Synthesis and glycosidase inhibition of Australine and its fluorinated derivatives

Article abstract of DOI:10.1021/ol503728e

Australine (1), 7-epi-australine (2), and their C-7-fluorinated derivatives 4 and 5 have been synthesized efficiently from d-arabinose-derived cyclic nitrone 11. Fluorination at the C-7 position enhanced the inhibition against A. niger ?±-glucosidase, and

Full text of DOI:10.1021/ol503728e

Letter
pubs.acs.org/OrgLett
Synthesis and Glycosidase Inhibition of Australine and Its
Fluorinated Derivatives
Yi-Xian Li,† Yousuke Shimada,‡ Kasumi Sato,‡ Atsushi Kato,*,‡ Wei Zhang,† Yue-Mei Jia,†
George W. J. Fleet,§,⊥ Min Xiao,∥ and Chu-Yi Yu*,†,⊥
†Beijing National Laboratory for Molecular Science (BNLMS), CAS Key Laboratory of Molecular Recognition and Function,
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
‡Department of Hospital Pharmacy, University of Toyama, 2630 Sugitani, Toyama 930-0194, Japan
§Chemistry Research Laboratory, Department of Chemistry, University of Oxford, Mansfield Road, Oxford, OX1 3TA, U.K.
∥State Key Laboratory of Microbial Technology and National Glycoengineering Research Center, Shandong University, Jinan 250100,
P. R. China
⊥National Engineering Research Center for Carbohydrate Synthesis, Jiangxi Normal University, Nanchang 330022, P. R. China
S
* Supporting Information
ABSTRACT: Australine (1), 7-epi-australine (2), and their C-7-
fluorinated derivatives 4 and 5 have been synthesized efficiently
from D-arabinose-derived cyclic nitrone 11. Fluorination at the C-7
position enhanced the inhibition against A. niger α-glucosidase, and
this constitutes the first example of fluorination substitution for a
hydroxyl increasing the inhibition of any glycosidases. The
enantiomers synthesized from nitrone ent-11 showed no inhibition
of the corresponding enzymes.
ustraline (1) was isolated from seeds of Castanospermum
1
Aaustrale in 1988 and can be viewed as a conformationally
fixed 2,5-dideoxy-2,5-imino-D-mannitol (DMDP, 6) analogue.
As one of the most investigated iminosugars, australine (1) is a
potent inhibitor of α-glucosidase,2 a competitive inhibitor of
amyloglucosidase,1,3 and the first polyhydroxylated pyrrolizi-
dine inhibiting glycoprotein processing enzyme glucosidase I,3b
which also displays antiviral4 and anti-HIV activity.5 Compared
to DMDP (6),6 the glycosidase inhibition profile of australine
(1) was believed to arise from the locked bicyclic structure
which abolished the C2v symmetry of the system.6d,7 In the
context of our continuing interest in the synthesis and
biological activities of DMDP-related iminosugars,8 we became
interested in the synthesis and structure−activity relationship
study of australine and its derivatives, especially C-7-fluorinated
derivatives (Figure 1).
Figure 1. Iminosugars related to australine.
Fluorination is a powerful tool in structure−activity relation-
ship studies.8d,9 The importance of the C-6 hydroxyl group of
related iminosugars. Moreover, synthetic enantiomers of some
naturally occurring iminosugars are more potent glycosidase
inhibitors.10 For example, L-DMDP (ent-6) is a more powerful
and specific α-glucosidase inhibitor than DMDP (6).8d,10a,f
Therefore, the synthesis and glycosidase inhibition evaluation
of enantiomers of australine (ent-1) and its derivatives (ent-2)-
(ent-5) could help to provide a better understanding of the
SAR of australine and DMDP-related iminosugars.
DMDP in glycosidase inhibition had been reported by Stutz
̈
and co-workers.9b Study of the role of the C-7 hydroxyl of the
bicyclic system of australine could prove insightful, since the C-
7 hydroxyl of australine corresponds to the C-6 hydroxyl of
DMDP. In this regard, fluorination of iminosugar hydroxyls
usually decreases glycosidase inhibition except for a few
examples.9e
The synthesis and glycosidase inhibition evaluation of 7-
deoxy-7-fluoro-australine (3), 7-deoxy-7-fluoro-7-epi-australine
(4), and 7,7-difluoro-7-deoxyaustraline (5) could help us to
understand the SAR of australine and that of the DMDP-
Received: December 26, 2014
© XXXX American Chemical Society
A
DOI: 10.1021/ol503728e
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Synthetic methods for accessing australine or its derivatives
have been well documented in the literature via a range of
approaches.11 Our strategy was based on the sugar-derived
nitrone 1112 which can be easily converted to aldehyde 10,
establishing the bicyclic structure in an efficient way (Scheme
1).8d,12h The aldehyde can then be converted to australine
Scheme 2. Synthesis of Australine and Its Derivatives
Scheme 1. Retrosynthesis of Australine and Its Derivatives
fluoride, affording the configuration-retained product 13. As for
alcohol 7, fluorination proceeded in an ordinary SN2 pattern,
giving the configuration-inverted product, i.e., 13. After
hydrogenation of the fluorinated derivative 13, 7-deoxy-7-
fluoro-7-epi-australine (4) was obtained in high yield.
To prepare the C7-difluorinated derivative of australine, i.e.
7,7-difluoro-7-deoxyaustraline (5), the alcohols 7 and 8 were
transformed into ketone 14 via Swern oxidation in 87% yield.
Treatment of ketone 14 with DAST afforded the desired C-7-
difluorinated derivative 15 together with a tricyclic byproduct
16. Debenzylation of 15 by hydrogenation gave the expected
difluorinated product 5 in 96% yield (Scheme 3).
derivatives 7 and 8 by Grignard or Barbier reactions,
ozonization, and reductive amination. Direct hydrogenation
of 7 and 8 would give australine (1) and its C-7 epimer 2, while
fluorination of the suitably protected 7 and 8 and subsequent
deprotection would afford fluorinated australine derivatives 3−
5.
Thus, the key intermediate, aldehyde 10, which was prepared
via a reported method from nitrone 11 in 81% overall yield
within 4 steps,8d,12h reacted with allylzinc bromide under
Barbier conditions13 to give 12 as an inseparable mixture of two
Scheme 3. Synthesis of 7,7-Difluoro-7-deoxyaustraline (5)
1
isomers (dr = 35/65, determined by H NMR). Sequential
ozonation and reductive amination of 12 afforded tri-O-
benzylated australine derivatives 7 and 8 in 89% yield (dr =
23/77) which were separated by silica gel chromatography. The
configuration of the C-7 hydroxyl was determined by
comparing the optical rotation and NMR data of 7 and 8
20
with those reported for 1,2,8-tri-O-benzyl-australine [7: [α]D
+9.0 (c 1.11 in CH2Cl2); lit. of 7:11d [α]D +8.4 (c 1.6 in
25
11l
20
20
EtOH); 8: [α]D −9.8 (c 1.02 in CH2Cl2); lit. of 8: [α]D
−27 (c 0.33 in CHCl3)]. Further hydrogenolysis of 7 and 8 led
to australine (1) and 7-epi-australine (2), respectively, in high
yields. The H and 13C NMR spectra of compounds 1 and 2
1
were identical to those reported for australine and 7-epi-
20
australine with optical rotations [1: [α]D +9.0 (c 1.12, H2O);
20
2: [α]D −12.0 (c 1.84, H2O)] consistent with literature
[australine: [α]D +8.0 (c 0.35, H2O);11d 7-epi-australine:
25
[α]D −14.1 (c 0.22, H2O), [α]D −13.2 (c 1.2, H2O)14b].
Treatment of the alcohols 7 and 8 (Scheme 2) with DAST
(diethylaminosulphur trifluoride) unexpectedly gave the same
fluorinated product 13. The structure of 13 was determined by
spectroscopic data, and its C-7 configuration was determined as
an R-configuration through a NOESY experiment (13: H1 and
H7; Supporting Information). Fluorination of 8 with DAST
may have occurred via an aziridinium ion intermediate,11a,15
which opened with a second inversion when attacked by
14a
24
23
A plausible reaction pathway for formation of the C-7
difluorinated product 15 and the tricyclic byproduct 16 is
shown in Figure 2. Reaction of ketone 14 with DAST formed
the intermediates 17a and 17b, where both would give product
15 by an SN2 attack of fluoride.16 It is also possible to finish the
reaction in another way; that is, an azidirinium system can be
formed by losing the leaving group, and the ion is then opened
B
DOI: 10.1021/ol503728e
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: kato@med.u-toyama.ac.jp.
*E-mail: yucy@iccas.ac.cn.
Notes
The authors declare no competing financial interest.
Figure 2. Possible intermediate in formation of 15 and 16.
ACKNOWLEDGMENTS
■
Financial support from National Basic Research Program of
China (No. 2012CB822101), the National Natural Science
Foundation of China (Nos. 21202173 and 21272240), National
Science and Technology Major Projects for “Major New Drugs
Innovation and Development” (2013ZX09508104), Develop-
ment Fund for Collaborative Innovation Center of Glyco-
science of Shandong University, and National Engineering
Research Center for Carbohydrate Synthesis of Jiangxi Normal
University is gratefully acknowledged. This work was also
supported in part by a Grant-in-Aid for Scientific Research (C)
(No. 26460143) from the Japanese Society for the Promotion
of Science (JSPS).
by a second fluoride to give product 15.11a,15 Since the C-8
benzyloxy group can also act as a nucleophile in 17a and 17b,
an intramolecular SN2 reaction in 17b would furnish the cyclic
ether in product 16,14b,17 while such a reaction is not favorable
in 17a.
Through similar procedures, the enantiomers of australine
(ent-1) and its derivatives ent-2, ent-4, and ent-5 were also
synthesized from the D-xylose-derived nitrone ent-11 smoothly.
These compounds were then assayed as potential inhibitors
for a range of glycosidases (see the Supporting Information).
Australine (1) and its C-7 diastereomer (2) proved to be
potent inhibitors of A. niger α-glucosidase (IC50 = 6.0 and 1.5
μM, respectively) and A. niger amyloglucosidase (IC50 = 37 and
8.0 μM, respectively). By contrast, ent-1 and ent-218 showed
virtually no inhibition of the tested enzymes, which is in sharp
contrast to the more potent glycosidase inhibition of L-DMDP
(ent-6) than DMDP (6).8d,10a,f For fluorinated derivatives,
compound 4 turned out to be a potent inhibitor of A. niger α-
glucosidase (IC50 = 0.63 μM), which is 10 times better than
australine (1). Compound 4 also behaved as a moderate
inhibitor of porcine kidney trehalase (IC50 = 31 μM), while
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