Stereocontrolled synthesis of β-D-mannuronic acid esters: Synthesis of an alginate trisaccharide

Article abstract of DOI:10.1021/ja064787q

A facile synthesis route toward β-linked mannuronic acid oligomers using the corresponding 1-thiomannuronic acid esters in combination with the Ph₂SO/Tf₂O or NIS/TMSOTf reagent combinations is presented. The presence of the remotely

Full text of DOI:10.1021/ja064787q

Published on Web 09/20/2006
Stereocontrolled Synthesis of â-D-Mannuronic Acid Esters: Synthesis of an
Alginate Trisaccharide
Leendert J. van den Bos, Jasper Dinkelaar, Herman S. Overkleeft, and Gijsbert A. van der Marel*
Leiden Institute of Chemistry, Leiden UniVersity, P.O. Box 9502, 2300 RA Leiden, The Netherlands
Received July 6, 2006; E-mail: marel_g@chem.leidenuniv.nl
Table 1. Stereocontrolled Glycosidation of 1-Thio â-Mannuronic
Alginates are linear and polyanionic polysaccharides that occur
in Nature and are composed of D-mannuronic acid and L-guluronic
acid residues.1 The uronic acid monomers are interconnected
through 1,4-interglycosidic linkages that have a 1,2-cis configura-
tion. Three main alginate polymers have been reported, one
containing mainly â-D-mannuronic acid building blocks, one
containing mainly R-L-guluronic acid building blocks, and the third
composed of alternating sequences of â-D-mannuronic and R-L-
guluronic acid motifs. On the basis of their thickening and gel-
forming capacities, alginates have found wide and diverse appli-
cations as additives in food industry and coating materials.
Recently, it was discovered that mixtures of alginate oligomers
have immunomodulating properties by binding to Toll-like receptors
(TLRs) 2 and 4.2 TLRs are key players in mammalian immunity.3
They recognize molecules, mostly from microbial origin, and
instigate activation of inflammatory and antimicrobial innate
immune responses. Additionally, recognition of microbial products
by TLRs expressed on dendritic cells triggers functional maturation
of dendritic cells and leads to initiation of antigen-specific adaptive
immune responses. The availability of well-defined fragments of
alginates and functionalized derivatives thereof would be highly
useful in studying the mechanism of TLR-mediated recognition and
signal transduction leading to cytokine production. However, to
date, no synthesis of alginate oligomers has been reported.
Both â-D-mannuronic and R-L-guluronic acid oligomers have a
1,2-cis equatorial relationship, which is difficult to attain in a
stereocontrolled manner. The group of Crich has found an elegant
solution for the introduction of the similarly challenging â-D-
mannoside linkage by application of a 4,6-O-benzylidene protection
at 1-thio and 1-sulfoxide functionalized mannoside donors bearing
nonparticipating groups at the O2 and O3 positions.4,5 Upon
activation of these donors, the 4,6-O-benzylidene moiety destabilizes
the mannosyl oxacarbenium ion relative to the covalent R-mannosyl
triflate intermediate.6 The latter is thought to undergo an SN2-type
displacement with the incoming acceptor. The influence on the
reactivity (disarming effect) and the associated stereochemical
steering of the benzylidene protective group pattern can be explained
by the locking of the mannose C6-O6 bond in the tg conformation,
leading to torsional strain and more dominant electronic effects.7
The importance of these electronic effects is further highlighted
by the use of nonparticipating, powerfully electron-withdrawing
protecting groups for the stereocontrolled introduction of 1,2-cis
equatorial glycosidic linkages.8
Acids Esters Using Ph2SO/Tf2O or NIS/TMSOTf
Ph2SO/Tf2O
yield (
NIS/TMSOTf
yield (
entry
donor
acceptor
R
/
â
)
R
/
â
)
disaccharide
1
2
3
4
5
6
1
2
3
1
1
1
4
4
5
5
6
7
81% (1/>10)
66% (1/0)
55% (1/>10)
69% (1/>10)
71% (1/>10)
91% (1/>10)
8
9
10
11
12
13
58% (1/>10)
99% (1/2)
a sequential glycosylation strategy in which 1-hydroxyl and 1-thio
donors were activated with sulfonium ion based promoter systems.10
We here demonstrate that 1-thio mannuronic acid esters are
viable starting compounds for the construction of â-(1f4)-linked
mannuronic acid oligomers.
Thiomannuronic acid derivatives 1, 2, and 3 were selected to
establish their glycosylating properties in terms of yield and
stereoselectivity (Table 1).11 The 4-O-acetyl donor 1 was condensed
with primary alcohol acceptor 4 following preactivation of 1 using
in situ generated diphenylsulfonium bistriflate at -50 °C and
subsequent slow addition of 4, affording â-linked disaccharide 8
(1JCH ) 156 Hz) in 81% yield. Condensation of the 3-O-acetylated
donor 2 with acceptor 4 led to R-disaccharide 9 (1JCH ) 175 Hz),
whereas condensation of tri-O-benzyl donor 3 with the sterically
more demanding acceptor 5 furnished â-disaccharide 10 (1JCH
)
159 Hz).12 The stereochemical outcome of these condensations
indicates that the 3-O-acetyl moiety may participate through a six-
membered transition state, thereby preventing â-side attack.13 The
glycosylating properties of 4-O-acetyl donor 1 were further evalu-
ated by executing condensation reactions with the less reactive and
therefore more R-directing glycosyl acceptors 5, 6, and 7. The
condensations of 1 with 5 and 6 led to the isolation of â-disac-
charides 11 (69%) and 12 (71%), respectively. Subjection of the
sterically and electronically deactivated acceptor 7 to the glycosyl-
ation protocol did not lead to a productive coupling, indicating that
the reactivity of the acceptor plays a decisive role on the outcome
of a glycosylation.8c
Recently, we disclosed an efficient entry to suitably protected
uronic acid donors by the application of 2,2,6,6-tetramethyl-
piperidinyloxy free radical (TEMPO)/ [bis(acetoxy)iodo]benzene
(BAIB), as a regio- and chemoselective oxidation system.9
The presence of an electron-withdrawing carboxyl moiety at C5
marks such uronic acid donors as highly unreactive. A study toward
the glycosylating properties of protected 1-thio glucuronic and
galacturonic donors showed that these donors could be applied in
The finding that the introduction of 1,2-cis equatorial glycosidic
linkages can be attained with different types of donors and promoter
systems guided us to explore NIS/TMSOTf14 mediated glycosyl-
ation reactions.15 In a first attempt, a mixture of donor 1, acceptor
9
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J. AM. CHEM. SOC. 2006, 128, 13066-13067
10.1021/ja064787q CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1. Synthesis of the Alginate Trisaccharide 21a
HW40 gel filtration afforded deprotected trisaccharide 21 in 35%
yield over the three steps.
In summary, a facile synthesis route toward â-linked mannuronic
acids is presented. The carboxylic ester function in the mannuronic
acid donors sufficiently influences the electronic environment
around the anomeric center to allow good to excellent â-selectivities
in Ph2SO/Tf2O and NIS/TMSOTf mediated glycosylation events.
Further research is devoted to the synthesis of extended â-
mannuronic acid oligomers, the development of a similar strategy
to the assembly of all types of alginate oligomers, and the biological
evaluation of these compounds.
a Reagents and conditions: (a) Lev2O, pyridine, rt, 86%; (b) 3-azido-
propanol, Ph2SO, Tf2O, TTBP, DCM, -75 °C to rt, 80% (R/â ) 1/5); (c)
cat. NaOMe, MeOH, rt, 60%; (d) NIS, TMSOTf, 0 °C, 2 h, 18: 78% (R/â
) 1/>10), 20: 50% (R/â ) 1/>10); (e) N2H4‚H2O, pyridine/AcOH (4/1),
rt, 95%; (f) N2H4‚H2O, pyridine/AcOH (4/1), rt then KOH, THF/H2O (1/
1) then H2, Pd(black), MeOH/AcOH (10/1), 35%.
Acknowledgment. This work was supported by the Council
for Chemical Sciences of The Netherlands Organization for
Scientific Research (CW-NWO). The authors thank C. Erkelens
and F. Lefeber for recording NMR spectra. H. van den Elst is kindly
acknowledged for technical assistance.
4, and NIS in dichloromethane was cooled to -40 °C and a catalytic
amount of TMSOTf was added to afford â-disaccharide 8 in 91%
yield. In another experiment, donor 1 was preactivated with NIS
and an equimolar amount of TMSOTf.15a However, no complete
activation of donor 1 could be attained as monitored by TLC
analysis. After addition of acceptor 4, the â-linked disaccharide 8
was isolated in the same yield (91%). These results suggest that,
contrary to the Ph2SO/Tf2O conditions, the donor is not transformed
into an R-triflate intermediate. Presumably, direct SN2-type dis-
placement of the initially formed iodosulfonium species is at the
basis of the observed â-selectivities in the NIS/TMSOTf mediated
glycosylation reactions. The standard NIS/TMSOTf protocol was
also applied for the condensation of donor 1 and benzylidene
protected acceptor 5 to give â-linked disaccharide 11 in 58% yield.
Partial cleavage of the acid labile benzylidene functionality explains
this moderate yield. Finally, the earlier unproductive coupling of
acceptor 7 with donor 1 was executed using NIS/TMSOTf.
Disaccharide 13 was isolated in excellent yield as a mixture of
anomers (R/â ) 1/2), probably as a result of the sterical bias in
the acceptor.
The stage was now set for the assembly of the spacer containing
mannuronic acid trisaccharide 21 using the NIS/TMSOTf activation
system (Scheme 1). Levulinoylation of compound 14 using Lev2O
in pyridine afforded donor 15 in excellent yield. The requisite
acceptor 17 was synthesized under Ph2SO/Tf2O mediated conditions
of 1 with 3-azidopropanol, affording fully protected 16 in 80% yield
as an inseparable mixture of anomers.16 Ensuing basic hydrolysis
of the acetate ester using NaOMe/MeOH delivered the target
â-linked acceptor 17 in 60% yield. It was found that hydrolysis of
the â-anomer 16â occurred much faster than hydrolysis of the
corresponding R-oriented anomer 16R, leading to facile separation
of the anomers. Donor 15 and acceptor 17 were subjected to the
NIS/TMSOTf mediated condensation, giving â-linked disaccharide
18 in 78% yield. Standard deprotection of the levulinoyl group using
hydrazine hydrate yielded acceptor disaccharide 19, which was
subjected to the same coupling cycle giving trisaccharide 20 in 50%
yield. Deprotection of the mannuronic acid trisaccharide 20 was
accomplished following levulinoyl deprotection, hydrolysis of the
methyl ester, and final reduction of the azide and benzyl groups.
Supporting Information Available: All general procedures, the
synthesis and characterizations of all new compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
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