Rapid synthesis of oligomannosides with orthogonally protected monosaccharides

Article abstract of DOI:10.1039/c2cc37099a

We developed a facile synthesis to yield orthogonally protected mannose building blocks with high overall yields. The protection/glycosylation steps can be carried out in a successive manner without purification of intermediate products. This developed synthesis led to formation of linear/branched tri-, penta- and heptasaccharides.

Full text of DOI:10.1039/c2cc37099a

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Rapid synthesis of oligomannosides with orthogonally
protected monosaccharides†‡
Sue-Ming Chang,ab Zhijay Tu,b Hau-Ming Jan,ab Jia-Fu Panb and Chun-Hung Lin*ab
Cite this: Chem. Commun., 2013,
49, 4265
Received 29th September 2012,
Accepted 23rd October 2012
DOI: 10.1039/c2cc37099a
www.rsc.org/chemcomm
We developed a facile synthesis to yield orthogonally protected do not employ orthogonal protecting groups,8 which restricts the
mannose building blocks with high overall yields. The protection/ possibility of further derivatization. We herein report a general
glycosylation steps can be carried out in a successive manner without and practical method for facile preparation of two mannose
purification of intermediate products. This developed synthesis led to building blocks that were orthogonally protected. Both building
formation of linear/branched tri-, penta- and heptasaccharides.
blocks were further derived to afford glycans 1–4 and 5 (the fully
deprotected product of 4) (Fig. 1) that are linear/branched tri-,
Oligomannosides are a major component of asparagine-linked penta- and heptasaccharides. The retrosynthetic analysis of 4
oligosaccharides (N-glycans). In N-glycans, mannose is usually represents one example demonstrating how mannose building
attached to either D-mannose or N-acetyl-D-glucosamine via a1,2-, blocks 6 and 7 are applied for the glycan assembly.
a1,3-, a1,6- and b1,4-linkages. Many bioactive glycoconjugates
Our development started with 6-O-t-butyldiphenylsilyl-1-
also contain multiple mannose residues in correlation with their thio-p-tolylmannopyranoside (10, Scheme 1),9 which was treated
physiological activities, such as GPI anchors,1 fungal cell wall with trimethyl orthobenzoate and a catalytic amount of camphor
mannans,2 and high affinity sugar ligands of concanavalin A3 and sulfonic acid to protect C2- and C3-hydroxyl groups with formation
cyanovirin N.4 Among various mannose-containing oligosaccharides, of orthoesters. Without purification, the product was acetylated with
the 3,6-branched trimannosaccharide (Manal,3[Mana1,6]Man) and acetic anhydride and a catalytic amount of N,N-dimethylamino-
pentamannoside [(Manal,3[Mana1,6]Man)al,6Man3,1aMan] units pyridine (DMAP) at O4, followed by the acidic hydrolysis with 2 N
have been shown to exist in a wide range of important glycoproteins. HCl to generate the protected mannoside (68% in three steps) with
For instance, Mycobacterium tuberculosis is the main cause of O2-benzoate and O4-acetate groups. Further reaction with levulinic
tuberculosis. The pathogen possesses mannose-rich glycophos- acid, EDC and DMAP afforded the fully protected product (11) in
pholipids that were found to not only have the 1,6-linear 92% yield. Consistent with our previous development,10 the condi-
trimannosaccharide, but also effectively avoid the immune tions of these protection reactions are all compatible to be carried
surveillance.5
out sequentially without chromatography of intermediate products.
Since mannose residues can be assembled into a myriad of Interrupted by removal of reaction solvent and simple workup, the
oligomannosides via formation of various glycosidic linkages, conversion from compound 10 to 11 led to the total yield of 64%
there are several concerns associated with the synthesis, such (Scheme 1, also entry 3 of Table 1). Likewise, compounds 6 and 7
as regioselective masking and unmasking of hydroxyl groups, can be obtained in 73% and 67% yields, respectively (entries 1 and 2
stereoselective glycosylation, and retrosynthetic analysis6 (i.e. to of Table 1). The consecutive syntheses of 6, 7 and 11 have been
determine what fragments of glycosides need to be prepared operated at a scale of >15 g. This synthetic approach is flexible,
first). The synthetic efforts thus become labor intensive. Despite allowing for different modifications to generate a large number of
many reports on preparation of oligomannosides,7 most of them protected mannose monosaccharides. The aforementioned acetyla-
tion, for example, can be substituted with benzylation (see entry 2 of
Table 1) or benzoylation (compound SI-13 in Fig. S1, ESI‡) or others.
Trimethyl orthoacetate is applicable to the protection of C2- and
C3-hydroxyl groups, resulting in the formation of O2-acetate after
the partial hydrolysis with 2 N HCl (SI-6–SI-13). Meanwhile,
O3-levulinate ester can also be replaced with other ester- or
ether-groups (e.g. SI-9).11 So far we have prepared sixteen
orthogonally protected mannosides that include compounds
6, 7 and 11 (see Fig. S1, ESI‡).
a Institute of Biochemical Sciences, National Taiwan University, Taipei 10617,
Taiwan
b Institute of Biological Chemistry and the Genomics Research Center, Academia
Sinica, No. 128, Academia Road Section 2, Nan-Kang, Taipei 11529, Taiwan.
E-mail: chunhung@gate.sinica.edu.tw; Fax: +886-2-26514705
† This article is part of the ChemComm ‘Emerging Investigators 2013’ themed issue.
‡ Electronic supplementary information (ESI) available: General experimental
procedures, characterization, NMR spectra, supporting figures and scheme. See
DOI: 10.1039/c2cc37099a
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Fig. 1 Structures of target molecules 1–4 and the retrosynthetic analysis of 4 in which 6 (blue) and 7 (red) serve as the building blocks for the synthesis via proper
deprotection and glycosylation steps. Blue and red colours help to demonstrate how 1–4 are derived from 6 and 7.
Scheme 1 Compound 10 was consecutively protected without purification of intermediate products to give mannoside 11. The operation can include the
subsequent glycosylation to obtain the final product with a high overall yield (see Table 1). Each arrow represents one reaction. Colours designate different reactions
and the resulting groups.
Compounds 11 and 6 then served as glycosyl donors to molecular sieves, the glycosidation produced heptasaccharide 4 in
couple with 6-azidohexanol in the presence of N-iodosucciniimide 82% yield. The remaining reactions (from 4 to 5) correspond to
(NIS), triflic acid (TfOH) and 3 Å molecular sieves to afford the deprotection steps and the introduction of sulfates into C3 of two
glycosylated products 12 and 13 in 71% and 79% yields, respectively terminal Gal residues. These steps include (1) reduction of the
(entries 4 and 5 of Table 1). Combining the glycosylation reaction N-trichloroacetyl group and the terminal azide to produce N-acetyl
with the aforementioned protection procedure, we successfully and the amino groups, respectively, (2) protection of the resulting
carried out five-step consecutive syntheses to generate products amino group to give the benzyl carbamate (82% in two steps), (3)
12–14 and 2 in 50–52% total yields (corresponding to entries 4–7 hydrolysis of the levulinate esters (85%), (4) sulfation of the resulting
of Table 1, respectively).
hydroxyl groups (80%), (5) removal of the TBDPS ethers, (6) hydro-
To prepare mannose oligosaccharides, treatment of compound lysis of acetate and benzoate esters, and (7) hydrogenolysis to
14 with tetrabutylammonium fluoride (TBAF) afforded 18 (84%) remove the benzyl groups (42% in three steps). The procedures
under acidic conditions (Scheme 2), and was followed by NIS–TfOH- are all listed in ESI.‡ Additionally, the structures of these synthesized
promoted glycosylation with donor 7 to give linear trimannoside 1 saccharides (including target molecules 1–5) were determined in
(81%). Meanwhile, O3- and O30-levulinate esters of 18 were hydro- detail by HRMS and several NMR methods, including 1H–1H COSY,
lyzed by using hydrazine monoacetate. The resulting product 19 1H–13C HMQC, 1D-selective TOCSY and DEPT (see Fig. S2, ESI‡
(84%) then served as the acceptor for the triple glycosylation with (A–G) for a detailed explanation).
donor 7 to produce the branched pentamannoside 3 (81%), forming
In summary, we developed a consecutive synthetic procedure to
one al,6- and two al,3-mannosidic linkages at the same time. prepare mannose mono-, di- and trisaccharides. The sugars were
Furthermore, O3b- and O3c-levulinate esters of trimannoside 2 were orthogonally protected, allowing specific deprotection at desirable
replaced by benzoate esters (see the conversion from 2 to 21 with a site(s) at a later stage, as well as fast assembly of glycans. This
total 82% yield in Scheme 2). The subsequent deprotection of silyl method also highlights three features valuable to the field of
ethers by HF–pyridine led to formation of product 8 (85%) that carbohydrate chemistry, including versatility (protecting groups at
contains two unmasked primary alcohols. Compound 9, previously C2, C3 and/or C4-positions can be changed to increase the diver-
employed in the synthesis of LacNAc-containing tetrasaccharide,10 sity), practicability (several compounds can be prepared at a scale of
acted as the donor to transfer the Gal-bl,4-GlcNAc disaccharide to >15 g), and flexibility (several mentioned sugars are glycosyl donors
C6b- and C6c-hydroxyl groups of 8. In the presence of TfOH and 3 Å and can be converted into acceptors after selective deprotection).
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Table 1 Consecutive protection/glycosylation reactions. It is noted that protec-
tion products 6, 7 and 11 can function as glycosyl donors or as acceptors after
specific deprotection (e.g. conversion of 6 to 16/17). Different colours are used to
designate individual reactions
Isolated
yield
Entry
1
Product
Bz2O,
Et3N
—
—
—
73%
2
3
4
BnBr,
NaH
67%
64%
50%
Ac2O,
Et3N
Ac2O,
Et3N
5
6
Bz2O,
Et3N
52%
50%
Scheme 2 Synthetic procedures to prepare oligomannosides 1, 3, 4 and 5.
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J. B. Matson, C. Krishnamurthy, M. Rawat and L. C. Hsieh-Wilson,
BnBr,
NaH
7
BnBr,
NaH
52%
¨
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a
Thiomannoside 10 (1.0 eq.) in CH2CI2 was treated with PhC(OMe)3
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b
(3.0 eq.), CSA. Two methods were applied to protect the C4-hydroxyl
group. To form an ester, Ac2O (or Bz2O, 2.0 eq.) and Et3N (3.0 eq.) in
CH2CI2 were used. To form an ether, BnBr (2.0 eq.) and NaH (4.0 eq.) in
THF were utilized. The reaction mixture was treated with LevOH
(1.5 eq.) and EDC (1.5 eq.) in CH2CI2. For entries 4–7, 1.0 eq. of glycosyl
donor (11, 6, 7 or 7) reacted with an acceptor [15 (1.5 eq.), (15 (1.5 eq.), 16
(0.5 eq.) or 17 (0.25 eq.), respectively] in CH2CI2 at ꢀ40 1C in the presence
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c
d
On the basis of this development, we have been preparing various
fully deprotected oligomannosides (e.g. 5) for examining their
biological activities. The result will be reported in due course.
´
´
Notes and references
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of decomposition if acetate or benzoate existed in the same
molecule.
c
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Chem. Commun., 2013, 49, 4265--4267 4267