Amine-linked diquercitols as new α-glucosidase inhibitors

Article abstract of DOI:10.1016/j.bmcl.2014.09.064

Two new diastereomeric amine-linked diquercitols 7 and 8 were synthesized by reductive amination of ketoquercitol 4 and epimeric aminoquercitols 3 and 6. The ketone and amines were successfully prepared, without the formation of byproducts, from naturally available (+)-proto-quercitol (1). The amine-linked diquercitols showed inhibitory effect against α-glucosidases with more pronounced potency than their original aminoquercitol monomers.

Full text of DOI:10.1016/j.bmcl.2014.09.064

Bioorganic & Medicinal Chemistry Letters 24 (2014) 5530–5533
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Amine-linked diquercitols as new
a
-glucosidase inhibitors
⇑
Wisuttaya Worawalai, Sumrit Wacharasindhu, Preecha Phuwapraisirisan
Natural Products Research Unit, Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new diastereomeric amine-linked diquercitols 7 and 8 were synthesized by reductive amination of
ketoquercitol 4 and epimeric aminoquercitols 3 and 6. The ketone and amines were successfully pre-
pared, without the formation of byproducts, from naturally available (+)-proto-quercitol (1). The
Received 13 August 2014
Revised 8 September 2014
Accepted 24 September 2014
Available online 6 October 2014
amine-linked diquercitols showed inhibitory effect against
potency than their original aminoquercitol monomers.
a-glucosidases with more pronounced
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Quercitol
Glucosidase
Diabetes
Aminocyclitol
Reductive amination
OH
Carbasugars are glycomimics in which the oxygen atom of the
pyranose ring is replaced by carbon. They involve several biological
events by inhibiting glycosidases, leading to potential applications
in the therapy of viral infections,1 cancer2 and diabetes.3 Prominent
examples of carbasugars include aminocyclitols or pseudoami-
nosugars, whose structures comprise one or more amino groups
in place of hydroxy moieties. Aminocyclitols are exclusively pro-
duced by Actinomycetes bacteria. The presence of amino group
as N-linked glycosidic bond in various aminocyclitols has been
noted for pronounced glycosidase inhibition, which was later elab-
orated by the unusually tight binding between amino linkage of
inhibitor and acidic group of the enzyme.4 The most recognized
aminocyclitols (Fig. 1) that mimic polysaccharide include acarbose,
whose structure contains one aminocyclitol connected with trisac-
charide. Its potent inhibition against human intestinal glucosidase
and wide use for diabetes therapy have triggered the synthesis of
aminocyclitols connected with sugar or cyclitol moieties such as
oligobiosaminide.5
O
O
OH
HO
HO
Polysaccharide (amylose)
O
O
OH
HO
HO
O
O
HO
OH
HO
O
O
HO
HO
HO
HO
HO
O
H3C
HO
O
NH
OH
OH
aminocyclitol
HO
O
O
HO
OH
HO
O
O
acarbose
HO
HO
HO
OH
OH
NH
HO
HO
H3C
O
HO
OH
HO
OCH3
In our previous studies, we have synthesized diastereomeric
aminocyclitols from naturally available (+)-proto-quercitol.6
Subsequent investigation by installing a variety of aryl and alkyl
moieties onto amino group led to the discovery of more
oligobiosaminide
Figure 1. Polysaccharide-mimic aminocyclitols.
pronounced a-glucosidase inhibitors; some of which are 2–8 times
more potent than antidiabetic drug acarbose.6 To date, over 40
aminoquercitols6,7 and related analogues8,9 have been generated
from (+)-proto-quercitol.
The success of preparing diverse aminocyclitol derivatives is
due mainly to exclusive formation of single bisacetonide in the first
step; in contrast, up to 50% of material is lost if other stereoisomers
of quercitol are employed.10 With the success of our approach in
hand, we herein plan to synthesize new aminocyclitols containing
two (+)-proto-quercitol residues connected through N-linked gly-
⇑
Corresponding author. Tel.: +66 2 2187624; fax: +66 2 2187598.
E-mail address: preecha.p@chula.ac.th (P. Phuwapraisirisan).
cosidic bond and evaluate their
a-glucosidase inhibition. In fact,
http://dx.doi.org/10.1016/j.bmcl.2014.09.064
0960-894X/Ó 2014 Elsevier Ltd. All rights reserved.

W. Worawalai et al. / Bioorg. Med. Chem. Lett. 24 (2014) 5530–5533
5531
OH
O
OH
the idea of N-linked dicyclitols was introduced by Hudlicky and co-
workers since 2002;11 however, all synthesized targets (Fig. 2)
showed weak inhibition (IC50 370–2,000 M). In the recent study,
we hope that the replacement of the above cyclitols by (+)-proto-
quercitol would promote inhibitory effect more potent than other
N-linked dicyclitols previously reported.
OH
OH
a
O
O
O
O
e
1
5
3
l
75%
HO
54%
O
O
OH
O
O
1
4
2
The starting material (+)-proto-quercitol (1) utilized in this
investigation was obtained from the stems of Arfeuillea arborescens
using the procedure described elsewhere.6a Crucial to the success-
ful synthesis of amine-linked diquercitols (7 or 8) was the use of
reductive amination of aminoquercitols 3 or 6 and ketoquercitol
4 (Scheme 1), in which high diastereoselectivity was observed in
each step.
b, 78%; c, 79%; d, 71%
f, 93%
OH
NH2
O
O
O
O
O
O
O
O
5
All key coupling intermediates 3, 4 and 6 were synthesized from
(+)-proto-quercitol (1) (Scheme 2), using our previous methodol-
ogy.6a Initially, hydroxyl groups in 1 were protected by reaction
with Me2C(COMe)2 in the presence of p-TsOH as a catalyst, yielding
exclusively bisacetonide 2 in 75% yield. This observation could be
rationalized by initially favored formation of cis-acetonide at C-2
and C-3 followed by inevitable trans-acetonide formation at C-4
and C-5, resulting in the free hydroxyl group at C-1 for further
functionalization. The remaining hydroxyl group in 2 was con-
verted to a better leaving group (a mesyl moiety), which was fur-
ther substituted by azide group (NaN3). The desired
aminoquercitol 3 was eventually obtained by hydride reduction
using LiAlH4. Noticeably, transformation of 2–3 proceeded stereo-
specifically with inversion of configuration.
3
b, 82%; c, 84%; d, 43%
NH2
O
O
O
O
6
Scheme 2. Reagents and conditions: (a) Me2C(OMe)2, DMF, p-TsOH; (b) MeSO2Cl,
Et3N, DMAP; (c) NaN3, DMF, 15-crown-5-ether, 100 °C; (d) LiAlH4; (e) Ac2O, DMSO;
(f) LiAlH4.
Having aminoquercitol 3 in hand, we turned our plan to gener-
ate another coupling motif, ketoquercitol 4. Starting from 2, the
target ketone 4 was synthesized in moderate yield (54%) by Alb-
right-Goldman oxidation12 using Ac2O-DMSO. At this step, we also
have an idea to produce another aminoquercitol that is epimeric to
3, which would be coupled to ketone 4 to produce diastereomeric
N-linked diquercitol. Ketone 4 was first reduced by LiAlH4, afford-
OH
OH
OH
OH
H
N
1) NaBH3CN, AcOH,
MeOH, 31%
HO
HO
OH
OH
3
4
2) HCl/MeOH, Dowex
50W-X8 (H+), 70%
7
OH
OH
OH
1) NaBH3CN, AcOH,
MeOH, 21%
OH
H
HO
HO
N
OH
OH
6
4
RN
OH
2) HCl/MeOH, Dowex
50W-X8 (H+), 35%
HO
HO
OH
OH
8
OH
R
IC50 ( M)
µ
Scheme 3.
amine-linked diconduritol
H
1,400
1,350
acetamide-linked diconduritol Ac
ing the hydroxybisacetonide 5 with excellent yield (93%) as sole
product. The desired aminoquercitol 6 was thus generated using
the similar conditions applied for 3.
HO
HO
RN
OH
OH
Having aminoquercitols 3 and 6 together with ketone 4 in hand,
the required N-linked diquercitols were synthesized by reductive
amination13 using NaBH3CN14 (Scheme 3). Coupling of 3 and 4 fol-
lowed by deprotection under acid condition afforded amine-linked
diquercitol 7 while compound 8 was generated from 6 and 4.
Although amine-linked diquercitols 7 and 8 were obtained as sin-
gle product in each synthetic route, the configuration of newly gen-
erated chiral center (C-1) remained unclear.
R'
HO
HO
OH
OH
OH
R
R'
IC50 (µM)
acetamide-linked inositol
amine-linked inositol
diaminoinositol dimer
Ac OH
H
H
370
>2,000
>2,000
OH
NH2
The severely overlapped 1H NMR signals, particularly in diag-
nostic region (d 3.0–4.0 ppm), made impossible determination of
configuration. We therefore turned our attempt to inspect 1H
NMR data of the bisacetonides (7a and 8a) of 7 and 8 because they
displayed well-separated spectra. We have demonstrated, in our
previous report,6a that 1H NMR pattern and coupling constants of
methylene protons in 3 and 6 are distinct enough to apply for
predicting configuration of amino-connected chiral carbon as
Figure 2.
O
OH
O
OH
1'
OH
H
N
O
O
O
OH
OH
O
O
NH2
HO 3'
HO
1
OH
OH
*
3
*
+
HO
5'
5
O
OH
O
OH
OH
a
- or b-oriented. Apparently, bisacetonide 7a revealed a single set
7
8
3
6
4
1
or
or
of methylene protons (H2-6/and H2-60) identical to those of 3 whereas
bisacetonide 8a contained two different patterns of 3 and 6 (Fig. 3).
Scheme 1.

5532
W. Worawalai et al. / Bioorg. Med. Chem. Lett. 24 (2014) 5530–5533
O
O
O
O
H
N
O
O
O
O
H-6'ax
H-6ax
6
6
'
H-6'eq
H-6eq
7a
2
O
O
O
O
H
N
O
O
O
O
H-6ax
6
6
'
H-6'ax
H-6'eq
1
H-6eq
8a
3.1
2.6
2.1
1.6
1
Figure 3. Partial H NMR spectra of 7a and 8a (CDCl3) focusing on signals of methylene protons (H2-6 and H2-60).
consistent with only six carbon signal observed in 13C NMR
spectrum.
Based on aforementioned data, the proposed mechanism on the
formation of amine-linked diquercitols 7 and 8 would be rational-
ized in Scheme 4. Noticeably, a more steric hindrance on the bot-
tom face of imine, contributed by sterical control of acetonides,
RNH2
favoured
H
O
O
O
O
O
-H2O
O
O
O
disfavoured
O
RN
4
imine
H
allowed
Amine-linked diquercitols 7 and 8 were evaluated for inhibitory
effect toward -glucosidases (Table 1) from two different sources;
baker’s yeast (type I) and rat intestine (maltase & sucrase, type
II).15 They showed moderate inhibition (32.3–40.1
M) against
yeast -glucosidase while 10-time more potent inhibition (3.1–
4.0 M) toward rat intestine was observed. Although 7 and 8 are
diastereomers different in configuration of C-10, they revealed
comparable inhibition against all -glucosidases tested. Compared
a-attack of hydride (Na BH3CN).
1) NaBH3CN
2) deprotection
a
OH
OH
HO
HO
l
OH
OH
OH
or
a
HN
OH
NH
OH
OH
l
OH
OH
HO
OH
OH
HO
a
8
7
to the original aminoquercitols 3a and 6a (Fig. 4) obtained from
acetonide deprotection of 3 and 6, the amine-linked diquercitols
7 and 8 showed slightly improved inhibitory effect against maltase
Scheme 4. Proposed mechanistic formations of 7 and 8.
and sucrase. In addition, amine-linked diquercitols
7 and 8
revealed much more potent inhibition (ca. 93–500 times) than
other related N-linked dicyclitols.11 It is likely that structural motif
of cyclitol would also critically participate in exerting the inhibi-
tory effect, in addition to the presence of N-linked glycosidic bond.
In conclusion, we reported the first synthesis of diastereomeric
amine-linked diquercitols 7 and 8 through reductive amination of
ketone 4 and epimeric amines 3 and 6. The coupling intermediates
were successfully prepared, without the formation of byproducts,
from naturally available (+)-proto-quercitol (1). The synthesized
amine-linked diquercitols 7 and 8 showed comparable inhibition
Table 1
a
-Glucosidase inhibitory effect of synthesized compounds
Compound
IC50 (lM)
Baker’s yeast
Maltase
Sucrase
3a
6a
7
8
2,890.0
12.5
32.3
40.1
403.9
5.8
4.4
3.1
3.6
1.5
7.3
6.8
3.7
4.0
2.4
Acarbose
against a-glucosidases, while the inhibitory effect toward maltase
and sucrase were more enhanced than the original aminoquercitol.
Interestingly, they inhibited glucosidase function much more
potent than other related N-linked dicyclitols, therefore suggesting
pivotal role of quercitol residue in addition to the presence of
nitrogen-linkage.
NH2
OH
NH2
OH
OH
HO
OH
OH
HO
OH
3a
6a
Figure 4.
Acknowledgments
This project was supported by the Thailand Research Fund
(DBG5380037). W.W. acknowledges the Royal Golden Jubilee
Ph.D. Program (PHD/0025/2553) for fellowship.
Therefore, the newly generated chiral C-6s in 7a and 8a would be
-oriented as shown. Noticeably, the occurrence of -oriented C-6
and C-60 in 7a resulted in its meso compound character, which was
a
a

W. Worawalai et al. / Bioorg. Med. Chem. Lett. 24 (2014) 5530–5533
5533
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Supplementary data
Supplementary data (experimental procedure and NMR spec-
tra) associated with this article can be found, in the online version,
at http://dx.doi.org/10.1016/j.bmcl.2014.09.064.
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(1R,2S,3S,4S,5S)-5-((10R,20R,30R,40R,50S)-2,3,4,5-tetrahydroxycyclohexylamino)-
cyclohexane-10,20,30,40-tetraol (7) or amine-linked diquercitol 7: white solid, 1H
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d, J = 9.6 Hz, 1H), 1.88 (br d, J = 12.0 Hz, 1H), 1.58 (q, J = 11.2 Hz, 1H); 13C NMR
(D2O, 100 MHz) d 73.7, 71.9, 69.2, 67.8, 50.8, 30.2; HRMS m/z 310.1499 [M+H]+
(calcd for C12H24NO8, 310.1502). (1R,2S,3S,4S,5S)-5-((10S,20R,30R,40R,50S)-
2,3,4,5-tetrahydroxycyclohexylamino)cyclohexane-10,20,30,40-tetraol (8) or
amine-linked diquercitol 8: pale yellow oil, 1H NMR (D2O, 400 MHz) d 3.88
(br s, 1H), 3.63–3.66 (m, 3H), 3.51 (m, 1H), 3.25–3.34 (m, 3H), 2.99 (br s, 1H),
2.76 (br d, J = 8.0 Hz, 1H), 1.67–1.80 (m, 3H), 1.39 (m, 1H); 13C NMR (D2O,
100 MHz) d 74.4, 73.7, 72.8, 71.8, 71.5, 70.2, 69.5, 69.2, 52.3, 51.3, 32.6, 30.8;
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