β-Mannosylation of N-acetyl glucosamine by propargyl mediated intramolecular aglycon delivery (IAD): synthesis of the N-glycan core pentasaccharide

Article abstract of DOI:10.1016/j.tetlet.2007.02.108

An efficient and completely stereocontrolled synthesis of the N-glycan Manβ(1-4)GlcNAc disaccharide is achieved by propargyl mediated intramolecular aglycon delivery (IAD). Isomerisation of the 2-O-progargyl group of a manno thioglycoside to an allene is

Full text of DOI:10.1016/j.tetlet.2007.02.108

Tetrahedron Letters 48 (2007) 3061–3064
b-Mannosylation of N-acetyl glucosamine by propargyl
mediated intramolecular aglycon delivery (IAD): synthesis
of the N-glycan core pentasaccharide
Emanuele Attolino and Antony J. Fairbanks*
Chemistry Research Laboratory, Oxford University, Mansfield Road, Oxford OX1 3TA, UK
Received 25 January 2007; revised 8 February 2007; accepted 23 February 2007
Available online 28 February 2007
Abstract—An efficient and completely stereocontrolled synthesis of the N-glycan Manb(1–4)GlcNAc disaccharide is achieved by
propargyl mediated intramolecular aglycon delivery (IAD). Isomerisation of the 2-O-progargyl group of a manno thioglycoside
to an allene is followed by iodonium ion mediated mixed acetal formation with the 4-OH of a protected GlcNAc derivative, and
subsequent intramolecular glycosylation with complete control of anomeric stereochemistry. Access to this key disaccharide inter-
mediate allows completion of the total synthesis of the core N-glycan pentasaccharide.
Ó 2007 Elsevier Ltd. All rights reserved.
Control of anomeric stereochemistry during the course
of a glycosylation reaction is a key consideration during
the synthesis of any oligosaccharide. Whilst much work
has been done to facilitate the speed of assembly of
oligosaccharide structures, the issue of absolute control
of anomeric stereochemistry still remains unanswered.
Whilst 1,2-trans glycosidic linkages can usually be syn-
thesised with high levels of stereocontrol by taking
advantage of neighbouring group participation of 2-O-
acyl protected glycosyl donors1 the synthesis of 1,2-cis
glycosidic linkages is considerably more difficult. The
current situation is that there is still no generally appli-
cable method available,2 and considerable research
effort continues to be focussed on the development of
new and improved stereocontrolled glycosylation proce-
dures. For example, very recently Boons and co-workers
described a new and elegant approach to the synthesis of
a-glucosides by the use of novel neighbouring group
participation.3 However this methodology, whilst repre-
senting an extremely welcome development is, as it
stands, limited in its applicability. For example, this
approach does not currently allow the formation of b-
1,2-cis linkages, such as the notorious b-mannosides.
Hence there is still an urgent need for the development
of reliable and truly generally applicable methodology
that will allow the formation of any 1,2-cis glycosidic
linkage, irrespective of the configuration of the glycosyl
donor.
Conceptually, perhaps the most elegant solution to this
‘1,2-cis glycoside problem’ is to apply the technique of
intramolecular aglycon delivery, or IAD, which is an
intramolecular glycosylation strategy4 wherein the gly-
cosyl acceptor is temporarily appended to the 2-hydr-
oxyl group of a glycosyl donor through a short linker.
The first so-called ‘tethering’ step, that is, the linking
of donor and acceptor, is followed by activation of the
glycosyl donor which subsequently furnishes the 1,2-cis
glycoside in a completely stereoselective fashion. The
two key advantages of the IAD approach over other
intramolecular glycosylation reactions in which the do-
nor and acceptor are either connected by other hydroxyl
groups, through longer linkers, or other remote posi-
tions, are the predictability and strict control of the ano-
meric stereochemistry of the product. However, even
using the IAD approach, one has to be careful to avoid
problems of regiochemical control, that is, ensure that
only the desired hydroxyl is glycosylated,5 and also to
avoid the use of longer linkers, which can lead to unde-
sired non-stereoselective reactions.6
Keywords: Carbohydrates; Glycosylation; Intramolecular aglycon
delivery; b-Mannosides; N-Glycan; Pentasaccharide.
Corresponding author. Tel.: +44 1865 275647; fax: +44 1865
Amongst several IAD methodologies that have been
reported,7–9 the approach developed by Ogawa and
co-workers, based on the use of 2-O-para-methoxybenzyl
*
275674; e-mail: antony.fairbanks@chem.ox.ac.uk
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.02.108

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E. Attolino, A. J. Fairbanks / Tetrahedron Letters 48 (2007) 3061–3064
(PMB) protected glycosyl donors,10 is the only method-
ology that has been really successfully applied to the
synthesses of complex targets, such as the core N-glycan
pentasaccharide. This pentasaccharide, which contains
the crucial Manb(1–4)GlcNAc linkage, can be consid-
ered a benchmark test for potential widespread utility
of any 1,2-cis glycosylation procedure. Whilst the
Ogawa approach has also been applied to the synthesis
of a selection of other 1,2-cis glycosides, with varying
degrees of success,11 it is notable that it has not yet
found routine applicability.
We recently reported the development of an allyl pro-
tecting group mediated IAD approach (allyl IAD),12
based on either thioglycoside13 or glycosyl fluoride
donors.14 This methodology relies on iodonium ion
mediated formation of a mixed acetal between a glycosyl
acceptor alcohol and an enol ether, itself derived from a
2-O-allyl protected glycosyl donor by Wilkinson’s cata-
lyst mediated isomerisation15 of the double bond. How-
ever, attempted application of this methodology to the
synthesis of complex oligosaccharides, such as the core
N-glycan pentasaccharide, met with frustration. For
example, although an effective intramolecular glycosyl-
ation of a fully ‘armed’ benzyl protected manno-thio-
glycoside was achieved with the 4-hydroxyl of a
glucosamine acceptor, when this reaction sequence was
attempted with the more advanced manno-thioglycoside
1, using a previously applied glucosamine acceptor 2,
investigations met with considerable difficulties.
Although optimised conditions6 for mixed acetal forma-
tion allowed the synthesis of intermediate 3 in high yield,
it was found that 3 resisted almost all attempts to effect
intramolecular glycosylation. A variety of different con-
ditions commonly used for thioglycoside activation were
investigated. The only successful procedure involved the
use of a Me2S2/Tf2O mixture, as reported originally by
Scheme 1. Reagents and conditions: (a) (Ph3P)3RhCl, n-BuLi, THF,
reflux, 73%; (b) 2, I2, AgOTf, DTBMP, CH2Cl2, À78 °C to rt, 64%; (c)
Me2S, Tf2O, DTBMP, CH2Cl2, 0 °C–rt, 29%.
of propargyl ethers20 as sterically unobtrusive groups for
the protection of the 2-hydroxyl of manno thioglycosides
as part of their b-mannosylation procedure. As the basis
of an IAD glycosylation approach it was reasoned that
base-mediated isomerisation of 2-O-progargyl protected
glycosyl donors would allow access to 2-O-allenyl pro-
tected carbohydrates.21 Such allenes could then undergo
a similar sequence as had previously been applied as the
basis of the allyl IAD—namely, iodonium ion catalyzed
mixed acetal formation with an acceptor alcohol, and
subsequent intramolecular glycosylation with complete
control of stereochemistry. An envisaged advantage of
the propargyl based approach over the allyl based meth-
odology was that the oxonium ion produced subsequent
to glycosylation would be stabilised by conjugation with
the side-chain alkene; a factor which may facilitate agly-
con delivery.
Fugedi and Tatai,16 together with added di-tert-butyl-
¨
methylpyridine (DTBMP), which was found to further
improve the efficiency of glycosylation.17 However, even
these conditions were only able to produce the desired
b-disaccharide 4a in 29% yield, albeit with complete
control of anomeric stereochemistry (Scheme 1).
In order to investigate the feasibility of such an ap-
proach, propargyl ether 7 was synthesised by regioselec-
tive silylation of diol 5 to yield 6, which then underwent
subsequent etherification using propargyl bromide and
sodium hydride in DMF to give propargyl ether 7.
Isomerisation by treatment with t-butoxide22 then
yielded the desired intermediate allene 8. Iodonium ion
mediated mixed acetal formation with glucosamine
acceptor 2 gave mixed acetals 9 in an excellent 88%
yield. Subsequent intramolecular glycosylation, using
We were therefore unable to access disaccharide 4a
satisfactorily by our allyl IAD procedure. Moreover, it
should also be noted at this point that the Crich direct
b-mannosylation procedure does not work well with
glucosamine acceptors such as 2.18 It was thought that
the difficulty in obtaining a satisfactory yield for this
step, as compared to previous intramolecular glycosyl-
ation reactions using the allyl IAD approach, could in
part be due to the fact that the manno donor is partially
‘disarmed’19 due to torsional deactivation by the 4,6-
benzylidene group. With the reasoning that an increase
in stability of the oxonium ion produced subsequent
to the intramolecular glycosylation step may actually
facilitate the glycosylation process it was decided to
investigate the use of 2-O-propargyl ethers for a similar
intramolecular glycosylation strategy.
the previously optimised Fugedi activation conditions,17
¨
proceeded smoothly to give the desired Manb(1–4)Glc-
NAc disaccharide 4a in an excellent 81% yield, with
complete stereocontrol (Scheme 2).
The above process represented a significant improve-
ment on the allyl IAD system for glycosylation of the
disarmed manno donor, and could be performed reliably
on a significant scale. With substantial quantities of
disaccharide 4a in hand attention then turned to the
completion of the total synthesis of the N-glycan penta-
saccharide. Deprotection of the silyl group of 4a with
Although not widely used in carbohydrate chemistry,
recent reports from Crich et al. have detailed the utility

E. Attolino, A. J. Fairbanks / Tetrahedron Letters 48 (2007) 3061–3064
3063
Scheme 2. Reagents and conditions: (a) ButPh2SiCl, imidazole, CH2Cl2, rt, 84%; (b) propargyl bromide, NaH, DMF, rt, 72%; (c) ButOK, Et2O, 66%;
(d) 2, I2, AgOTf, DTBMP, CH2Cl2, À78 °C to rt, 88%; (e) Me2S, Tf2O, DTBMP, CH2Cl2, 0 °C–rt, 81%.
TBAF gave diol 4b, which was then glycosylated regio-
selectively at position 3 with the known trichloracetimi-
date donor 10,23 to give a trisaccharide which was
immediately acetylated to give 11a. Removal of the
4,6-benzylidene protection to yield trisaccharide diol
11b proved slightly problematic. A simple treatment
with aqueous acetic acid invariably resulted in the
formation of small quantities of the 6-O-acetate as a side
product. An optimised procedure was therefore devel-
oped involving treatment with trifluoroacetic acid in
Scheme 3. Reagents and conditions: (a) TBAF, THF, rt, 60%; (b) 10, TMSOTf, CH2Cl2, À78 °C to rt; (c) Ac2O, pyridine, rt, 86% over two steps; (d)
10% TFA in 10:1 MeCN:H2O, 90%; (e) 10, TMSOTf, CH2Cl2, À78 °C to rt, 76%; (f) K2CO3, MeOH, rt; (g) NaH, BnBr, DMF, rt, 77% over two
steps; (h) (NH4)2Ce(NO3)6, toluene, MeCN, H2O, rt, 77%; (i) Cl3CCN, DBU, CH2Cl2, rt, 96%; (j) 2, TMSOTf, CH2Cl2, À78 °C to rt, 85%.

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E. Attolino, A. J. Fairbanks / Tetrahedron Letters 48 (2007) 3061–3064
9. (a) Ennis, S. C.; Fairbanks, A. J.; Tennant-Eyles, R. J.;
Yeates, H. S. Synlett 1999, 1387–1390; (b) Ennis, S. C.;
Fairbanks, A. J.; Slinn, C. A.; Tennant-Eyles, R. J.;
Yeates, H. S. Tetrahedron 2001, 57, 4221–4230; (c)
Chayajarus, K.; Chambers, D. J.; Chughtai, M. J.;
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aqueous acetonitrile. In this case, although some of the
corresponding 6-O-trifluoroacetate was formed initially,
this material was readily hydrolysed upon work-up,
yielding diol 11b in 90% yield. A second regioselective
glycosylation reaction with donor 10 then smoothly
gave tetrasaccharide 12a. Further protecting group
manipulations and activation of the anomeric centre as
the trichloroacetimidate then allowed glycosylation with
acceptor 2 to finally yield the desired pentasaccharide 13
(Scheme 3).
10. (a) Lergenmuller, M.; Nukada, T.; Kuramochi, K.; Dan,
¨
A.; Ogawa, T.; Ito, Y. Eur. J. Org. Chem. 1999, 1367–
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In conclusion, the development of propargyl mediated
IAD appears to represent a considerable improvement
over the allyl IAD approach in terms of efficiency of
the intramolecular glycosylation step. The work detailed
in this Letter demonstrates that an efficient intra-
molecular glycosylation can be achieved even using a
disarmed glycosyl donor, together with a hindered N-
acetyl glucosamine acceptor, allowing completion of
the total synthesis of the core N-glycan pentasaccharide.
Further investigations exploring the scope of use and
efficiency of propargyl mediated IAD are currently in
progress, and the results will be reported in due course.
12. Fairbanks, A. J. Synlett 2003, 1945–1958.
13. (a) Seward, C. M. P.; Cumpstey, I.; Aloui, M.; Ennis, S.
C.; Redgrave, A. J.; Fairbanks, A. J. Chem. Commun.
2000, 1409–1410; (b) Aloui, M.; Chambers, D.; Cumpstey,
I.; Fairbanks, A. J.; Redgrave, A. J.; Seward, C. M. P.
Chem. Eur. J. 2002, 8, 2608–2621.
14. (a) Cumpstey, I.; Fairbanks, A. J.; Redgrave, A. J. Org.
Lett. 2001, 3, 2371–2374; (b) Cumpstey, I.; Fairbanks, A.
J.; Redgrave, A. J. Monatsh. Chem. 2002, 133, 449–466.
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Acknowledgement
The authors gratefully acknowledge the European
Union (Marie Curie Intra-EU Fellowship to E.A.) for
financial support.
16. Fugedi, P.; Tatai, J., OP58, 13th European Carbohydrate
¨
Symposium, August 21–26, 2005; Bratislava, Slovakia.
17. Attolino, E.; Cumpstey, I.; Fairbanks, A. J. Carbohydr.
Res. 2006, 341, 1608–1618.
18. A referee required us to clarify this point. The direct b-
mannosylation procedure developed by Crich has been
used to access a trisaccharide precursor to the core N-
glycan pentasaccharide (a) and also the pentasaccharide
itself (b). However, in order for the process to work
effectively the nitrogen on the glucosamine acceptor must
either be carried through as an azide (a), or as a
sulfonamide (b), and in the latter case the mannosylation
gives an inseparable anomeric mixture of products (b:a,
8:1). See: (a) Dudkin, V. Y.; Crich, D. Tetrahedron Lett.
2003, 44, 1787–1789; (b) Dudkin, V. Y.; Miller, J. S.;
Danishefsky, S. J. Tetrahedron Lett. 2003, 44, 1791–1793.
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2006, 71, 3064–3070; (c) Crich, D.; Wu, B. Org. Lett. 2006,
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