Stereocontrolled synthesis of the D- and L-glycero-β-D-manno- heptopyranosides and their 6-deoxy analogues. Synthesis of methyl α-L-rhamno-pyranosyl-(1→3)-D-glycero-β-D-manno-heptopyranosyl- (1→3)-6-deoxy-glycero-β-D-manno-heptopyranosyl-(1→4) -α-L-rhamno-pyranoside, a tetrasaccharide subunit of the lipopolysaccharide from Plesimonas shigelloides

Article abstract of DOI:10.1021/ja061594u

The synthesis of D- and L-glycero-α-manno-thioheptopyranosides, protected with 4,6-O-alkylidene-type acetals is described. In glycosylations carried out with preactivation with the 1-benzenesulfinylpiperidine/ trifluoromethanesulfonic anhydride couple, both the D- and L-glycero series exhibit excellent β-selectivity with a range of glycosyl acceptors. In contrast, a 4,7-O-alkylidene acetal was found not to afford β-selectivity. With a 4,6-O-[1-cyano-2-(2-iodophenyl)ethylidene] acetal protected thioglycoside, excellent β-selectivity was obtained in glycosylation reactions, and subsequent treatment with tributyltin hydride and azoisobutyronitrile brought about clean fragmentation to the 6-deoxy-glycero-β-D-manno-heptopyranosides. This chemistry was applied to the stereocontrolled synthesis of methyl α-L-rhamnopyranosyl-(1→3)-D- glycero-β-D-manno-heptopyranosyl-(1→3)-6-deoxy-glycero-β-D-manno- heptopyranosyl-(1→4)-α-L-rhamno-pyranoside, a component of the lipopolysaccharide from Plesimonas shigelloides.

Full text of DOI:10.1021/ja061594u

Published on Web 05/26/2006
Stereocontrolled Synthesis of the D- and
L-glycero-â-D-manno-Heptopyranosides and Their 6-Deoxy
Analogues. Synthesis of Methyl
r-L-Rhamno-pyranosyl-(1f3)-D-glycero-â-D-manno-heptopyranosyl-
(1f3)-6-deoxy-glycero-â-D-manno-heptopyranosyl-(1f4)-r-L-
rhamno-pyranoside, a Tetrasaccharide Subunit of the
Lipopolysaccharide from Plesimonas shigelloides
David Crich* and Abhisek Banerjee
Contribution from the Department of Chemistry, UniVersity of Illinois at Chicago,
845 West Taylor Street, Chicago, Illinois 60607-7061
Received March 7, 2006; E-mail: dcrich@uic.edu
Abstract: The synthesis of D- and L-glycero-R-manno-thioheptopyranosides, protected with 4,6-O-alkylidene-
type acetals is described. In glycosylations carried out with preactivation with the 1-benzenesulfinylpiperidine/
trifluoromethanesulfonic anhydride couple, both the D- and L-glycero series exhibit excellent â-selectivity
with a range of glycosyl acceptors. In contrast, a 4,7-O-alkylidene acetal was found not to afford â-selectivity.
With a 4,6-O-[1-cyano-2-(2-iodophenyl)ethylidene] acetal protected thioglycoside, excellent â-selectivity
was obtained in glycosylation reactions, and subsequent treatment with tributyltin hydride and azoisobu-
tyronitrile brought about clean fragmentation to the 6-deoxy-glycero-â-D-manno-heptopyranosides. This
chemistry was applied to the stereocontrolled synthesis of methyl R-L-rhamno-pyranosyl-(1f3)-D-glycero-
â-D-manno-heptopyranosyl-(1f3)-6-deoxy-glycero-â-D-manno-heptopyranosyl-(1f4)-R-L-rhamno-pyrano-
side, a component of the lipopolysaccharide from Plesimonas shigelloides.
century,4 and several methods have been developed.2,4,5 The
synthesis of a large number of complex oligosaccharides
Introduction
Structurally complex lipopolysaccharides (LPS) are amphi-
pathic and microheterogeneous glycolipids and essential com-
ponents of the outer membrane of Gram-negative bacteria.1
Numerous immunological and pathophysiological effects in
bacterial infections are mediated by LPS. Due to the increasing
resistance of many bacterial strains against conventional anti-
biotics, a detailed understanding of the immunological responses
is necessary in order to develop novel drugs, which may prevent
the assembly of a functionally intact bacterial cell wall by
inhibition of its biosynthesis.2 Thus, well-defined syntheses of
complex oligosaccharides and glycoconjugates corresponding
to the native bacterial structures are essential, as the synthetic
derivatives can be employed to (i) locate immunodominant
motifs, (ii) determine the specificity of the antibody response
obtained from native structures, and (iii) investigate the process-
ing of glycoproteins by the immune systems. Moreover, such
synthetic oligosaccharides could act as vaccine candidates.3 The
L-glycero-D-manno- and D-glycero-D-manno-heptoses, are com-
mon constituents of both the inner- and outer-core regions of
LPS of many pathogenic bacteria, where they are present mostly
in the R-anomeric configuration.2 The synthesis of higher-carbon
sugars such as heptoses has been investigated for more than a
containing R-L,D- or R-D,D-heptopyranosides (Hepp), has been
achieved by Oscarson and co-workers.3,6
Plesimonas shigelloides is a Gram-negative, flagellated, rod-
shaped bacterium which has been isolated from a variety of
sources such as fresh water, surface water, and many wild and
domestic animals. The infections correlate strongly with the
surface water contamination and are particularly common in
tropical and subtropical habitats. The structure of the O-specific
side chain of the LPS, strain CNCTC 113/92 (serotype 54) has
been recently elucidated by a combination of 1H and 13C NMR
spectroscopy, mass spectrometry, monosaccharide analysis, and
immunological methods.7 Interestingly, the polysaccharide is
(4) Gyo¨rgydea´k, Z.; Pelyva´s, I. F. Monosaccharide Sugars-Chemical Synthesis
by Chain Elongation, Degradation and Epimerization; Academic Press:
San Diego, 1998.
(5) (a) Brimacombe, J. S.; Kabir, A. B. K. M. S. Carbohydr. Res. 1986, 152,
329, and references therein. (b) Boons, G. J. P. H.; van der Marel, G. A.;
van Boom, J. H. Tetrahedron Lett. 1989, 30, 229. (c) Jørgensen, M.; Iversen,
E. H.; Madsen, R. J. Org. Chem. 2001, 66, 4625, and references therein.
(d) Read, J. A.; Ahmed, R. A.; Morrison, J. P.; Coleman, W. G., Jr.; Tanner,
M. E. J. Am. Chem. Soc. 2004, 126, 8878. (e) Read, J. A.; Ahmed, R. A.;
Tanner, M. E. Org. Lett. 2005, 7, 1457. (f) Guzlek, H.; Graziani, A.; Kosma,
P. Carbohydr. Res. 2005, 340, 2808.
(6) (a) Oscarson, S.; Ritzen, H. Carbohydr. Res. 1994, 254, 81. (b) Ekeloef,
K.; Oscarson, S. J. Carbohydr. Chem. 1995, 14, 299. (c) Bernlind, C.;
Oscarson, S. Carbohydr. Res. 1997, 297, 251. (d) Bernlind, C.; Oscarson,
S. J. Org. Chem. 1998, 63, 7780. (e) Bernlind, C.; Bennett, S.; Oscarson,
S. Tetrahedron: Asymmetry 2000, 11, 481. (f) Segerstedt, E.; Manerstedt,
K.; Johansson, M.; Oscarson, S. J. Carbohydr. Chem. 2004, 23, 443.
(1) Raetz, C. R. H.; Whitfield, C. Annu. ReV. Biochem. 2002, 71, 635.
(2) Zamyatina, A.; Gronow, S.; Puchberger, M.; Graziani, A.; Hofinger, A.;
Kosma, P. Carbohydr. Res. 2003, 338, 2571.
(3) Oscarson, S. Carbohydr. Polym. 2001, 44, 305.
9
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J. AM. CHEM. SOC. 2006, 128, 8078-8086
10.1021/ja061594u CCC: $33.50 © 2006 American Chemical Society

D- and
L
-glycero-
â
-D-manno-Heptopyranosides
A R T I C L E S
Scheme 1. 4,6-O-[1-Cyano-2-(2-iodophenyl)ethylidene]
Benzylidene Radical Fragmentation
Figure 1. Repeating unit of the O-specific polysaccharide from CNCTC
113/92 LPS (serotype 54).
composed of a novel hexasaccharide repeating unit (Figure 1)
containing two unusual â-linked heptose units, one of which is
6-deoxy.
These unique structural features of the repeating unit prompted
our investigation of stereocontrolled â-glycosylation of the D,D-
and L,D-heptoses. The synthetic challenge was heightened by
the presence of the 6-deoxy-glycero-â-D-manno-hepto-
pyranoside. To design a concise synthesis of the hexasaccharide
repeating unit, we needed to address two issues: (i) stereo-
selective â-glycosydation in D,D- and L,D-heptoses and (ii)
efficient generation of the 6-deoxy-â-Hepp unit with full
regiocontrol at the 6-position and stereocontrol at the anomeric
position. The first problem is conceptually similar to that of
stereocontrolled â-mannosylation, for which we developed a
powerful method using 4,6-O-benzylidene-blocked thiomanno-
sides,8a,b and/or their sulfoxides.8c Our interest in these molecules
was further spurred by the insight that might be gained into the
underlying reasons for the beneficial influence of the 4,6-O-
benzylidene acetal on the stereocontrolled preparation of â-man-
nosides. Originally, working in the gluco-series, Fraser-Reid and
co-workers suggested that trans-fused 4,6-O-benzylidene pro-
tecting groups restrict the flexibility of the pyranose ring,
resulting in an oxacarbenium ion intermediate with a computed
(PM3) 20° twist in the ideally syn-coplanar C5-O5-C1-C2
system.9 In a subsequent paper, differential solvation was also
computed to be of significance in these so-called torsional
disarming effects.10 More recently, Bols and co-workers pro-
vided experimental evidence in support of the notion that the
disarming effect of the 4,6-O-acetal group is mainly due to the
locking of the C5-C6 bond in the deactivating tg-conforma-
tion.11 We hypothesized that the inclusion of either an extra
axial C6 substituent, as in the 4,6-O-benzylidene-protected
L-glycero-D-manno-heptoses, or a corresponding equatorial
substituent, as in the D-glycero-D-manno series, would influence
both torsional and solvation considerations differently, whereas
the tg conformation of the C5-C6 bond would be unchanged.
Comparisons of either reactivity or stereoselectivity between
the two diastereomeric series might therefore differentiate
between the two rationales for the 4,6-O-benzylidene group
effect.
derivatives at the 6-position, which is best achieved after
glycosylation so as to take advantage of the directing effect of
a 4,6-O-benzylidene acetal-type protecting group.12 In view of
the susceptibility of benzyl ethers to hydrogen atom abstraction,
neither the Hanessian-Hullar13 reaction nor the related Roberts’
protocol14 for the cleavage of benzylidene acetals are applicable
to the type of complex oligosaccharide synthesis envisaged. With
this problem in mind, we initially developed the 4,6-O-[R-(2-
(2-iodophenyl)ethylthiocarbonyl)benzylidene] group as a sur-
rogate for the 4,6-O-benzylidene acetal15a and applied it in the
total synthesis of the LPS E. hermanii ATCC 33650/33652.15b
However, this method suffers from two limitations: the lengthy
synthesis of the thiol and the transesterification reaction, required
to introduce the thiol ester, which reduces functional group
compatibility. An improved second-generation system, the
1-cyano-2-(2-iodophenyl)ethylidene group, was developed and
employed successfully in the synthesis of â-D-rhamno-pyrano-
sides (Scheme 1).16 This new 4,6-O-benzylidene surrogate can
be easily prepared and introduced under mild conditions.
Moreover, it is compatible with a wide variety of protecting
groups. However, one question remained in the application of
this second-generation system to the synthesis of the 6-deoxy-
â-Hepp unit, namely the regioselectivity of the radical frag-
mentation, which might potentially give either a 4- or a 6-deoxy
system. Nevertheless, based on the understanding of the radical
(12) (a) Crich, D.; Picione, J. Org. Lett. 2003, 5, 781. (b) Crich, D.; Vinod, A.
U. J. Org. Chem. 2003, 68, 8453. (c) Crich, D.; Hutton, T.; Banerjee, A.;
Jayalath, P.; Picione, J. Tetrahedron: Asymmetry 2005, 70, 8064.
(13) (a) Hanessian, S.; Plessas, N. R. J. Org. Chem. 1969, 34, 1035. (b)
Hanessian, S.; Plessas, N. R. J. Org. Chem. 1969, 34, 1045. (c) Hanessian,
S.; Plessas, N. R. J. Org. Chem. 1969, 34, 1053. (d) Chana, J. S.; Collins,
P. M.; Farnia, F.; Peacock, D. J. J. Chem. Soc Chem. Commun. 1988, 2,
94. (e) Binkley, R. W.; Goewey, G. S.; Johnston, J. C. J. Org. Chem. 1984,
49, 992. (f) Hanessian, S. Org. Synth. 1987, 65, 243. (g) Hullar, T. L.;
Siskin, S. B. J. Org. Chem. 1970, 35, 225. (h) Hanessian, S. AdV. Chem.
Ser. 1968, 74, 159. (i) Szarek, W. A. AdV. Carbohydr. Chem. Biochem.
1973, 28, 225. (j) Paulsen, H. AdV. Carbohydr. Chem. Biochem. 1971, 26,
127. (k) Gelas, J. AdV. Carbohydr. Chem. Biochem. 1981, 39, 71.
(14) (a) Roberts, B. P.; Smits, T. M. Tetrahedron Lett. 2001, 42, 3663. (b)
Roberts, B. P.; Smits, T. M. Tetrahedron Lett. 2001, 42, 137. (c) Dang,
H.-S.; Roberts, B. P.; Sekhon, J.; Smits, T. M. Org. Biomol. Chem. 2003,
1, 1330. (d) Fiedling, A. J.; Franchi, P.; Roberts, B. P.; Smits, T. M. J.
Chem. Soc., Perkin Trans. 2 2002, 155. (e) Cai, Y.; Dang, H.-S.; Roberts,
B. P. J. Chem. Soc., Perkin Trans. 1 2002, 2449. (f) Jeppesen, L. M.; Lundt,
I.; Pedersen, C. Acta Chem. Scand. 1973, 27, 3579.
The second issue perhaps is more intriguing and is analogous
to the problems associated with the synthesis of â-D-rhamno-
pyranosides. The problem of the â-D-rhamno-pyranosides
reduces to one of the regioselective deoxygenation of D-mannose
(7) (a) Niedziela, T.; Lukasiewicz, J.; Jachymek, W.; Dzieciatkowska, M.;
Lugowski, C.; Kenne, L. J. Biol. Chem. 2002, 277, 11653. (b) Czaja, J.;
Jachymek, W.; Niedziela, T.; Lugowski, C.; Aldova, E.; Kenne, L. Eur. J.
Biochem. 2000, 267, 1672.
(8) (a) Crich, D.; Smith, M. J. Am. Chem. Soc. 2001, 123, 9015. (b) Crich,
D.; Lim, L. B. L. Org. React. 2004, 64, 115. (c) Crich, D.; Sun, S.
Tetrahedron 1998, 54, 8321.
(9) Fraser-Reid, B.; Wu, Z.; Andrews, C. W.; Skowronski, E.; Bowen, J. P. J.
Am. Chem. Soc. 1991, 113, 1434.
(10) Andrews, C. W.; Rodebaugh, R.; Fraser-Reid, B. J. Org. Chem. 1996, 61,
5280.
(15) (a) Crich, D.; Yao, Q. Org. Lett. 2003, 5, 2189. (b) Crich, D.; Yao, Q. J.
Am. Chem. Soc. 2004, 126, 8232.
(16) Crich, D.; Bowers, A. A. J. Org. Chem. 2006, 71, 3452.
(11) Jensen, H. H.; Nordstrøm, L. U.; Bols, M. J. Am. Chem. Soc. 2004, 126,
9205.
9
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A R T I C L E S
Crich and Banerjee
Scheme 3. Synthesis of D-glycero-D-manno-Heptothioglycoside
Figure 2. Tetrasaccharide moiety in the repeating unit of the polysaccha-
ride.
Scheme 4. Inversion of Configuration at C6
fragmentation step and keeping the steric environment in mind,
we were confident that the desired 6-deoxy product would be
the preferred one. We report here on the successful implementa-
tion of this strategy and illustrate it with a concise, fully
stereocontrolled synthesis of the tetrasaccharide moeity (Figure
2), containing all the unique and unusual features present in
the hexasaccharide repeating unit of the LPS Plesimonas
shigelloides.
concomitant formation of diene 4 in 14% yield.24 Use of the
Nysted reagent25 for this transformation also generated com-
pound 4, along with compound 3, however, in higher yield,
whereas the Petasis reagent26 gave predominantly decomposition
products. Treatment of olefin 3 with OsO4 (5 mol%) and NMNO
at 0 °C, and room temperature, furnished diols 5 and 6 in 5:1
and 3:1 diastereomeric ratios and in 79% and 81% yields,
respectively (Scheme 2). The stereochemical outcome of this
dihydroxylation follows Kishi’s empirical rule;27 the relatively
poor diastereoselectivity starting from the sugar with 2,3-erythro
configuration is also consistent with literature precedent.5c
Application of the Sharpless asymmetric dihydroxylation pro-
tocol was unsatisfactory for this transformation in our hands.5f,23,28
Compound 5 was converted uneventfully to the D-glycero-D-
manno-heptothioglycoside 8 by oxidative ring closure with DDQ
in 87% yield29 and subsequent silylation of the residual hydroxyl
group in 7 with TBDPSCl in 98% yield (Scheme 3). To obtain
a significant quantities of the L,D-epimer, the primary hydroxyl
group of 5 was silylated, after which Mitsunobu inversion23
afforded the inverted ester 9 in 92% yield over two steps.
Hydrolysis of the ester function then gave the epimeric L,D-
heptose 10 in 95% yield (Scheme 4).
Results and Discussion
Stereoselective Glycosylation in the glycero-manno-Hep-
topyranoside Series.17 The synthesis of a diastereomeric
glycosyl donor pair (Scheme 2) began with the known 4,6-O-
p-methoxybenzylidene-protected thiomannoside 1,18 which was
regioselectively opened to the primary alcohol 2 exclusively in
90% yield with DIBAL-H in dichloromethane.19 Comparable
results were obtained with scandium triflate-catalyzed, BH3-
THF-mediated reduction;20 all other standard protocols21 gave
mixtures of isomers and considerable cleavage of the acid-labile
p-methoxybenzyl group. Swern oxidation22 of 2 gave the
corresponding unstable23 aldehyde, which was carried on to the
next step, Wittig olefination,22 without purification. The yield
(65%) of the olefination product 3 was compromised by the
Scheme 2. Preparation of Heptoses 5 and 6
Oxidative cyclization of the diol 6 in the L,D-series was
difficult due to the developing 1,3-diaxial interaction in the
acetal ring. The result was the consistent formation of the
hydrolysis product 12 and the less-strained 4,7-O-benzylidene
aetal 13, together with the desired acetal 11. The structures of
these acetals (11 and 13) were confirmed by transforming the
residual hydroxyl groups to the acetate esters (14 and 15), when
the anticipated chemical shift changes were observed (Scheme
5).
The oxidative cyclization of compound 10 was also com-
paratively slow, and oxidative cleavage of the p-methoxybenzyl
(17) For preliminary communication, see: Crich, D.; Banerjee, A. Org. Lett.
2005, 7, 1395.
(18) Crich, D.; Li, W.; Li, H. J. Am. Chem. Soc. 2004, 126, 15081.
(19) (a) Takano, S.; Akiyama, M.; Sato, S.; Ogasawara, K. Chem. Lett. 1983,
1593. (b) Mikami, T.; Asano, H.; Mitsunobu, O. Chem. Lett. 1987, 2033.
(20) Wang, C.-C.; Luo, S.-Y.; Shie, C.-R.; Hung, S.-C. Org. Lett. 2002, 4, 847.
Note: This method gave 84% yield but generated more diol compared to
the DIBAL-H reaction.
(23) Dong, L.; Roosenberg, J. M., II; Miller, M. J. J. Am. Chem. Soc. 2002,
124, 15001.
(24) Epimerization at C5 was also observed, but was substantially eliminated
by carrying out the reaction at low temperature.
(25) (a) Nysted, L. N. U.S. Patent 3,865,848, 1975. (b) Matsubara, S.; Sugihara,
M.; Utimoto, K. Synlett 1998, 313.
(21) (a) Johansson, R.; Samuelsson, B. J. Chem. Soc., Chem. Commun. 1984,
201. (b) Liptak, A.; Jodal, I.; Nanasi, P. Carbohydr. Res. 1975, 44, 1. (c)
Sakagami, M.; Hamana, H. Tetrahedron Lett. 2000, 41, 5547. (d) Jiang,
L.; Chan, T.-K. Tetrahedron Lett. 1998, 39, 355. (e) Garegg, P. J. In
Preparative Carbohydrate Chemistry; Hanessian, S., Ed.; Marcel Dekker:
New York, 1993; p 53.
(26) Petasis, N. A.; Bzowei, E. I. J. Am. Chem. Soc. 1990, 112, 6392.
(27) (a) Cha, J. K.; Christ, W. J.; Kishi, Y. Tetrahedron 1984, 40, 2247. (b)
Cha, J. K.; Kim, N.-S. Chem. ReV. 1995, 95, 1761.
(28) Gurjar, M. K.; Talukdar, A. Tetrahedron 2004, 60, 3267.
(29) (a) Oikawa, Y.; Yoshioka, T.; Yonemitsu, O. Tetrahedron. Lett. 1982, 23,
889. (b) Lergenmu¨ller, M.; Nukada, T.; Kuramochi, K.; Dan, A.; Ogawa,
T.; Ito, Y. Eur. J. Org. Chem. 1999, 1367. (c) Mulzer, J.; Berger, M. J.
Org. Chem. 2004, 69, 891.
(22) Leeuwenburgh, M. A.; Kulker, C.; Duynstee, H. I.; Overkleeft, H. S.; van
der Marel, G. A.; van Boom, J. H. Tetrahedron 1999, 55, 8253.
9
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D- and
L
-glycero-
â
-D-manno-Heptopyranosides
A R T I C L E S
Scheme 5. Oxidative Cyclization of L-glycero-D-manno-Heptose
Figure 3. Diagnostic NOE correlations for donors 8 and 16.
Following our standard thioglycoside activation protocol, a
series of glycosylation reactions were then conducted with
donors 8 and 16 with the aid of 1-benzenesulfinyl piperidine
and trifluoromethanesulfonic anhydride (BSP/Tf20)8a in the
presence of the hindered base 2,4,6-tri-tert-butylpyrimidine
(TTBP)30 in dichloromethane at -60 °C (Scheme 7). All
couplings were highly â-selective and proceeded in high yields
(Table 1). A minor exception to the otherwise exclusive
â-selectivity observed with the 4,6-O-p-methoxybenzylidene
protection was the coupling of donor 16 to the acceptor 19
(Table 1, entry 2). This may represent a small difference in
reactivity between the two donors, which only becomes apparent
with less reactive acceptors such as 19.31 Interestingly, the
coupling of donor 18, with its 4,7-O-benzylidene acetal, with
acceptor 19 yielded the R-disaccharide predominantly (Table
1, entry 7), once again demonstrating the critical influence of
the 4,6-O-benzylidene protecting group in the stereoselective
â-glycosylation. The assignment of anomeric stereochemistry
of all the glycosides in Table 1, and all subsequent schemes,
was made on the basis of 1JCH coupling constants.32 Furthermore,
in all products arising from donors 8 and 16, a relatively upfield
chemical shift (δ 3.25-3.50 in CDCl3) of the heptose H5
resonance was observed, as is characteristic of 4,6-O-ben-
zylidene-protected â-mannosides.8c
group was again a competing reaction producing diol 17 in 48%
yield along with the desired L-glycero-D-manno-heptothiogly-
coside 16 in 43% yield. In the face of this difficulty, a protocol
was devised whereby the reaction mixture from treatment of
10 with DDQ was treated with p-methoxybenzaldehyde di-
methyl acetal and catalytic camphorsulfonic acid (CSA) to give
16 in 80% yield over two steps (Scheme 6). To investigate the
role of a bulky protecting group adjacent to the free hydroxyl
group involved in the oxidative cyclization reaction, we silylated
the primary hydroxyl group of 5 with TBDPSCl and treated
the resulting silyl ether with DDQ. As expected, no significant
change in the yield or reaction time was observed and 8 was
obtained in 84% yield over two steps (Scheme 6). To investigate
the influence of a seven-membered acetal on the anomeric
stereoselectivity in the glycosylation reaction, the 4,7-O-p-
methoxybenzylidene acetal 13 was also converted into a glycosyl
donor. Thus, 13 was silylated with TBDPSCl to produce 18 in
91% yield (Scheme 6). At this stage, the NOE correlations
(Figure 3) served to confirm both the configuration at C6 in
compounds 8 and 16 and the equatorial nature of the p-
methoxyphenyl group in the two acetals.
Scheme 7. Glycosylation with the BSP/Tf2O Couple
In donor 18, with its more pliable trioxa[5.4.0]bicycloundecene
core, the C5-C6 bond is not held rigidly in the tg-conformation
but is forced to adopt an approximate gg-conformation. Extend-
ing the logic of Bols, this reduces its destabilizing influence on
any oxacarbenium ion intermediate and leads, ultimately, to the
R-selectivity. Alternatively, it can be argued that the greater
conformational mobility of this system minimizes any torsional
effect on formation of the oxacarbenium ion. Thus, this system
alone does not provide a means of differentiating between the
two rationales for the benzylidene effect. However, the com-
parable â-selectivity obtained with donors 8 and 16, both of
which have the rigid tg-conformation about the C5-C6 bond
but which differ in the position of the C6 substituent, suggests
that torsional effects are not the major contributors to the
directing effect of the benzylidene acetal.
Scheme 6. Preparation of Glycosyl Donors
Turning to deprotection, glycosides 23, 24â, and 25 were first
exposed to TBAF in THF. This was followed by the removal
of the p-methoxybenzylidene acetals and the acetonides with
(30) Crich, D.; Smith, M.; Yao, Q.; Picione, J. Synthesis 2001, 323.
(31) Paulsen, H. Angew. Chem., Int. Ed. Engl. 1982, 21, 155.
(32) Bock, K.; Pedersen, C. J. Chem. Soc., Perkin Trans. 2 1974, 293.
9
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A R T I C L E S
Crich and Banerjee
Table 1. Diastereoselective Glycosylation Reaction
catalytic CSA in methanol at reflux. Finally, hydrogenolysis
over Pd/C afforded the fully deprotected disaccharides (Table
2).
determining the regiochemistry of this reaction, as does the
nature of the 3-O-protecting group. In the case of 3-O-allyl-
protected thiomannoside 37, when the reaction was conducted
at -78 °C, a 3:1 mixture of the regioisomers was obtained
favoring the undesired secondary alcohol. Fortunately, at 0 °C,
the primary alcohol was obtained exclusively. It was also
necessary to add the solution of DIBAL-H slowly to avoid any
removal of the acid-labile p-methoxybenzyl group. Conve-
niently, in the case of the 3-O-benzyl-protected thiomannoside
1, no other regioisomer was obtained at -78 °C. Moreover,
the p-methoxybenzyl group is less susceptible to removal at
this temperature.
A number of one-carbon nucleophiles were surveyed with
the aldehyde, obtained from 38 by Swern oxidation, in an
attempt to shorten the synthesis of the heptose unit. These
included organometallic reagents from benzyloxymethyl
chloride,6d,34 (benzyloxymethyl)tributylstannane,35 and benzyl-
The synthesis of the tetrasaccharide (Figure 2), in principle,
can be achieved in a linear fashion by a sequence involving
three glycosylation reactions, with intermediate deprotection
steps for unmasking the 3-OH group at the nonreducing end,
and one radical fragmentation reaction. Two different L-
rhamnose building blocks and one common heptose building
block are required to achieve the task. Thus, diol 3618 was
converted to the 3-O-allyl ether by treatment with dibutyltin
oxide33 and then allyl bromide. Standard benzylation with benzyl
bromide and sodium hydride next afforded the protected
thioglycoside 37 in 74% yield over two steps. Treatment with
DIBAL-H in dichloromethane opened the 4,6-O-benzylidene
acetal regioselectively to primary alcohol 38 exclusively in 89%
yield (Scheme 8). Temperature plays an important role in
(33) (a) David, S.; Hanessian, S. Tetrahedron 1985, 41, 643. (b) David, S. In
PreparatiVe Carbohydrate Chemistry; Hanessian, S., Ed.; Dekker: New
York, 1997; p 69.
(34) (a) Zamojski, A. Polish J. Chem. 2003, 71, 257. (b) Oscarson, S. Top.
Curr. Chem. 1997, 186, 171. (c) Blomberg, C.; Hartog, F. A Synthesis
1977, 18.
9
8082 J. AM. CHEM. SOC. VOL. 128, NO. 24, 2006

D- and
L
-glycero-
â
-D-manno-Heptopyranosides
A R T I C L E S
Table 2. Deprotection of the Disaccharides
a Treatment with TBAF in THF followed by CSA in methanol. b Hydrogenolysis over Pd/C in methanol.
Scheme 8. Preparation of Heptoses 39 and 40
Scheme 9. Preparation of glycero-manno-Heptoside Donors 42
and 43
Table 3. Chain Elongation with Various One-Carbon Nucleophiles
reagent
% yield
39 (D,D):40 (L,D)
BnOCH2Cl, Mg, HgBr2
Bu3SnCH2OBn, nBuLi
44
41
38
1:1.15
1:1.15
1:1
glycero-D-manno-heptothioglycoside 43 in 87% yield (Scheme
9). The equatorial orientations of the hydroxymethyl group and
benzylidene group in 43 were confirmed by NOE correlations.
On the basis of stereoelectronic considerations42 and previous
X-ray crystollagraphic analysis,16 it is inferred that the nitrile
group in 42 adopts the axial orientation. Donor 42 was then
glycosylated with 20 by prior activation with Ph2SO/Tf2O
combination:43 a stronger activator than BSP/Tf2O required due
to the strong electron-withdrawing, disarming nature of the
nitrile group.16 For the same reason, a higher temperature (-20
°C) is also required for complete activation. Under these
conditions, disaccharide 44 was obtained with complete â-se-
lectivity, albeit in moderate yield (42%) (Scheme 10). In the
NMR spectrum of the reaction mixture, no other glycosylated
product was observed. However, a complex mixture of com-
pounds, in which both terminal and internal olefinic signals
derived from the allyl group were clearly invisible, made up
the mass balance, indicating incompatibility of the allyl group
oxymethyl 2-pyridyl sulfone.36 The results of these reactions37
are presented in Table 3.
Treatment of the L,D-diol 40 with ortho ester 4141 in the
presence of catalytic CSA followed by BF3-OEt2-promoted
cyanation furnished the thioglycoside donor 42 in 62% yield
over two steps. Similarly, the D,D-isomer 39 was treated with
benzylidene dimethyl acetal and catalytic CSA to give D-
(35) (a) Ferna´ndez-Meg´ıa, E.; Ley, S. V. Synlett 2000, 4, 455. (b) Still, W. C.
J. Am. Chem. Soc. 1978, 100, 1481.
(36) Skrydstrup, T.; Jespersen, T.; Beau, J.-M.; Bols, M. Chem Commun. 1996,
515. The SmI2 was freshly prepared prior to the reaction following
Imamoto’s protocol. Imamoto, T.; Ono, M. Chem. Lett. 1987, 16, 501.
(37) The reaction yields are presented for a three-step protocol involving Swern
oxidation, chain elongation, and removal of the p-methoxybenzyl group
with TFA.38 Considering the aldehyde to be a poor electrophile,39 anhydrous
40
CeCl3 was used in the case of Grignard reaction, but no significant
improvement was observed.
(42) (a) Eliel, E.; Nadar, F. W. J. Am. Chem. Soc. 1970, 92, 584. (b) Eliel, E.;
Nadar, F. W. J. Am. Chem. Soc. 1970, 92, 3050. (c) Rychnovsky, S. D.;
Kim, J. Tetrahedron Lett. 1991, 32, 7223. (d) Wardrop, D. J.; Velter, A.
I.; Forslund, R. E. Org. Lett. 2001, 3, 2261. (e) Wardrop, D. J.; Forslund,
R. E.; Landrie, C. L.; Velter, A. I.; Wink, D.; Surve, B. Tetrahedron:
Asymmetry 2003, 14, 929.
(43) (a) Code´e, J. D. C.; Litjens, R. E. J. N.; den Heeten, R.; Overkleeft, H. S.;
van Boom, J. H.; van der Marel, G. A. Org. Lett. 2003, 5, 1519. (b) van
den Bos, L. J.; Code´e, J. D. C.; van Boom, J. H.; Overkleeft, H. S.; van
der Marel, G. A. Org. Biomol. Chem. 2003, 1, 4160.
(38) The diastereomeric mixture was separated only after removal of the
p-methoxybenzyl group. Yan, L.; Kahne, D. Synlett 1995, 523.
(39) Achkar, J.; Sanchez-Larraza, I.; Johnson, C. A.; Wei, A. J. Org. Chem.
2004, 70, 214.
(40) (a) Dimitrov, V.; Kostava, K.; Genov, M. Tetrahedron Lett. 1996, 37, 6787.
(b) Evans, W. J.; Feldman, J. D.; Ziller, J. W. J. Am. Chem. Soc. 1996,
118, 4581.
(41) The ortho ester 41 was prepared from 2-iodophenyl acetonitrile in 51%
yield16 and used without further purification.
9
J. AM. CHEM. SOC. VOL. 128, NO. 24, 2006 8083

A R T I C L E S
Crich and Banerjee
Scheme 10. Preparation of the Disaccharide 45
Scheme 12. Preparation of Donors 53 and 54
with the glycosylation conditions applied.44 Dropwise addition
of tributyltin hydride and AIBN to a solution of the disaccharide
44 in xylene at reflux and subsequent removal of the tin residues
46
which furnished the diol 52 in 82% yield (Scheme 12).
Treatment of thioglycoside 52 with ortho ester 41 in the presence
of catalytic CSA followed by BF3-OEt2-promoted cyanation
gave the thioglycoside donor 53, precursor to the deoxy-
heptoside unit, in 86% yield. Similarly, D-glycero-D-manno-
heptothioglycoside 54 was obtained by reacting 52 with
benzaldehyde dimethyl acetal and catalytic CSA in 89% yield
(Scheme 12). NOE correlations were used to verify the
equatorial orientations of the benzyloxymethyl group and
benzylidene group in 54 and, by implication, 53.
with NaBH4 afforded the 6-deoxy product 45 exclusively in
52% yield (Scheme 10). In the 1H NMR spectrum of the reaction
mixture, there was no indication of the formation of the
regioisomeric 4-deoxy product. The hydrolysis of the 4-O-(2-
cyanophenylacetyl) group, generated after the radical fragmenta-
tion, was the result of the NaBH4 work up.
Synthesis of the Tetrasaccharide. On the basis of the above
studies, a final strategy was evolved employing a benzyl-type
ether protecting group on O3 of the eventual 6-deoxy heptoside
donor, with all acetal protecting groups being introduced from
the more favorable D,D-series. Moreover, the original Wittig
olefination/osmoylation sequence for homologation was con-
sidered more practical than any of the organometallic methods
investigated. Thus, the thioglycoside 46, obtained by treatment
of diol 36 with dibutyltin oxide and 2-naphthylmethyl bromide47
followed by benzyl bromide and sodium hydride, was subjected
to the sequence of reactions shown in Scheme 11 to give diols
50 and 51. The stereochemistry at the C6 position of the minor
L-glycero-D-manno-heptothioglycoside 51 was inverted by the
Mitsunobu protocol (Scheme 11).
The donor 53 was then glycosylated with methyl rhamnoside
20, following preactivation with the Ph2SO/Tf2O combination
in the presence of TTBP in dichloromethane at -20 °C, to give
an 86% yield of the disaccharide 55 with complete â-selectivity
(Scheme 13). Exposure of 55 to DDQ removed the 2-naphth-
ylmethyl ether,47c providing alcohol 56 in 82% yield. Dropwise
addition of tributyltin hydride and AIBN to a solution of the
disaccharide 56 in xylene at reflux and subsequent removal of
the tin residues with NaBH4 afforded the 6-deoxy product 57
exclusively in 61% yield (Scheme 13). The NMR spectrum of
the reaction mixture again showed no indication of the formation
of the regioisomeric 4-deoxy product. Unfortunately, the NaBH4
treatment hydrolyzed the 2-cyanophenyl acetyl ester generated
on radical fragmentation. To avoid this problem, a solution of
55 in xylene at reflux was treated with tributyltin hydride and
AIBN to afford the 6-deoxy-manno-heptopyranoside 58 as a
single regioisomer. The reaction mixture was subsequently
treated with DDQ to remove the 2-naphthylmethyl group. This
oxidative treatment also facilitated chromatographic purification
and eliminated the need for the NaBH4 treatment. The disac-
charide acceptor alcohol 58 was obtained in 57% yield over
the two steps (Scheme 14).
Scheme 11. Preparation of Heptoses 50 and 51
Scheme 13. Preparation of the Disaccharide 57
It is noteworthy that 50 is protected by means of three benzyl-
type ethers primed for selective removal. The sequence began
with the removal of the p-methoxybenzyl group with TFA,38
(44) This is somewhat unexpected as allyl ethers had previously been shown to
be compatible with the sulfoxide glycosylation.8b However, it should be
noted that Mukaiyama has applied the Ph2SO/Tf2O combination to the
functionalization of olefins.45
9
8084 J. AM. CHEM. SOC. VOL. 128, NO. 24, 2006

D- and
L
-glycero-
â
-D-manno-Heptopyranosides
A R T I C L E S
Scheme 14. Preparation of the Tetrasaccharide
Scheme 15. Completion of the Synthesis
Synthesis of the desired trisaccharide was performed by
activating the D-glycero-D-manno-heptothioglycoside 54 with
BSP and Tf2O in the presence of TTBP in dichloromethane at
-60 °C, followed by addition of 58 in dichloromethane.
Subsequent removal of the 2-naphthylmethyl ether with DDQ
furnished the trisaccharide alcohol 59 in 88% yield (Scheme
14). The stage was now set for the final glycosylation reaction
to furnish the tetrasaccharide. Therefore, the rhamnosyl donor
6048 was activated under standard BSP/Tf2O conditions and
(47) (a) Gaunt, M. J.; Yu, J.; Spencer, J. B. J. Org. Chem. 1998, 63, 4172. (b)
Wright, J. A.; Yu, J.; Spencer, J. B. Tetrahedron Lett. 2001, 42, 4033. (c)
Xia, J.; Abbas, S. A.; Locke, R. D.; Piskorz, C. F.; Alderfer, J. L.
Tetrahedron Lett. 2000, 41, 169.
(48) Zegelaar-Jaarsveld, K.; van der Marel, G. A.; van Boom, J. H. Tetrahedron
1992, 48, 10133.
(45) (a) Matsuo, J-i.; Yamanaka, H.; Kawana, A.; Mukaiyama, T. Chem. Lett.
2003, 32, 392. (b) Yamanaka, H.; Mukaiyama, T. Chem. Lett. 2003, 32,
1192. (c) Yamanaka, H.; Matsuo, J-i.; Kawana, A.; Mukaiyama, T. Chem.
Lett. 2003, 32, 626.
(46) Crich, D.; Sun, S. J. Org. Chem. 1996, 61, 7200.
9
J. AM. CHEM. SOC. VOL. 128, NO. 24, 2006 8085

A R T I C L E S
Crich and Banerjee
Table 4. Comparison of Anomeric 1H and 13C NMR Chemical Shifts (ppm)
1H NMR
13C NMR
residues
isolateda
synthetic
isolateda
synthetic (1JCH
)
-4)-R-L-rhap-(1-(reducing end)
-3)-6d-â-D-manno-Hepp-(1-b
-3)-D-glycero-â- D-manno-Hepp-(1-
-4)-R-L-rhap-(1-(nonreducing end)
4.85 (4.83)
5.02 (5.02)
4.76 (4.74)
4.95 (4.91)
4.56
4.70
4.77
4.92
102.1 (101.9)
100.5 (100.3)
97.9 (97.8)
101.3 (163.0 Hz)
100.8 (157.6 Hz)
96.8 (157.4 Hz)
96.1 (166.1 Hz)
97.3 (96.7)
a The data collected from ref 7a, whereas the data in parentheses have taken from ref 7b. b The O2 is acylated in the polysaccharide.
Table 5. Comparison of 1H and 13C NMR Chemical Shifts (ppm) at the 6-Position
1H NMR
13C NMR
C6
H6
H6′
residues
isolateda
synthetic
isolateda
synthetic
isolateda
synthetic
-4)-R-L-rhap-(1-(reducing end)
-3)-6d-â-D-manno-Hepp-(1-
-4)-R-L-rhap-(1-(nonreducing end)
1.26 (1.24)
1.75 (1.75)
1.29 (1.32)
1.25
1.68-1.75
1.33
-
-
17.7 (17.6)
34.4 (34.4)
17.9 (18.1)
16.6
34.5
17.0
2.17 (2.18)
2.14-2.21
-
-
a The data collected from ref 7a, whereas the data in parentheses have taken from ref 7b.
allowed to react with 59 to afford the tetrasaccharide as a single
R-stereoisomer in 73% yield, following the established pattern.49
Saponification of the (2-cyanophenyl)acetyl ester with sodium
methoxide in methanol and subsequent treatment with TFA49
in dichloromethane followed by quenching the excess TFA with
tris(2-aminoethyl) amine50 afforded a mixture of 62 and 63 in
85% yield over two steps (Scheme 15).51 These were subjected
individually to hydrogenolysis over Pearlman’s catalyst giving
the tetrasaccharide 64 in 96% and 94% yields, respectively.
In summary, a method for the stereocontrolled synthesis of
both â-D-glycero-D-manno- and â-L-glycero-D-manno-heptopy-
ranosides has been developed, making possible a total synthesis
of the core tetrasaccharide unit present in the hexasaccharide
repeating unit of the LPS Plesimonas shigelloides. The 1-cyano-
2-(2-iodophenyl)ethylidene group was successfully employed
as a 4,6-O-benzylidene surrogate to construct the 6-deoxy-â-
Hepp unit by a reductive radical fragmentation method.
1
Representative H and 13C NMR chemical shift data for this
unit contained within the LPS from Plesimonas shigelloides7
are presented in Tables 4 and 5. The differences between the
two sets of data most evident toward the reducing end, probably
arise from the synthetic material being a methyl glycoside rather
than the much more hindered glycosidic bond in the natural
isolate.
Acknowledgment. We thank the NIH (GM 57335) for
support of this work and the University of Illinois at Chicago
for the award of the Moriarty Fellowship to A.B.
Supporting Information Available: Full experimental and
characterization details for all new compounds. This material
is available free of charge via the Internet at http://pubs.acs.org.
(49) Crich, D.; Li, H. J. Org. Chem. 2002, 67, 4640.
(50) Booth, R. J.; Hodges, J. C. J. Am. Chem. Soc. 1997, 119, 4882.
(51) Extended reaction time led to the onset of decomposition.
JA061594U
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8086 J. AM. CHEM. SOC. VOL. 128, NO. 24, 2006