Direct chemical synthesis of the β-D-mannans: The β-(1→2) and β-(1→4) series

Article abstract of DOI:10.1021/ja047194t

The direct syntheses of β-(1→2)-mannooctaose and of β-(1→4)-mannohexaose are reported by means of 4,6-O-benzylidene- protected β-mannosyl donors. The synthesis of the (1→2)-mannan was achieved by means of the sulfoxide coupling protocol, whereas the (1→4)

Full text of DOI:10.1021/ja047194t

Published on Web 10/20/2004
Direct Chemical Synthesis of the â-D-Mannans:
The â-(1f2) and â-(1f4) Series
David Crich,* Abhisek Banerjee, and Qingjia Yao
Contribution from the Department of Chemistry, UniVersity of Illinois, 845 West Taylor Street,
Chicago, Illinois 60607-7061
Received May 13, 2004; E-mail: dcrich@uic.edu
Abstract: The direct syntheses of a â-(1f2)-mannooctaose and of a â-(1f4)-mannohexaose are reported
by means of 4,6-O-benzylidene-protected â-mannosyl donors. The synthesis of the (1f2)-mannan was
achieved by means of the sulfoxide coupling protocol, whereas the (1f4)-mannan was prepared using
the analogous thioglycoside/sulfinamide methodology. In the synthesis of the (1f4)-mannan, the
glycosylation yields and stereoselectivities remain approximately constant with increasing chain length,
whereas those for the (1f2)-mannan consist of two groups with the formation of the tetra- and higher
saccharides giving yields and selectivities consistently lower than those of the lower homologues. The
decrease in yield after the trisaccharide in the (1f2)-mannan synthesis is attributed to steric interference
by the n-3 residue and is consistent with the collapsed, disordered structure predicted by early computational
work. The consistently high yields and selectivities seen in the synthesis of the (1f4)-mannan are congruent
with the more open, ordered structure originally predicted for this polymer. The lack of order in the structure
of the (1f2)-mannan, as compared to the high degree of order in the (1f4)-mannan, is also evident from
a comparison of the NMR spectra of the two polymers and even from their physical nature: the (1f2)-
mannan is a gum and the (1f4)-mannan is a high melting solid.
Introduction
raised against â-(1f2)-oligomannosides have been shown to
be protective against C. albicans in rodent models of systemic
The â-mannans are a group of homopolymers of D-mannopy-
ranose featuring the 1,2-cis-equatorial glycosidic bond. The
structure and function of these oligosaccharides depend heavily
on the linkage isomer. The â-(1f4)-mannan is a primary
constituent of alimentary gums and of hard and soft woods
according to the type and degree of glycosylation.1 The
galactomannans, â-(1f4)-mannans decorated at the 6-position
with varying amounts of R-galactopyranosides, are major
components of guar and other gums used in the food and
pharmaceutical industries.2 The glucomannans, linear copoly-
mers of â-(1f4)-linked mannose with â-glucopyranoside, make
up 3-5% of the dry weight of most hardwoods, whereas
partially acetylated glucomannans and galactoglucomannans (6-
O-galactosylated glucomannans) are found in soft woods.3 The
â-(1f2)-mannans, on the other hand, are critical components
of the Candida albicans cell wall phosphopeptidomannan. The
C. albicans â-(1f2)-mannans are immunogenic and elicit
specific antibodies in both humans and animals.4 Antibodies
and vaginal candidosis.5 It has been shown that â-(1f2)-
mannosides derived from C. albicans phosphopeptidomannans
induce TNF1-R synthesis from cells of the macrophage lineage
and bind to macrophage cell membranes.6 Furthermore, an
antigenic phospholipomannan has been isolated, purified, and
shown to be a strong TNF-R inducer in vivo and in vitro.7 In
addition to their role in candidosis, â-(1f2)-mannans have been
demonstrated to be components of the yeast cell wall mannan
of Torulaspora delbrueckii.8 An intriguing â-mannan in which
(1f3) and (1f4) linkages alternate is a major part of the
antigenic oligosaccharides from Leptospira biflexa.9
Any chemical synthesis of the â-mannans designed to provide
them as predetermined homogeneous samples, as opposed to
(4) (a) Faille, C.; Michalski, J. C.; Strecker, G.; Mackenzie, D. W. R.; Camus,
D.; Poulain, D. Infect. Immun. 1990, 58, 3537-3544. (b) Shibata, N.; Arai,
M.; Haga, E.; Kikuchi, T.; Najima, M.; Satoh, T.; Kobayashi, H.; Suzuki,
S. Infect. Immun. 1992, 60, 4100-4110. (c) Tojo, M.; Shibata, N.;
Kobayashi, M.; Mikami, T.; Suzuki, M.; Suzuki, S. Clin. Chem. 1988, 34,
539-543. (d) Poulain, D. C. F.; Delaunoy, C.; Jacquinot, P. M.; Trinel, P.
A.; Camus, D. Infect. Immun. 1993, 61, 1164-1166.
(5) (a) De Bernardis, F.; Boccanera, M.; Adriani, D.; Spreghini, E.; Santoni,
G.; Cassone, A. Infect. Immun. 1997, 65, 3399-3405. (b) Caesar, T. T. T.
C. J. Infect. Immun. 1997, 65, 5354-5357.
(1) Aspinall, G. O. The Polysaccharides; Academic Press: London, 1982; Vol.
1.
(2) (a) Joshi, H.; Kapoor, V. P. Carbohydr. Res. 2003, 338, 1907-1912. (b)
Whistler, R. L. Industrial Gums; Academic Press: New York, 1973; Vol.
2. (c) Dea, I. C. M.; Morrison, A. AdV. Carbohydr. Chem. Biochem. 1975,
31, 241-312.
(3) (a) Sjo¨stro¨m, E. Wood Chemistry: Fundamentals and Applications, 2nd
ed.; Academic Press: New York, 1993. (b) Fengel, D.; Wegener, G.
Wood: Chemistry, Ultrastructure, Reactions; Walter de Gruyter: Berlin,
1984. (c) Shimizu, K. In Wood and Cellulosic Chemistry; Hon, D. N.-S.,
Shiraishi, N., Eds.; Marcel Dekker: New York, 1991; pp 177-214. (d)
Hannuksela, T.; Herve´ du Penhoat, C. Carbohydr. Res. 2004, 339, 301-
312. (e) Teleman, A.; Nordstrom, M.; Tenkanem, M.; Jacobs, A.; Dahlman,
O. Carbohydr. Res. 2003, 338, 525-534.
(6) (a) Jouault, T.; Lepage, G.; Bernigauda, A.; Trinel, P. A.; Fradin, C.;
Wieruszeski, J.; Strecker, G.; Poulain, D. Infect. Immun. 1995, 63, 2378-
2381. (b) Li, R. K.; Cutler, J. E. J. Biol. Chem. 1993, 268, 18293-18299.
(c) Fradin, C.; Jouault, T.; Mallet, A.; Mallet, J. M.; Camus, D.; Sinay¨, P.;
Poulain, D. J. Leukocyte Biol. 1996, 60, 81-87.
(7) Trinel, P.-A.; Plancke, Y.; Gerold, P.; Jouault, T.; Delplace, F.; Schwarz,
R. T.; Strecker, G.; Poulain, D. J. Biol. Chem. 1999, 274, 30520-30526.
(8) Okawa, Y.; Monma, K.; Shibata, N.; Kobayashi, H.; Yamada, Y. Carbo-
hydr. Res. 2003, 338, 1175-1182.
(9) Matsuo, K.; Isogai, E.; Araki, Y. Carbohydr. Res. 2000, 328, 517-524.
9
14930
J. AM. CHEM. SOC. 2004, 126, 14930-14934
10.1021/ja047194t CCC: $27.50 © 2004 American Chemical Society

Direct Chemical Synthesis of the
â-D-Mannans
A R T I C L E S
Table 1. â-(1f2) Linkage Yields and Selectivities in the Synthesis
of 28
the heterogeneous natural isolates, must confront the issue of
the stereoselective synthesis of the challenging â-mannosidic
bond.10 In fact, the â-mannans might be seen as the ultimate
proving ground for any technology aspiring to the synthesis of
â-mannosides. Indeed, the challenge of the â-(1f2)-mannans
has been met by two groups, both using indirect methods. Thus,
Bundle and Nitz first synthesized the â-(1f2)-linked manno-
tetraose,11 then the corresponding hexaose,12 employing the
Lichtenthaler strategy of silver salt-mediated stereoselective
â-mannosylation with a 2-ulosyl bromide donor,13 thereby
allowing them to determine the solution structure of these
fascinating oligomers.14 Fraser-Reid and co-workers, more
recently, completed a synthesis of the â-(1f2)-linked man-
nooctaose, employing a strategy of stereoselective â-glucosy-
lation, selective deprotection, and inversion by a two-step
oxidation-reduction protocol.15 The â-(1f4)-linked manno-
triose has been synthesized by Nikolaev and co-workers, again,
by an indirect route involving the gluco- to manno-inversion,
by a two-step triflation-displacement sequence, following
glycosylation.16 In this paper, we present our direct syntheses
of a â-(1f2)-mannooctaose17 and a â-(1f4)-mannohexaose,
applying a stereoselective â-mannosylation sequence developed
in this laboratory.18
â
-glycoside
R
-glycoside
â
ratio
/
R
freed alcohols
(% yield)
acceptor
(% yield)
(% yield)
C6H11OH
5
7
10
13
16
19
22
4 (77)
6 (94)
8 (89)
11 (77)
14 (69)
17 (68)
20 (64)a
24 (64)a
- (0)
â only
â only
9.9/1
3.9/1
4.3/1
4.5/1
4.9/1
4.5/1
5 (85)
7 (97)
10 (91)
13 (85)
16 (80)
19 (85)
22 (67), 23 (14)
26 (71), 27 (13)
- (0)
9 (9)
12 (20)
15 (16)
18 (15)
21 (13)a
25 (14)a
a Anomeric pairs 20 and 21, and 24 and 25, isolated in 77 and 78%
yields, respectively, could not be separated chromatographically. The ratios
given were therefore determined after removal of the PMB groups when
separation was possible.
romethane at low temperature with trifluoromethanesulfonic
anhydride in the presence of the hindered base, 2,4,6-tri-tert-
butylpyrimidine (TTBP).22 This protocol results in the rapid,
clean formation of an R-mannosyl triflate,23 which is the true
glycosyl donor in an SN2-like process possibly involving a
transient contact ion pair. Cyclohexyl mannoside 4 was obtained
with high yield and selectivity (Table 1), and the â-stereochem-
istry was assigned on the basis of the unusual, somewhat upfield,
H-5 chemical shift of δ 3.33, which is diagnostic of the
â-stereochemistry in the 4,6-O-benzylidene-protected man-
nosides.18b The anomeric stereochemistry in all subsequent
coupling products was similarly assigned on the basis of the
number of upfield H-5 resonances, which were always one less
in the minor R-product compared to the major â-isomer.
Removal of the PMB group with DDQ then provided alcohol
5, thereby completing the first cycle of the two-step iterative
sequence. Repeated iteration of this protocol, with the yields
and selectivities reported in Table 1, ultimately provided the
protected mannooctaose 26 with the all â-configuration.
Results and Discussion
The synthesis of the â-(1f2)-linked mannooctaose began
with the known thioglycoside 1, which was converted to the
corresponding 2-O-p-methoxybenzyl ether 2 and then to sul-
foxide 3. This compound was obtained as a single stereoisomer
at sulfur, consistent with the precedent in the R-series,19 and
was assigned the (R)S configuration by analogy with that of
crystallographically determined structures.20 Donor 3 was
coupled to cyclohexanol, arbitrarily chosen as a capping group
for the reducing end of the octaose, by the standard protocol
involving prior activation18,21 of the sulfoxide in dichlo-
(10) (a) Barresi, F.; Hindsgaul, O. In Modern Methods in Carbohydrate
Synthesis; Khan, S. H., O’Neill, R. A., Eds.; Harwood Academic Publish-
ers: Amsterdam, 1996; pp 251-276. (b) Demchenko, A. V. Synlett 2003,
1225-1240. (c) Pozsgay, V. In Carbohydrates in Chemistry and Biology;
Ernst, B., Hart, G. W., Sinay¨, P., Eds.; Wiley-VCH: Weinheim, Germany,
2000; Vol. 1, pp 319-343. (d) Gridley, J. J.; Osborn, H. M. I. J. Chem.
Soc., Perkin Trans. 1 2000, 1471-1491.
(11) Nitz, M.; Purse, B. W.; Bundle, D. R. Org. Lett. 2000, 2, 2939-2942.
(12) Nitz, M.; Bundle, D. R. J. Org. Chem. 2001, 66, 8411-8423.
(13) (a) Lichtenthaler, F. W.; Kla¨res, U.; Szurmai, Z.; Werner, B. Carbohydr.
Res. 1998, 305, 293-303. (b) Lichtenthaler, F. W.; Lergenmu¨ller, M.;
Peters, S.; Varga, Z. Tetrahedron: Asymmetry 2003, 14, 727-736.
(14) Nitz, M.; Ling, C.-C.; Otter, A.; Cutler, J. E.; Bundle, D. R. J. Biol. Chem.
2002, 277, 3440-3446.
(15) Mathew, F.; Mach, M.; Hazen, K. C.; Fraser-Reid, B. Synlett 2003, 1319-
Inspection of Table 1 reveals that yields and selectivities in
the coupling reaction decreased dramatically after the formation
of trisaccharide 8. In other words, the glycosylation of trisac-
charide alcohol 10, and all subsequent homologues, was less
selective than that of cyclohexanol, monosaccharide 5, and
disaccharide 7. Despite the dramatic decrease in selectivity
between alcohols 7 and 10, the selectivities and yields of
subsequent couplings are all clustered in a similar range. The
implication is that there is a structural change between alcohol
7 and its homologue 10, which impacts the chemistry in a
deleterious manner. Fortuitously, alcohol 13 was obtained as
crystals suitable for X-ray diffraction; analysis of the ensuing
1322.
(16) Twaddle, G. W. J.; Yashunsky, D. V.; Nikolaev, A. V. Org. Biomol. Chem.
2003, 1, 623-628.
(17) For a preliminary communication on the 1,2-mannooctaose, see: Crich,
D.; Li, H.; Yao, Q.; Wink, D. J.; Sommer, R. D.; Rheingold, A. L. J. Am.
Chem. Soc. 2001, 123, 5826-5828.
(18) (a) Crich, D.; Sun, S. J. Org. Chem. 1997, 62, 1198-1199. (b) Crich, D.;
Sun, S. Tetrahedron 1998, 54, 8321-8348. (c) Crich, D.; Smith, M. J.
Am. Chem. Soc. 2001, 123, 9015-9020. (d) Crich, D. In Glycochemistry:
Principles, Synthesis, and Applications; Wang, P. G., Bertozzi, C. R., Eds.;
Dekker: New York, 2001; pp 53-75.
(19) Crich, D.; Sun, S.; Brunckova, J. J. Org. Chem. 1996, 61, 605-615.
(20) This selectivity is a direct result of the exo-anomeric effect, which in the
R-series, exposes one sulfur lone pair to solvent thereby predisposing it to
oxidation but shields the second underneath the pyranose ring: (a) Crich,
D.; Mataka, J.; Sun, S.; Lam, K.-C.; Rheingold, A. R.; Wink, D. J. J. Chem.
Soc., Chem. Commun. 1998, 2763-2764. (b) Crich, D.; Mataka, J.;
Zakharov, L. N.; Rheingold, A. L.; Wink, D. J. J. Am. Chem. Soc. 2002,
124, 6028-6036.
(21) (a) Kahne, D.; Walker, S.; Cheng, Y.; Engen, D. V. J. Am. Chem. Soc.
1989, 111, 6881-6882. (b) Yan, L.; Kahne, D. J. Am. Chem. Soc. 1996,
118, 9239-9248. (c) Gildersleeve, J.; Pascal, R. A.; Kahne, D. J. Am. Chem.
Soc. 1998, 120, 5961-5969.
(22) Crich, D.; Smith, M.; Yao, Q.; Picione, J. Synthesis 2001, 323-326.
(23) (a) Crich, D.; Sun, S. J. Am. Chem. Soc. 1997, 119, 11217-11223. (b)
Callam, C. S.; Gadikota, R. R.; Krein, D. M.; Lowary, T. L. J. Am. Chem.
Soc. 2003, 125, 13112-13119.
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A R T I C L E S
Crich et al.
Table 2. Glycosidic Bond Torsion Angles in Tetrasaccharide 13a
Cyc
−a
a
−b
b
−c
c−d
æ (H1-C1-O-C2′)
ψ (C1-O-C2′-H2′)
43.7
-15.3
44.4
10.7
18.3
-8.7
31.3
13.6
a Where a and d are the sugars at the reducing and nonreducing ends of
the chain, respectively.
Table 3. â-(1f4) Linkage Yields and Selectivities in the Synthesis
of 56
â
-glycoside
R
-glycoside
R
ratio
/
â
deprotection
(% yield)
esterification
(% yield)
acceptor
(% yield)
(% yield)
MeOH
36
40
44
48
30 (82)
37 (80)
41 (77)
45 (72)
49 (71)
53 (73)
31 (8)
38 (9)
42 (8)
46 (8)
50 (8)
54 (7)
1/10
1/9
1/9
1/9
1/9
35 (89)
39 (85)
43 (85)
47 (87)
51 (88)
36 (98)
40 (88)
44 (88)
48 (89)
52 (86)
52
1/10
with the synthesis described here in which the donor was never
employed in greater than a 2-fold excess.
Finally, hydrogenolysis of 26 over palladium charcoal cleanly
afforded mannooctaose 28 in high yield, whose NMR spectra
reveal the presence of eight distinct glycosidic linkages,
consistent with the persistence of the disordered structure in
aqueous solution.
In principle, the synthesis of the â-(1f4)-mannan, like that
of â-(1f2)-mannan 28, may be achieved by iteration of a two-
step protocol involving 4,6-O-benzylidene-mediated â-manno-
sylation followed by unmasking of the 4-OH group at the
reducing end of the chain by regioselective reductive cleavage
of the benzylidene acetal. This latter operation is typically
wrought with sodium cyanoborohydride and hydrogen chloride
in dry ether,27 although more recent reagent combinations are
also available for the same task.28 In the synthesis of the
â-(1f4)-mannan, preference was also given to the activation
of a thioglycoside donor with the combination of 1-benzene-
sulfonyl piperidine (BSP)18c and triflic anhydride over the triflic
anhydride-mediated sulfoxide coupling used in the chronologi-
cally earlier synthesis of â-(1f2)-mannan 28. With these
considerations in mind, activation of thioglycoside 29 with BSP
and Tf2O in the presence of TTBP, followed by the addition of
methanol, gave the methyl glycoside 30 in 82% yield along
with 8% of the R-anomer 31 in excellent agreement with similar
couplings to methanol conducted by the sulfoxide method (Table
3). Reduction of 30 with sodium cyanoborohydride and
hydrogen chloride in ether proceeded according to plan and
afforded 6-O-benzyl-4-hydroxy sugar 32 in 72% yield. When
this alcohol was coupled to donor 29 with BSP and Tf2O in the
usual manner, â-disaccharide 33 was obtained in 73% yield
together with 7% of the R-anomer 34. Unfortunately, when the
cyanoborohydride-hydrogen chloride protocol was applied to
the regioselective cleavage of the benzylidene acetal in 34, very
structure confirmed the all â-structure, initially assigned spec-
troscopically on the basis of the H-5 chemical shifts, and
revealed 13 to adopt a somewhat disordered, collapsed helical
structure in which the fourth pyranose ring is situated ap-
proximately above the first.24 In terms of reactivity, this means
that the introduction of the forth residue (i.e., the formation of
11) and of all subsequent residues is subject to additional steric
constraints arising from the proximity of the fully protected n-3
pyranose ring. The irregular nature of the collapsed helical
structure, evident from the glycosidic torsion angles in Table
2, presumably continues as the chain grows and is responsible
for the minor fluctuations in yield and selectivity apparent in
subsequent couplings.
The decrease in yield and selectivity at the level of formation
of the tetrasaccharide is not unique to the synthesis presented
here. At the outset of our work on 28, Bundle and Nitz reported
a synthesis of an analogous tetrasaccharide,11 which was
subsequently taken through two further iterations of their
sequence to the hexasaccharide,12 when similar problems
became apparent.25 In a later synthesis of an analogous
mannooctaose by Fraser-Reid and co-workers, coupling yields
remained largely constant throughout the complete sequence,
but it was specified that generous excesses of donor were
employed in order to achieve the high yields.26 This contrasts
(24) Details of the crystal structure may be found in the Supporting Information
for the original communication (ref 17).
(25) In the Bundle and Nitz synthesis, the reported yields for the formations of
the â-di-, tri-, tetra-, penta-, and hexasaccharides were 78, 60-65, 48, 51,
and 48%, respectively, with the reactions typically conducted with an
approximately 4-fold excess of glycosyl donor (refs 11 and 12).
(26) The coupling yields reported are all in the range of 83-92%, with the
specification of 7 equiv of donor in an experimental footnote (ref 15).
(27) (a) Garegg, P. J.; Hultberg, H.; Wallin, S. Carbohydr. Res. 1982, 108, 97-
101. (b) Garegg, P. J. In PreparatiVe Carbohydrate Chemistry; Hanessian,
S., Ed.; Dekker: New York, 1993; pp 53-67.
(28) Sakagami, M.; Hamana, H. Tetrahedron Lett. 2000, 41, 5547-5551.
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Direct Chemical Synthesis of the
â-D-Mannans
A R T I C L E S
large excesses of both reagents, coupled with long reaction
times, were required to bring about the complete conversion of
34. Not too surprisingly, the long reaction times and excess of
strong acid employed for this conversion ensured that the
formation of the desired product was marred by the onset of
decomposition. The increased requirement for acid and long
reaction times in the reduction of 34 is presumably linked to
the presence of 10 ether oxygens of similar basicity, as opposed
to the 6 in monosaccharide 30, which significantly reduces the
overall level of protonation at the acetal required for reduction.29
Inspection of the yields and anomeric selectivities in Table
3 shows that there is no significant decrease as the chain length
grows, which is in stark contrast to the synthesis of â-(1f2)-
mannan 28 (Table 1). This difference is in full accord with the
extended ribbonlike structure anticipated30 for â-(1f4)-mannans
as opposed to the collapsed helical structure of the â-(1f2)-
mannan.30
Deprotection of hexasaccharide 53 was achieved by saponi-
fication with sodium methoxide in hot methanol, affording
pentanol 55 in 94% yield. The synthesis was completed by
hydrogenolysis over palladium charcoal, which provided pure
56 in 87% yield.
Reasoning that this problem could only get worse as the
synthesis advanced, we turned to an alternative sequence
involving complete removal of the benzylidene group followed
by selective reprotection of the primary 6-OH group. Although
this necessitated the addition of an extra step in each repetition
of the cycle, it did provide us with the opportunity to examine
the effect of a sterically bulky O-6 protecting group in the
glycosyl acceptor.
Accordingly, treatment of methyl glycoside 30 with neopentyl
glycol and camphorsulfonic acid in dichloromethane afforded
diol 35 in 89% yield. This was then converted to monopivalate
36 in 98% yield with pivaloyl chloride, DMAP, and triethyl-
amine in dichloromethane. Coupling of 36 to donor 29 under
the standard BSP conditions afforded disaccharide 37 and it’s
R-anomer 38 in 80 and 9% yield, respectively, thereby
demonstrating the coupling reaction to be unaffected by the
bulky ester protecting O-6 of the acceptor. Removal of the
benzylidene acetal from 37 and selective introduction of the
pivalate group, ultimately giving 40, proceeded uneventfully
(Table 3), thereby validating the three-step iterative protocol.
The synthesis of hexasaccharide 53 was completed accordingly,
with the yields and selectivities for each iteration set out in Table
3. In the synthesis of 53, stereochemical assignment of the newly
formed glycosidic bond in each cycle was greatly facilitated
by the presence of only a single benzylidene group, unlike the
synthesis of 28 wherein each step introduced a further ben-
zylidene acetal. Thus, at each stage, the â-glycoside clearly
exhibited a single mannose H-5 in the region δ 3.1-3.3, whereas
the corresponding R-anomer had no resonance in this region.
In the absence of confirmatory crystal structures, we also
determined the 1JCH coupling constants for the R- and â-anomers
at each stage and found them to be in full agreement with the
assigned structures.
With the two mannans, 28 and 56, in hand, it is instructive
to compare their physical properties. In the 1H NMR spectrum
of 28, seven distinct anomeric hydrogen resonances are clearly
visible at 500 MHz, while the 13C spectrum at 125 MHz shows
eight separate anomeric carbon signals. The ability to distinguish
almost the complete set of anomeric proton and carbon
resonances in this manner is in full accord with the irregular
helical structure predicted in 197130 for this â-(1f2)-mannan,
with the structure proposed for the corresponding hexasaccharide
by Bundle based on NOE measurements14 and with the X-ray
structure of the protected tetrasaccharide 13. In stark contrast,
(30) (a) Rees, D. A.; Scott, W. E. J. Chem. Soc. B 1971, 469-479. (b) Rao, V.
S. R.; Qasba, P. K.; Balaji, P. V.; Chandrasekaran, R. Conformation of
Carbohydrates; Harwood Academic Publishers: Amsterdam, 1998.
(29) Note that the standard protocol for this cleavage already requires a
considerable excess of HCl, even for a monosaccharide (ref 28).
9
J. AM. CHEM. SOC. VOL. 126, NO. 45, 2004 14933

A R T I C L E S
Crich et al.
hexasaccharide 56, with the predicted30 more open, ordered
crystalline solid, with mp >300 °C, whereas 2 years of efforts
have so far failed to induce the gummy 28 to form crystals.
Efforts continue to obtain crystals of both mannans suitable for
X-ray analysis.
1
structure of the â-(1f4)-mannan, has very simple H and 13C
spectra resulting from extensive congruence between the various
residues in the chain. Remarkably, the 13C NMR spectrum of
56, recorded at 125 MHz, shows only two anomeric carbon
Acknowledgment. We thank the NIH (GM 57335) for
support of this work.
1
resonances in a 5:1 ratio. The H NMR spectrum has four
anomeric signals grouped into one resonance and two other
individual ones. Finally, the contrast between the disordered
28 and the highly ordered 56 is also reflected in the physical
state of the two compounds; simple evaporation of water from
a solution of 56 provides the mannohexaose as a white, micro-
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.
JA047194T
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14934 J. AM. CHEM. SOC. VOL. 126, NO. 45, 2004