Synthesis of α-D-GalpN3-(1-3)-D-GalpN3: α- and 3-O-selectivity using 3,4-diol acceptors

Article abstract of DOI:10.3762/bjoc.14.258

The motif α-D-GalpNAc-(1-3)-D-GalpNAc is very common in Nature and hence its synthesis highly relevant. The synthesis of its azido precursor has been studied and optimized in terms of steps, yields and selectivity. It has been found that glycosylation of the 3,4-diol acceptor is an advantage over the use of a 4-O-protected acceptor and that both regio- and anomeric selectivity is enhanced by bulky 6-O-protective groups. The acceptors and donors are made from common building blocks, limiting protective manipulations, and in this context, unavoidable side reactions.

Full text of DOI:10.3762/bjoc.14.258

Synthesis of α-D-GalpN3-(1-3)-D-GalpN3:
α- and 3-O-selectivity using 3,4-diol acceptors
Emil Glibstrup and Christian Marcus Pedersen*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2018, 14, 2805–2811.
Department of Chemistry, University of Copenhagen,
Universitetsparken 5, 2100 Copenhagen O, Denmark
doi:10.3762/bjoc.14.258
Received: 07 September 2018
Accepted: 30 October 2018
Published: 08 November 2018
Email:
Christian Marcus Pedersen* - cmp@chem.ku.dk
* Corresponding author
Associate Editor: S. Flitsch
Keywords:
© 2018 Glibstrup and Pedersen; licensee Beilstein-Institut.
License and terms: see end of document.
diastereoselectivity; glycosylation; regioselectivity
Abstract
The motif α-D-GalpNAc-(1-3)-D-GalpNAc is very common in Nature and hence its synthesis highly relevant. The synthesis of its
azido precursor has been studied and optimized in terms of steps, yields and selectivity. It has been found that glycosylation of the
3,4-diol acceptor is an advantage over the use of a 4-O-protected acceptor and that both regio- and anomeric selectivity is enhanced
by bulky 6-O-protective groups. The acceptors and donors are made from common building blocks, limiting protective manipula-
tions, and in this context, unavoidable side reactions.
Introduction
The disaccharide α-D-GalpNAc-(1-3)-β-D-GalpNAc is a wide- the Forssman antigen has remained an interesting target for
spread motif in glycobiology and present in several distinct vaccine development [9] and further biological evaluations [10].
entities. One of the most studied glycoconjugates, containing The disaccharide motif is also commonly found in viruses and
this sequence, is the Forssman antigen, in which this disaccha- bacteria. In bacteria, as an example, it has been found in patho-
ride is the terminal unit. The many biological roles attributed to genic bacteria such as in Salmonella [11], Shigella [12], several
this particular glycoconjugate and its appearance in some Burkholderia [13], Escherichia coli [14], Vibrio chlorae [15],
human tumors [1,2] has triggered the interest of carbohydrate Edwardsiella ictaluri [16], Enterrococcus [17], Proteus
chemists since the early days of complex oligosaccharides mirabilis [18], and Streptococcus [19-22]. With the wide spread
synthesis. The structure of the pentasaccharide part of the appearance of α-D-GalpNAc-(1-3)-β-D-GalpNAc in many
Forssman antigen was resolved in the seventies [3,4] and soon pathogens, this motif has been synthesized many times for
thereafter Paulsen and Bünsch finished the chemical synthesis various purposes, e.g., as part of the lipopolysaccharide found
of the pentasaccharide chain [5,6]. The pioneering work by in Shigella dysenteriae [23], as derivatives of the mucin
Paulsen was later followed up by a total synthesis by the Ogawa O-glycan core structures for glycosidase studies [24], for the
group [7] and an oligosaccharide synthesis by the Magnusson synthesis of T-antigen analogues [25], for the synthesis of
group [8]. With the increasing understanding of glycobiology, E. coli O-antigens [26-29], for the development of Burk-
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holderia vaccines [30-32], for the synthesis of PS A1 conjugate ture, surprisingly few reports were describing diol glycosyla-
vaccine [33], and the synthesis of E. faecium wall teichoic acid tions targeting the galacto-configured sugar [41-43] and none
fragments for vaccine development [34,35]. We have been for the synthesis of the α-D-GalpNAc-(1-3)-D-GalpNAc motif
interested in α-D-GalpNAc-(1-3)-β-D-GalpNAc as a part of our or its precursors.
ongoing syntheses of S. pneumoniae lipoteichoic acid (LTA)
derivatives [20,36,37]. During optimization of the synthesis, we We therefore set out to study this regioselective glycosylation in
realized that the synthesis of the α-D-GalpNAc-(1-3)-β-D- more detail and synthesized the necessary model donors and
GalpNAc part worked more efficiently, when using the 3,4-diol acceptors. Here, the simplicity of the approach showed its
acceptor instead of a fully protected 3-OH. As some protective potential, as both, donor and acceptor, could be obtained from a
group manipulations can be avoided and at the same time the common late stage building block, that in turn could be synthe-
yield and α-selectivity improved, we consider this finding im- sized anomerically pure as described previously [44]. Alterna-
portant for oligosaccharide synthesis. Here we describe this tively, these components could be obtained via an azido
regioselective glycosylation approach in detail.
phenylselenylation approach starting from D-galactose [45,46].
Results and Discussion
Starting from the common 2-azidoglucose precursor 7, the 4,6-
In the initial strategy for synthesizing the disaccharide, the fully O-benzylidene group was removed with TsOH to give the 4,6-
protected acceptor (1 or 2) was intended to be a key building diol in 95% yield, followed by selective 6-O-protection using
block. However, an unexpected problem during its preparation TBDPS-Cl and imidazole (IM) in 90% yield. The 4,6-O-benzyl-
occurred, i.e., benzylation of the axial 4-OH only gave the idene motif could also be regioselectively opened using triethyl-
desired 4-O-Bn product as a minor product, whereas the silane and BF3∙OEt2 to achieve the 6-O-Bn-protected variant in
benzoyl migration from the 3-O to the 4-O, followed by benzy- 78% yield (Scheme 2).
lation of the 3-O, was the major, if not the exclusive, product.
Despite several different benzylation procedures, i.e., NaH and In order to invert the 4-OH, to achieve the galacto configura-
BnBr, TriBOT [38] and TfOH or BnBr and Ag2O, the 4-O- tion, two approaches were attractive: one using our recently
benzylation remained inaccessible, with a 3-O-benzoyl group published method [44] or alternatively the Latrell–Dax inver-
present. Using the 6-O-Bn-protected variant (2 in Scheme 1), a sion using nitrite [47,48]. When having the 6-O-TBDPS protec-
1:2 mixture of the desired vs migrated product could be ob- tion the Latrell–Dax inversion provided slightly higher yields of
tained. When using the more bulky 6-OTBDPS as protective 1, whereas with the 6-O-Bn 8, the two methods provided
group (1 in Scheme 1) and benzylation with freshly prepared comparable yields. Migration of the benzoyl group under the
Ag2O and BnBr in the solvent mixture CH2Cl2/cyclohexane 1:4 4-O inversion conditions was irrelevant as it is removed in the
as described by Wang et al. [39], a 91% isolated yield following step. The proposed acceptors were obtained via
of the migrated product, i.e., 3-O-Bn, 4-O-Bz, was obtained Zemplén deprotection in 94% and 90% yield for the 6-O-
(Scheme 1).
TBDPS 9 and 6-O-Bn 10, respectively (Scheme 2). The accep-
tors could in turn be transformed into the corresponding donors
in three simple steps by first benzylating the 3,4-diol (11 and
12), hydrolyzing the thiophenol using NBS (13 and 14), thereby
making anomeric purity inconsequential, and finally formation
of the donors, 15 and 16, as trichloroacetimidates [49]. With
both, the donors and acceptors in hand, we set out to test our
hypothesis of regioselective 3-O-α-glycosylation on the 3,4-diol
9, which is based on the superior nucleophilicity and accessi-
bility of the 3-OH compared to the axial 4-OH. Initially, a small
screening of the glycosylation conditions expected to affect the
selectivity was performed.
Scheme 1: Undesired migration followed by benzylation of the 3-O-Bz
GalN3 using several different benzylation procedures (LG: leaving
group).
As the donor 15 stems from the acceptor 9, it was decided to
use a slight excess (1.2 equiv) of the “less precious” and recov-
erable acceptor 9, which, however, would result in lower yields
when compared to glycosylations using excess of donor. In the
initial experiment, using TMSOTf as the catalyst in CH2Cl2 at
0 °C, a pleasingly 8:1 3/4-O ratio and a 10:1 α/β-selectivity with
Following the apparent difference in nucleophilicity and acces-
sibility of the axial 4-OH versus the equatorial 3-OH [40], we
wondered, if this could be used to simplify the synthesis of the
α-D-GalpN3-(1-3)-D-GalpN3 unit. When consulting the litera-
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Beilstein J. Org. Chem. 2018, 14, 2805–2811.
Scheme 2: Simple synthesis of two acceptors and two donors from the same common and readily available building block.
a 57% isolated yield of the desired disaccharide 17, was ob- at either the donor, the acceptor or at both. With the 6-O-Bn
tained. Comparing the “normal procedure” with the “inverse donor 16 a slightly diminished, but still respectable, 49% yield
procedure” [50], i.e., slow addition of donor 15 to acceptor 9 of the desired product 19 was obtained (Scheme 3). However,
and catalyst, no discernable difference in the 3/4-O-regioselec- when using the 6-O-TBDPS donor 15 (1.2 equiv) a 60% isolat-
tivity or the α/β-ratio was observed, albeit a slightly lower iso- ed yield of the desired disaccharide 20 was obtained. When
lated yield was obtained using the inverse procedure (Table 1, using both the 6-O-Bn donor 10 and acceptor 16 we saw a
entry 1 vs entry 2). Changing the temperature, i.e., room tem- lower selectivity than observed with the TBDPS–TBDPS
perature, 0 °C and −50 °C, had no effects on the 3/4-O-ratio, but variant 17, with an isolated yield of 48% of 21, but as an insep-
resulted in a lower α/β-selectivity (5:1 at −50 °C; Table 1, arable 1.7:1 mixture of the 3/4-O-glycosylated products. To put
entries 3 and 4) [51]. The reaction was also shown to be scal- these results in to perspective, a few closely related glycosyla-
able giving comparable ratios and excellent isolated yields at tions are shown in Scheme 4. Firstly, our own glycosylations
both 1 mmol and 2.6 mmol scales (Table 1, entries 5 and 6). In- using the 4-O-Bz variants, gave a lower yield, when using the
creasing the excess of acceptor 9 to 1.7 equiv, showed no effect TBDPS–TBDPS variant, of 36% [44]. The less sterically
towards the ratios observed (Table 1, entry 6).
hindered Bn–TBDPS variant gave yields of 47%, which are
comparable to the 3,4-diol glycosylation. Glycosylation with a
With the conditions optimized, the selectivity was challenged closely related system having a 4-O-Bn, 6-O-TBDPS pattern
using less bulky 6-O-protecting groups, such as a benzyl group, gave 58% yield using 1.2 equiv of the donor, proving very
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Beilstein J. Org. Chem. 2018, 14, 2805–2811.
Table 1: Optimizing the α,3-O-selective glycosylation conditions.
entry
scale [mmol]
temp.
3/4-Oa
α/βa
isolated yields
1
0.1
0.1
0.1
0.1
1.0
2.6
0 °C
0 °C
−50 °C
rt
8:1
7:1
7:1
7:1
7:1
8:1
10:1
11:1
5:1
17 (57%)
2c
3
17 (47%) 18 (9%)
17 (51%) 18 (10%)
17 (66%) 18 (11%)
17 (67%) 18 (9%)
17 (64%) 18 (9%)
4
10:1
12:1
11:1
5
0 °C
0 °C
6b
aDetermined by 1H NMR on the crude; b1.7 equiv acceptor; cinverse procedure.
Scheme 3: Challenging the α,3-O-selectivity with the different 6-O-protecting group variants.
Scheme 4: Representative glycosylations with closely related systems [34,44].
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Beilstein J. Org. Chem. 2018, 14, 2805–2811.
comparable to our 60% yield for the same 4-O-Bn, 6-O-TBDPS uct 22 and a side-product 23, in which the adjacent TBDPS has
system (Scheme 3) [34]. If utilizing the 3,4-diol glycosylation been cleaved off and the 6-OH subsequently benzylated
herein reported, the acceptor 10 can be synthesized in 4 less (Scheme 5). This side reaction could, however, easily be
steps than otherwise required.
avoided by using a procedure in which BnBr and TBAI was
added and mixed before the addition of NaH. This modification
These results show, how the desired protecting group pattern gave a 91% yield of the product 22. Employing these condi-
can direct which glycosylation strategy to choose: In the less tions to the other three variants 24–26 allowed for much easier
sterically hindered cases, a 4-O-protecting group, such as the separation of the 3/4-O mixtures obtained from the glycosyla-
benzoyl or benzyl, can be preferable. However, when a large tions, especially for the Bn–Bn variant 26.
6-O-protective group, such as TBDPS, is used, it proves advan-
tageous to do the glycosylation directly on the 3,4-diol.
Ultimately, we employed the strategy to synthesize a pseudotri-
saccharide, found as the terminal part of the LTA from S. pneu-
Post-glycosylation modifications, i.e., benzylation of the free moniae. The deprotection of disaccharide 22 to give 27 fol-
4-OH on the TBDPS–TBDPS variant 17 using standard BnBr lowed by formation of the trichloroacetimidate 28 proceeded
followed by NaH resulted in a 1:1 mixture of the desired prod- smoothly, as shown in Scheme 6. The following glycosylation
Scheme 5: Capping the free 4-OH, allowing for easier separation of mixtures obtained during glycosylation.
Scheme 6: Pseudotrisaccharide synthesis for LTA elucidation.
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Supporting Information
Supporting Information File 1
Full description of experimental procedures and
characterization of new compounds.
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