An alternative and facile synthetic approach for the precursors of 3- and 6-aminosugar donors and study of one-pot glycosyltrasferation

Article abstract of DOI:10.1246/cl.190772

3- and 6-aminosugars are common motifs in bioactive compounds playing pivotal roles in the bioactivity of the core molecules to which they are attached. However, de novo synthesis of 3- and 6-aminosugars and their corresponding glycosyl donors can be time

Full text of DOI:10.1246/cl.190772

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An Alternative and Facile Synthetic Approach for the Precursors of 3- and 6-Aminosugar Donors and
Study of One-Pot Glycosyltrasferation
Uddav Pandey, Yagya Prasad Subedi, Madher N. Alfindee,
Taylor Shepherd, and Cheng-Wei Tom Chang*
Advance Publication on the web November 8, 2019
doi:10.1246/cl.190772
© 2019 The Chemical Society of Japan
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1An Alternative and Facile Synthetic Approach for the Precursors of 3- and 6-Aminosugar Donors and
Study of One-Pot Glycosyltrasferation
Uddav Pandey, Yagya Prasad Subedi, Madher N. Alfindee, Taylor Shepherd, Cheng-Wei Tom Chang*
Department of Chemistry and Biochemistry, Utah State University, 0300 Old Main Hill, Logan, UT, 84322-300, USA
E-mail: tom.chang@usu.edu
1
2
3
4
5
6
7
8
9
3- and 6-aminosugars are common motifs in bioactive
compounds playing pivotal roles in the bioactivity to which
the core molecules they are attached. However, de novo
synthesis of 3- and 6-aminosugars and their corresponding
glycosyl donors can be time and resource consuming. The
reported acid-catalyzed hydrolysis of kanamycin derivatives
offers an alternative route to the synthesis of these
molecules. Not only can the 3- and 6-aminosugars be
provided, but the direct chemical glycosyltransferation of
39 amino group manipulation. For example, regioselective
40 conversion of 6-OH of glucose to 6-NH2 takes 4-6 steps
41 depending on the availability of the starting material
42 (Scheme 1).9 The stereoselective synthesis of 3-
43 aminosugar usually started with commercially available
44 diacetone-D-glucose. Multiple synthetic steps and at least
45 2-3 column chromatography purification were needed to
46 offer the precursor of glycosyl door, 4 making the overall
47 process time consuming and expensive.10 In addition,
48 hazardous chromium-based oxidants were employed in
49 several reported syntheses.10,11 Therefore, there is a need
50 for an alternative route for 3-and 6-aminoglucopyransyl
51 donors. Herein, an alternative route involving the
52 hydrolysis of azidokanamycin and cbz-kanamycin is
53 developed for accessing these glycosyl donors in fewer
54 synthesis steps. In addition, the feasibility of one-pot
55 glycosyltransferation during the hydrolysis of acid-
56 catalyzed hydrolysis of kanamycin A derivatives is also
57 demonstrated.
10 these aminosugars is proven feasible.
11 Keywords: Kanamycin-hydrolysis, Aminosugars,
12 Glycosyltransferation
13 Aminosugars, such as 3- and 6-aminosugars are
14 prevalent in diverse naturally occurring or synthetic
15 bioactive compounds.1,2 For example, 3-aminosugar can
16 be found in antibacterial erythromycin and azithromycin,
17 anticancer daunorubicin, and antifungal amphotericin B
18 (Figure 1). A synthetic aminoglycoside, TC007,3 which
19 has been reported for its uses as the potential treatment of
20 spinal muscular atrophy (SMA), also contains a 3-
21 aminosugar. The glycolipid bearing 6-aminosugar has
22 been investigated for its antiviral activity.4 In addition,
58
23 many
bioactive
glycosides
often
employ
24 glycodiversification approach to incorporate various
25 aminosugars for revealing useful structure-activity
26 relationship.5-7 All these point to the essential need to
27 prepare corresponding aminosugar-based glycosyl
28 donors.
59
60 Scheme 1. Reported syntheses of aminosugars from
61 glucose.
62
63 Naturally occurring aminoglycosides, such as kanamycin
64 class of antibiotic, consist of various aminosugars that
65 are linked together via glycosidic bond (Figure 2).12-14
66 Thus, an alternative approach for the synthesis of
67 glycosyl donors of 3- and 6-aminosugars is to conduct
68 acid-catalyzed hydrolysis of aminoglycosides and yield
69 the desired aminosugars. Considering the synthetic
70 efficiency and cost, we selected kanamycin A, which has
71 6-aminosugar as the ring I and 3-aminosugar as ring III,
72 as the starting material for the initial study.
73
29
30 Figure 1. Bioactive compounds bearing 3- and 6-
31 aminosugars
32
33 Most of the reported chemical synthesis of glycosyl
34 donors carrying 3- and 6-aminosugar scaffolds start with
35 cost-effective
36 transformation can be labor-intensive, which involves
37 many protections and deprotection steps, stereoselective
38 and regioselective modification of hydroxyl groups, and
glucose.8
However,
the
overall
74 Acid-catalyzed hydrolysis of kanamycin offers two
75 aminosugars (rings I and III), which are difficult to

2
1
2
3
4
5
6
7
8
9
separate, especially the aminosugars from rings I and III
come in as a mixture of anomers.15,16 In addition, the
reported methods, commonly using hydrochloric acids
and sulfuric acids, cause the formation of brownish by-
products in the viscous form which makes the
purification of the desired products even more
35 Table 1. Different conditions for the hydrolysis of
36 tetraazidokanamycin A
37
Entry
Solvents
Acid
Product (%
yield)
1
MeOH
HCl
6 (34) + 7
(11) +8 (2)
challenging.
Thus,
we
decided
to
employ
(MeOH
and AcCl)
TMSOTf
azidokanamycin16 as the starting material since the
resulting azidosugars can be purified with traditional
2
3
4
DCM
DCM
No
Hydrolysis
No
10 silica gel-based column chromatography.
BF3-OEt2
TMSOTf
Hydrolysis
1,4-
6
(90)
Dioxane
+Inseparable
Mixture
5
Acetonitrile TMSOTf
6
(90)
+
Inseparable
Mixture
38
39 With the establishment of optimal conditions, we also
40 examined the acid-catalyzed hydrolysis using
11
12
41 pentaazidokanamycin
B and hexaazidoneomycin B
42 (Scheme 3). Excellent yields were observed from the
43 hydrolysis of both aminoglycoside derivatives using the
44 AcCl/MeOH condition. Interestingly, the yields were not
45 changed, even after changing the condition to
46 TMSOTf/MeCN. For “pentaazidokanamycin B (9)”, only
47 the glycosidic bond between rings II and III was cleaved,
48 resulting the formation of tetraazidoneamine and the
49 corresponding 2-azidosugar (ring II). For the hydrolysis
50 of “hexaazidoneomycin B (11)”, there was also no
51 hydrolysis of the glycosidic bond between rings I and II
52 leading to the formation of tetraazidoneamine and
53 mixture probably produced from breaking of glycosidic
54 bonds between rings II and III, and III and IV.
13 Figure 2. Structures of kanamycin class antibiotics.
14
15 Various solvents and acids were examined for the
16 hydrolysis of “tetraazidokanamycin (5)” (Scheme 2), and
17 the results are summarized in Table 1. For purification,
18 all of the reactions were quenched with methanol to yield
19 methyl glycosides except when methanol was used as a
20 solvent (Entry 1). From the results, HCl generated from
21 mixing AcCl with MeOH provided the best outcome to
22 obtain 3- and 6 amino sugar precursors, yielding
23 compounds 6 (34%), 7 (11%), and 8 (2%). The isolated
24 azidosugars, 7 and 8, can be converted into the
25 corresponding glycosyl donors using reported protocol.17
26 All other conditions generated either no hydrolysis or a
27 mixture of products that could not be separated. This
28 might be because of the solubility issue of starting
29 material in the solvent used or the acid not being strong
30 enough to hydrolyze the glycosidic bond.
31
32
33 Scheme 2. Hydrolysis of tetraazidokanamycin A
34

3
1
2
Scheme 3. Hydrolysis of different aminoglycosides
3
4
5
6
7
8
9
In azidokanamycin A, which has a hydroxyl group (-OH)
at 2′ position, resulted in the breaking of the glycosidic
bond between ring I and II. The presence of hydroxyl
group could have eased the intramolecular hydrogen
bonding to favor the hydrolysis of the glycosidic bond.
However, the absence of intramolecular hydrogen
16
17 Scheme 4. Synthesis of 3- and 6-amino sugar donors.
18
19 Although
20 tetraazidokanamycin
acid-catalyzed
hydrolysis
the
of
A
furnished
desired
21 aminosugars in just 2 synthetic steps compared to the
22 previously reported method17 which required 5 steps to
23 achieve the products , the cost of synthesis was high with
24 poor yield. Therefore, we also examined the use of
25 tetracarbobenzoxy (Cbz)-protected kanamycin A, 1219 to
26 provide aminosugars.
10 bonding in the presence of the azido group (N3) in 9
11 could have prohibited the breaking of the bond between
12 ring I and II in the aminoglycosides subjected to
13 hydrolysis. This result is consistent with the hydrolysis of
14 azidotobramycin that has been reported previously.18
15
27
28 The synthesis of compound 12 from kanamycin A was
29 much cheaper, resulted in higher yield, and involved no
30 safety issues compared to the synthesis of
31 azidokanamycin A. Compound 12 was subjected to
32 hydrolysis using MeOH/AcCl and various Lewis acids.
33 To our delight, not only the Cbz-protected aminosugars
34 can be obtained by using MeOH/AcCl, but the yields are
35 better than hydrolysis of tetraazidokanamycin A (Scheme
36 4). The subsequent acetylation of compound 13 and 15
37 resulted in the synthesis of amino sugar donor’s
38 precursors, compound 16 and 17 in an excellent yield.
39 Interestingly, no hydrolysis was observed when using
40 other acids like TsOH, TMSOTf, and BF3-OEt2.
41
42 Glycosyltransferation is common in an enzymatic
43 process catalyzed by glycosyltransferases. However, it is
44 rather unusual in chemical synthesis. To expand the
45 applications of the developed protocol, we attempted to
46 explore the possibility of utilizing alcohols other than
47 MeOH for the hydrolysis, so the methodology can be
48 feasible for possible chemical glycosyltransferation. In
49 this design, the aminosugars on aminoglycosides will

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2
3
4
5
6
7
8
9
undergo cleavage of glycosidic bonds and subsequent
glycosylation with provided alcohol in one-pot. Among
the alcohols and acids examined, isopropanol and 1-
octanol under 4 N HCl in 1,4-dioxane provided the
hydrolyzed adducts (Scheme 5). Although the only
modest yields were obtained, the outcome proved that it
is possible to conduct hydrolysis of 12 and use the
28 work was supported in part by NSF Award CHE-
29 1429195 for a 500 MHz Bruker NMR.
30 Supporting
Information
is
available
on
31 http://dx.doi.org/10.1246/cl.******.
32
33 References and Notes
hydrolyzed
aminosugars
for
chemical
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C. J. Thibodeaux, C. E. Melançon, H. Liu, Nature 2007, 446, 1008.
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13
14 Scheme 5. One pot glycosyltransferation.
76
15
16 In conclusion, we have developed convenient protocols
17 to furnish the protected 3- and 6-aminosugars. These
18 protocols utilizing the hydrolysis of aminoglycosides can
19 shorten the synthetic steps as compared to other methods
20 using unmodified pyranoses as the starting material. In
21 addition, our results suggest that the presence of
22 hydrogen bonding at the 2′ position can direct the
23 composition of products from acid hydrolysis of
24 aminoglycosides. Finally, we have proved that it is
25 possible to conduct glycosyltransferation in one-pot.
26 We acknowledge the support from the Department of
27 Chemistry and Biochemistry, Utah State University. This