How O-substitution of sialyl donors affects their stereoselectivity

Article abstract of DOI:10.1021/ol3000475

The profound effect of substituents at C-5 of glycosyl sialosides on their stereoselectivity is well-known although the exact nature of this effect is somewhat less understood. Presented herein is a comparative study of a range of novel sialyl donors with various O-substituents. It is demonstrated that O-substituents at C-4 and C-7 may also have a significant effect on the reactivity of sialyl donors and on the stereoselectivity of chemical sialylation.

Full text of DOI:10.1021/ol3000475

ORGANIC
LETTERS
2012
Vol. 14, No. 4
1126–1129
How O-Substitution of Sialyl Donors
Affects Their Stereoselectivity
Hemali D. Premathilake,† Chase P. Gobble,†,‡ Papapida Pornsuriyasak,†
Tramaine Hardimon,‡ Alexei V. Demchenko,† and Cristina De Meo*,‡
Department of Chemistry and Biochemistry, University of Missouri;St. Louis,
One University Boulevard, St. Louis, Missouri 63121, United States, and Department of
Chemistry, Southern Illinois University Edwardsville, Edwardsville, Illinois 62026,
United States
cdemeo@siue.edu
Received January 9, 2012
ABSTRACT
The profound effect of substituents at C-5 of glycosyl sialosides on their stereoselectivity is well-known although the exact nature of this effect is
somewhat less understood. Presented herein is a comparative study of a range of novel sialyl donors with various O-substituents. It is demonstrated that
O-substituents at C-4 and C-7 may also have a significant effect on the reactivity of sialyl donors and on the stereoselectivity of chemical sialylation.
Sialic acid containing glycoconjugates are natural pro-
ducts that, in the past two decades, have become extremely
important synthetictargets, duetotheir direct involvement
in numerous biological phenomena.1,2 Unfortunately, the
stereoselective synthesis of the glycosidic linkage between
sialic acid and a glycoconjugate chain is still a major
synthetic challenge. In fact, sialylation reactions are often
plagued by low yields of the desired target (R-anomer) due,
in part, to the low stereoselectivity of the reaction itself and
the concomitant elimination reaction that is sometimes
observed. Among the many sialyl donors that have been
investigated, those modified at C-5 have probably been the
most exploited.3 From the first modifications as N-acet-
ylacetamido and trifluoroacetamido by Boons et al.4 to the
azido modification of Wong et al.5 and the oxazolidinone
trans-fused ring and its N-acetyl derivative, introduced by
Takahashi,6,7 De Meo,8,9 and Crich,10,11 there has been a
general belief that C-5 modifications can dramatically
improve the synthesis of R-sialosides. Thus, we were
extremely intrigued when we found a series of results with
N-benzoylacetamido per-O-benzoylated sialyl donor 1a
reported by Ye et al.12 that led to the unnatural β-anomer
whencoupledwithglycosyl acceptors2 and 3 (Figure1 and
Scheme 1). In our opinion, these results posed the question
of whether it was the 5-N-benzoyl modification, or rather
the presence of O-benzoyl groups (or a combination of
both), that actually led to the unanticipated enhanced
β-stereoselectivity (the use of O-benzoylated building blocks13
is rather uncommon in sialic acid chemistry). It seems unlikely
that the high β-stereoselectivity obtained with sialyl donor
1a might be related to the use of CH2Cl2 as a replacement
for the common stereocontrolling solvent acetonitrile
† University of Missouri;St. Louis.
‡ Southern Illinois University Edwardsville.
(1) Biology of Sialic Acids; Rosenberg, A., Ed.; Plenum Press: New
York, London, 1995.
(6) Tanaka, H.; Nishiura, Y.; Takahashi, T. J. Am. Chem. Soc. 2006,
128, 7124–7125.
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4383–4386.
(8) Farris, M. D.; De Meo, C. Tetrahedron Lett. 2007, 48, 1225–1227.
(9) Harris, B. N.; Patel, P. P.; Gobble, C. P.; Stark, M. J.; De Meo, C.
Eur. J. Org. Chem. 2011, 2011, 4023–4027.
(2) Schauer, R. In Carbohydrates in Chemistry and Biology; Ernst, B.,
Hart, G. W., Sinay, P., Eds.; Wiley-VCH: Weinheim, NY, 2000; Vol. 2, pp
227À239.
(3) De Meo, C.; Priyadarshani, U. Carbohydr. Res. 2008, 343, 1540–
1552.
(10) Crich, D.; Li, W. J. Org. Chem. 2007, 72, 2387–2391.
(11) Crich, D.; Li, W. J. Org. Chem. 2007, 72, 7794–7797.
(12) Wang, Y.; Ye, X.-S. Tetrahedron Lett. 2009, 50, 3823–3826.
(13) Birberg, W.; Lonn, H. Tetrahedron Lett. 1991, 32, 7453–7456.
(4) Demchenko, A. V.; Boons, G. J. Chem.;Eur. J. 1999, 5, 1278–
1283.
(5) Yu, C. S.; Niikura, K.; Lin, C. C.; Wong, C. H. Angew. Chem., Int.
Ed. 2001, 40, 2900–2903.
r
10.1021/ol3000475
Published on Web 02/02/2012
2012 American Chemical Society

(Ye reported a lack of activation in acetonitrile). Recent
sialylations have been successfully performed in several
solvents, including dichloromethane.10,14,15 To help under-
stand the effect that N- or O-benzoyl groups may have
on stereoselectivity, we synthesized two sialyl donors bear-
ing mixed protecting group patterns: per-O-benzoylated
sialyl donor 1b and sialyl donor 1c benzoylated only at N-5
(Figure 1; see the Supporting Information (SI) for the
synthesis of all building blocks reported herein).
reaction conditions. In our hands, a modest R-stereoselec-
tivity for the synthesis of 4d (R/β = 1.5/1) and a notable
shift toward β-stereoselectivity for the synthesis of 5d
(R/β = 1/4.0) were noted (entries 7 and 8). It is noteworthy
thatwhile sialylations with benzoylatedcompounds1band
1c were completed within 20 min, sialylations with stan-
dard acetylated donor 1d occurred at a much slower rate
(6À9 h). In our opinion, the data summarized in Table 1
suggest that, while a concomitant per-N,O-benzoylation
has a strong β-directing effect (donor 1a), the major
β-directing contribution is probably due to per-O-benzo-
ylation as seen in the case of donor 1b.
Scheme 1. Unusually High β-Stereoselectivity Obtained with
Sialyl Donor 1a, as Reported by Ye et al.12
Table 1. Comparison of Sialyl Donors 1aÀd in Couplings with
Glycosyl Acceptors 2 and 3
The comparative testing of sialyl donors 1bÀd with
glycosyl acceptors 2 and 3 was performed under the same
experimental conditions as those reported by Ye for the
glycosidation of 1a, i.e. NIS/TfOH in dichloromethane
at À72 °C (entries 1 and 2, Table 1). Per-O-benzoylated
sialyl donor 1b lacking the N-benzoyl substituent still
showed a preference for β-stereoselectivity leading to the
corresponding disaccharides 4b (R/β=1/1.4) and5b (R/β=
1/6.0, entries 3 and 4). On the other hand, N-5 benzoylated
sialyl donor 1c gave the corresponding disaccharides 4c
and 5c with a shift toward R-stereoselectivity (R/β = 2.0/1
and 1.1/1, respectively, entries 5 and 6), although the
overall stereoselectivity remained unimpressive. It is note-
worthy that, as reported by Ye, the introduction of a
benzoyl group at C-5 creates two rotamers around the
C5ÀN bond. Hence, to achieve high resolution signals, all
1H NMR spectra of N-benzoylated mono- and disaccha-
rides had to be recorded at 80 °C in DMSO, whereas spectra
recorded at À40 °C clearly showed two distinctive groups
of signals corresponding to each rotamer.
a Reaction conditions: donor/acceptor ratio 1.5:1, NIS/TfOH,
o
CH2Cl2, À72 C. b The anomeric stereochemistry was assigned based
on the chemical shift of H-3eq. c The yield was determined after
Sephadex LH-20 size exclusion chromatography. d Donor/acceptor
ratio is 1:1.5.
In accordance with the initial goal of our investigation,
this preliminary comparative study suggests that it is not
the N-5 modification alone that is responsible for the high
β-stereoselectivity observed in the coupling of 1a. Instead,
this initial study (vide supra) clearly shows that the impact
of O-protection on the stereoselectivity of sialylations is
also of very high significance. It should be noted that while
the effect of O-protecting groups on the outcome of common
glycosylation reactions is well-known,16 to the best of our
knowledge, no systematic study of O-modifications has yet
been reported for sialyl donors. To begin a systematic
study of this effect, we decided to focus on the O-4 and
To acquire comparison data, traditional sialyl donor 1d
was also coupled to acceptors 2 and 3 under the same
(14) De Meo, C.; Farris, M.; Ginder, N.; Gulley, B.; Priyadarshani,
U.; Woods, M. Eur. J. Org. Chem. 2008, 3673–3677.
(15) Hanashima, S.; Sato, K.-i.; Ito, Y.; Yamaguchi, Y. Eur. J. Org.
Chem. 2009, 2009, 4215–4220.
Figure 1. Sialyl donors 1aÀd for comparative study.
Org. Lett., Vol. 14, No. 4, 2012
1127

O-7 positions. Our rationale is based on the assumption of the
existence of a conformational equilibrium of the oxacar-
benium ion, with one all-axial conformation that places O-7
in close proximity to the anomeric center and an axial O-4
with a potentially relevant stereoelectronic effect. The existence
of different conformations of the oxacarbenium ion for
simpler heterocyclic systems has been described by Woerpel
and others.17À20 Although no extensive mechanistic studies
have been reported for derivatives of sialic acid, the syntheses
of 2,7-anhydro, 1,7-lactone, and 1,5-lactam derivatives21À25
suggest the existence of a reaction intermediate conforma-
tion with an all-axial orientation of substituents.
steric bulk of benzoyl vs acetyl or the change in the
electronic profile with the benzoyl substituent being mar-
ginally more electron-withdrawing than acetyl26 remained
unclear. To expand upon these findings we synthesized a
range of new donors 1fÀh (Scheme 3), where we also
included protection at O-4 with a bulky tert-butyldi-
methylsilyl (TBDMS) group. Strategically, we chose to
introduce the 4-O-TBDMS group, taking into account the
experimental convenience, to eliminate the possible effect
of an electron-withdrawing substituent at O-4. Finally,
since all sialyl donors of the new series are equipped with a
4-O-TBDMS group it would still allow for direct side-by-
side comparison. However, direct comparison with the
previously investigated series can be only made between 1d
and 1f, which only differ by the pretecting group at O-4.
The latter comparison would then allow insight into the
effect of the substituent at O-4.
Scheme 2. Comparison of Stereoselectivity Provided by Sialyl
Donors 1b and 1e
The initial set of coupling reactions of new sialyl donors
1fÀh with acceptor 2 was performed in dichloromethane
at À72 °C. For the coupling of 7-O-acetylated 1f, we
observed a relatively long reaction time (3 h) along with
the preference toward the formation of the R-anomer of 4f
(R/β = 2.8/1, Table 2, entry 1). Interestingly, 7-O-benzoy-
lated donor 1g showed a dramatic increase in the reactivity
toward sialylation and also provided a further shift toward
R-sialylation. Thus, disaccharide 4g was obtained in only
15 min (R/β = 4.3/1, entry 2). Upon activation of 7-O-
TBDMS protected sialyl donor 1h, the slowest reaction
time (5 h) within this series was recorded along with the
bias toward β-linked disaccharide 4h (R/β = 1/2.2,
entry 3). Thus, upon activation of sialyl donor 1g, mainly
R-disaccharide 5g was obtained in 10 min (R/β = 2.5/1,
entry 5). Conversely, activation of donor 1h resulted in
the slow formation of 5h with the predominance of the
β-anomer (R/β = 1/2.3, entry 6). Interestingly, no reaction
of 1f and acceptor 3 took place (entry 4). This unusual
behavior of 1f may be related to the existence of a
mismatched donorÀacceptor pair27 or a double stereodif-
ferentiation effect.28 This observation cannot be explained
by the currently accepted monomolecular mechanism of
sialylation29 (the activation of a leaving group should not
be influenced by the nature of the nucleophile) and clearly
raises questions for future studies.
To obtain a general and very preliminary working
hypothesis on the nature of the effect of the O-7 position,
we designed sialyl donor 1e that has only one structural
variant, the 7-O-acetyl group, in comparison to that of per-
benzoylated sialyl donor 1b (Scheme 2). Sialylation of 1e
with acceptor 2 in dichloromethane gave the correspond-
ing disaccharide 4e in 15 min. The anomeric ratio observed
(R/β = 1.4/1) was a modest shift toward the R-anomer
formation in comparison to the stereoselectivity obtained
with sialyl donor 1b (R/β = 1/1.4). However, a more
dramatic enhancement in R-stereoselectivity was obtained
in acetonitrile, in which disaccharide 4e was obtained as a
R/β = 5/1 anomeric ratio vs R/β = 1.6/1 for 4b.
Inouropinion, this setofresultsclearly indicates thatthe
7-O-protecting group alone may play a significant role in
providing enhanced β-stereoselectivity obtained with per-
O-benzoylated donors 1a and 1b. However, whether the
nature of this effect shall be attributed to the increased
As mentioned earlier, the solvent effect in sialylations is
known to be the most significant factor influencing the
stereoselectivity (and often reactivity) of sialyl donors.
Since it is commonly accepted that CH3CN helps to obtain
high R-stereoselectivity, a vast majority of chemical sialy-
lations are performed using this solvent.30,31 As reported
by Ye et al., per-N,O-benzoylated sialyl donor 1a cannot
be activated in acetonitrile. Our further experimentation
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Org. Lett., Vol. 14, No. 4, 2012

Table 2. Comparison of Sialyl Donors 1fÀh in Couplings with
Acceptors 2 and 3 in CH2Cl2 and CH3CN
Scheme 3. Synthesis of a Series of 4-O-TBDMS-Protected Sialyl
Donors 1fÀh
showed that per-O-benzoylated donor 1b and N-benzoy-
lated donor 1c show similar reaction times and yields in
either acetonitrile or CH2Cl2. In terms of R-stereoselectiv-
ity, significant enhancement was recorded for donor 1b in
acetonitrile, whereas almost no changes were detected in
the case of donor 1c (see the SI for additional experimental
data). Hence, the investigation of whether CH3CN can
further impact the stereoselectivity of sialylation with
novel sialyl donors 1fÀh appealed to us as an important
subsequent step.
A very dramatic change in the sialylation outcome was
observed in couplings of sialyl donor 1f with acceptors 2
and 3 in CH3CN vs CH2Cl2. Previously, disaccharide 4f
was obtained as an R/β = 2.8/1 mixture of anomers in
CH2Cl2 (entry 1); herein, the R-anomer of 4f was formed
exclusively in CH3CN (entry 7). The coupling of 1f with
glycosyl acceptor 3 now proceeded rapidly (recall no
reaction in CH2Cl2, entry 4), and the resultant disaccharide
5f was obtained in 10 min as a pure R-anomer (entry 11).
These reactions stood out due to the exceptionally high
stereoselectivity, even in comparison with the standard
donor 1d leading to disaccharide 4d (R/β = 4.4/1, entry 8).
This was a very clear indication that the substituents on
O-4 have a very profound effect on sialylation. Results
obtained with 7-O-benzoyl donor 1g varied dramatically
depending on the nature of the glycosyl acceptor. For the
coupling of 7-O-benzoyl donor 1g, while a negligible
decrease in R-stereoselectivity was detected with acceptor
2 (compare entries 2 and 9), a 5-fold increase was noticed
with acceptor 3 (compare entry 5 vs 12). In all cases of
sialylation with 7-O-TBDMS donor 1h, lower R-stereo-
selectivity, in comparison to that of the standard per-
acetylated donor 1d, was recorded (entries 10 and 13).
Inconclusion, arangeof novel sialyldonorswithvarious
O-substituents have been investigated. Our preliminary
data suggests the following: (1) a bulky 7-O-benzoyl
group alone (e.g., donor 1g) cannot lead to preferential
β-sialylation; (2) the influence of acetonitrile seems to be in
direct correlation with the nature of the O-protecting
groups, ranging from the inversion of the anomeric con-
figuration for donors 1b and 1h and marginal effect on
donor 1c to complete R-stereoselectivity for donor 1g; (3)
the presence of a bulky substituent at C-4 can also exert a
strong influence on the stereoselectivity of sialylations. In
fact, the dramatic enhancement in R-stereoselectivity ob-
served with sialyl donor 1f is of high significance and may
lead to the development of new methodologies for R-
sialylation. To expand upon these findings and conduct a
further search of activating and/or R-directing groups at
C-4 and C-7, other studies are currently underway in our
laboratories.
Acknowledgment. A.V.D. thanks the NSF for support-
ing this work (award CHE-1058112). H.D.P. is grateful to
UMSL Graduate School for proving her with the Disser-
tation Fellowship.
Supporting Information Available. Experimental pro-
1
cedures, extended experimental data, H and 13C NMR
spectra for all new compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
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6126.
The authors declare no competing financial interest.
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