 73.5,
74.0, 74.4, 74.7, 77.2, 78.8, 78.9, 79.0, 98.8, 166.2, 166.3, 127.5,
127.6, 127.7, 127.9, 128.3, 128.6, 135.4, 135.5, 137.9, 138.0,
138.2; 31P NMR (202 MHz, CDCl3) δ -2.40, -2.16; ESI-
HRMS calcd for C38H42NO9PNa [M þ Na]þ 710.2495, found
710.2495.
Hz, J = 8.0 Hz, 1H), 4.68 (s, 1H), 5.51 (d, J = 7.5 Hz, 1H); 13
C
NMR (75 MHz, CD3OD) δ 55.4, 62.4, 62.8, 67.4, 68.4, 71.2,
71.9, 72.4 (d, J = 9.2 Hz), 74.4, 75.8, 77.8 (d, J = 4.8 Hz), 98.0
(d, J = 5.5 Hz), 102.5; 31P NMR (202 MHz, CD3OD) δ -1.34;
ESI-HRMS calcd for C13H24O14P [M-]- 435.0904, found
435.0918.
Tetrabutylammonium Benzyl(methyl 2,4,6-tri-O-benzyl-r-D-
mannopyranosid-3-yl)phosphate (6g): Colorless oil, 100%;
Acknowledgment. We thank the NIH (GM57335) for
partial support of this work.
1
[R]26 þ 26.8 (c 1.0, CHCl3); H NMR (500 MHz, CDCl3) δ
D
0.94 (t, J = 7.2 Hz, 12H), 1.31-1.44 (m, 8H), 1.48-1.62 (m, 8H),
3.21-3.28 (m, 8H), 3.31 (s, 3H), 3.66-3.84 (m, 3H), 3.94 (t, J =
9.5 Hz, 1H), 4.34 (s, 1H), 4.56-4.69 (m, 5H), 4.80 (d, J =12.6 Hz,
1H), 4.97 (dd, J =5.1 Hz, J =12.7 Hz, 1H), 5.02 (d, J =12.6 Hz,
1H), 5.03 (d, J = 12.4 Hz, 1H), 5.19 (d, J = 11.7 Hz, 1H),
Supporting Information Available: Preparation of 7, char-
1
acterization of 5a-f, 6a-f, 7, 8a-f, and 9a, H, 13C, and 31P
NMR spectra of 7, 5a-g, 6a-g, 8a-g, and 9a,b. This material is
available free of charge via the Internet at http://pubs.acs.org.
J. Org. Chem. Vol. 74, No. 24, 2009 9579