Synthesis and glycosidase inhibition of the enantiomer of (-)-steviamine, the first example of a new class of indolizidine alkaloid

Article abstract of DOI:10.1021/ol1007718

(+)-Steviamine, the enantiomer of the natural (-)-steviamine, and its corresponding C5 epimer have been synthesized from the d-ribose-derived cyclic nitrone. (-)-Steviamine was found to be the first naturally occurring iminosugar that causes any inhibition of α-galactosaminidases.

Full text of DOI:10.1021/ol1007718

ORGANIC
LETTERS
2010
Vol. 12, No. 11
2562-2565
Synthesis and Glycosidase Inhibition of
the Enantiomer of (-)-Steviamine, the
First Example of a New Class of
Indolizidine Alkaloid
Xiang-Guo Hu,†,‡ Barbara Bartholomew,§ Robert J. Nash,§ Francis X. Wilson,⊥
George W. J. Fleet,# Shinpei Nakagawa,| Atsushi Kato,| Yue-Mei Jia,†
Renate van Well,⊥ 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, Graduate UniVersity of The Chinese Academy of
Sciences, Beijing 100049, China, Phytoquest Limited, IBERS, Plas Gogerddan,
Aberystwyth SY23 3EB, Ceredigion, Wales, U.K., Department of Hospital Pharmacy,
UniVersity of Toyama, 2630 Sugitani, Toyama 930-0194, Japan, Summit PLC, 91,
Milton Park, Abingdon, Oxon OX14 4RY, U.K., and Chemistry Research Laboratory,
Department of Chemistry, UniVersity of Oxford, Mansfield Road, Oxford OX1 3TA, U.K.
yucy@iccas.ac.cn
Received April 2, 2010
ABSTRACT
(+)-Steviamine, the enantiomer of the natural (-)-steviamine, and its corresponding C5 epimer have been synthesized from the D-ribose-
derived cyclic nitrone. (-)-Steviamine was found to be the first naturally occurring iminosugar that causes any inhibition of r-galactosaminidases.
The absolute and relative stereochemistry of the five stereo-
genic centers in (-)-steviamine (1), recently isolated from
SteVia rebaudiana (Asteraceae) leaves,1 was established by
X-ray crystallographic analysis of the hydrobromide salt.2
This paper reports the synthesis of the enantiomer of (-)-
steviamine (1), (+)-steviamine (2), and of the corresponding
C5 epimer 3 and the glycosidase inhibition profile of 1-3
(Figure 1). (-)-Steviamine (1) can be viewed as the
indolizidine analogue of the pyrrolizidine, hyacinthacine A5
(4), isolated from Scilla sibirica;3 many hyacinthacines have
been isolated from a range of plants.4 (-)-Steviamine (1) is
† Institute of Chemistry, Chinese Academy of Sciences.
‡ Graduate University of The Chinese Academy of Sciences.
§ Phytoquest Limited.
(3) (a) Asano, N.; Kuroi, H.; Ikeda, K.; Kizu, H.; Kameda, Y.; Kato,
A.; Adachi, I.; Watson, A. A.; Nash, R. J.; Fleet, G. W. J. Tetrahedron:
Asymmetry 2000, 11, 1–8. (b) Yamashita, T.; Yasuda, K.; Kizu, H.; Kameda,
Y.; Watson, A. A.; Nash, R. J.; Fleet, G. W. J.; Asano, N. J. Nat. Prod.
2002, 65, 1875–1881.
⊥ Summit PLC.
# University of Oxford.
| University of Toyama.
(1) Michalik, A.; Hollinshead, J.; Jones, L.; Fleet, G. W. J.; Yu, C.-Y.;
Hu, X. G.; van Well, R.; Horne, G.; Wilson, F. X.; Kato, A.; Jenkinson,
S. F.; Nash, R. J. Phytochem. Lett. 2010, in press.
(2) Thompson, A. L.; Michalik, A.; Nash, R. J.; Wilson, F. X.; van Well,
R.; Johnson, P.; Fleet, G. W. J.; Yu, C.-Y.; Hu, X.-G.; Cooper, R. I.; Watkin,
D. J. Acta Crystallogr. 2009, E65, o2904.
(4) (a) Kato, A.; Adachi, I.; Miyauchi, M.; Ikeda, K.; Komae, T.; Kizu,
H.; Kameda, Y.; Watson, A. A.; Nash, R. J.; Wormald, M. R.; Fleet,
G. W. J.; Asano, N. Carbohydr. Res. 1999, 316, 95–103. (b) Kato, A.;
Kato, N.; Adachi, I.; Hollinshead, J.; Fleet, G. W. J.; Kuriyama, C.; Ikeda,
K.; Asano, N.; Nash, R. J. J. Nat. Prod. 2007, 70, 993–997.
10.1021/ol1007718  2010 American Chemical Society
Published on Web 05/03/2010

zymes;12 alternatively, they may be excellent inhibitors of
different glycosidases.13 For example, L-swainsonine (7)
inhibits R-rhamnosidase14 rather than R-mannosidases, and
the C-methyl analogue 8 is an even more potent inhibitor of
naringinase.15 (-)-Steviamine and its enantiomer may also
be viewed as bicyclic analogues of the iminolyxitols 9 and
ent-9; 9 is a very potent, and ent-9 a weak, competitive
inhibitor of R-galactosidases.16
Retrosynthesis for 2 (Scheme 1) suggested that, starting
from the D-ribose-derived cyclic nitrone 10,17 (+)-steviamine
Scheme 1. Retrosynthesis of (+)-Steviamine (2)
Figure 1. Iminosugars related to (-)-steviamine.
(2) could be synthesized efficiently through the diastereo-
selective addition of Grignard reagent 11, followed by
annulation, via either intramolecular reductive amination or
SN2 displacement.
According to the retrosynthetic analysis, we commenced
the synthesis by making the key intermediate, ketone 13,
the first example of a new class of indolizidine alkaloid in
which an alkyl group is attached to the piperidine ring.
Polyhydroxylated indolizidines, such as castanospermine (5)
from Castanospermum australe5 (an inhibitor of R-glucosi-
dases) and swainsonine (6) from Swainsona canescens6 (an
inhibitor of R-mannosidases), were among the first sugar
mimics recognized. Simple derivatives of castanospermine
are in development for the treatments of dengue virus7 and
of HCV infections;8 swainsonine (6) has potential as a
chemotherapeutic agent for the treatment of cancer.9
Nearly 200 iminosugars in which the ring oxygen of the
furanose or pyranose has been replaced by nitrogen have
been isolated from plants or bacteria.10 Many of the
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themselves even more potent inhibitors of the same en-
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Org. Lett., Vol. 12, No. 11, 2010
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starting from the D-ribose-derived cyclic nitrone 10 (Scheme
2). Thus, Grignard addition of 1118 to the all-cis cyclic nitrone
resulting alcohols were transformed into their corresponding
mesylates directly without further purification. Liberation of
the amino group by CF3CO2H and subsequent treatment of
the resulting amine with K2CO3 in methanol with catalytic
water afforded the two epimeric indolizidines 14 and 15 in
nearly 1:1 ratio and moderate yield.
Scheme 2. Synthesis of Ketone 13
In order to achieve better diastereoselectivity, annulation by
intramolecular reductive amination was also examined (Scheme
4). Thus, reduction by NaBH4 of the iminium intermediate after
Scheme 4
.
Synthesis of (+)-Steviamine (2) via Reductive
Amination
10 at 0 °C furnished the hydroxylamine 12 in excellent yields
(88-92%) with high diastereoselectivity (dr >95%). The trans-
selectivity was determined by NOESY experiment which
showed H5 and H6 correlation in 12. The gratifyingly high
trans-selectivity can be explained by a Felkin-Anh transi-
tion-state model19 and is in accordance with previous
reports.20 Reduction of the resulting hydroxylamine 12 by
Zn-Cu(OAc)2-AcOH system gave the corresponding amine
in quantitive yield, which was treated with (Boc)2O to form
the N-Boc derivative 13m in 82% yield. Compound 13m was
then converted to the key intermediate, N-Boc ketone 13, by
liberation of the carbonyl group under mild acidic condition.
With ketone 13 in hand, two parallel annulation approaches
were under consideration for the construction of the second
ring: (1) intramolecular nucleophilic displacement and (2)
intramolecular reductive amination. The annulation by in-
tramolecular nucleophilic displacement (Scheme 3) was first
acidic deprotection of N-Boc ketone 13 generated a mixture of
the two epimers 14 and 15 in good yields with relatively higher
diastereoselectivity (14/15 ) 76:24-84:16) which favored
amine 14. The stereochemistry of the newly generated stereo-
center was determined by the 600 MHz NOESY spectrum of
14 and 15 (14: H5 and H9, H5 and H8a; 15: H3 and H5, H1
and H5; Supporting Information).
Finally, hydrogenolysis of indolizidine 14 and 15 (Scheme
5) gave (+)-steviamine (2) and 5-epi-(+)-steviamine (3) in
Scheme 5
.
Completion of (+)-Steviamine (2) and
5-epi-(+)-Steviamine (3)
Scheme 3
.
Construction of (+)-Steviamine (2) via SN2
Displacement
quantitive yields, respectively. The 1H and 13C NMR spectra
of compound 2 were identical (see the Supporting Informa-
(18) Bellas, T. E.; Brownlee, R. G.; Silverstein, R. M. Tetrahedron 1969,
25, 5149–5153.
(19) Merino, P.; Revuelta, J.; Tejero, T.; Cicchi, S.; Goti, A. Eur. J.
Org. Chem. 2004, 776–782.
(20) (a) Yu, C.-Y.; Huang, M.-H. Org. Lett. 2006, 8, 3021–3024. (b)
Merino, P.; Delso, I.; Tejero, T.; Cardona, F.; Marradi, M.; Faggi, E.;
Parmeggiani, C.; Goti, A. Eur. J. Org. Chem. 2008, 2929–2947. (c) Hu,
X.-G.; Jia, Y.-M.; Xiang, J.; Yu, C.-Y. Synlett 2010, 982–986, and references
cited therein.
examined. Thus, reduction of N-Boc ketone 13 by NaBH4
furnished a mixture of diastereomeric alcohols, and the
2564
Org. Lett., Vol. 12, No. 11, 2010

tion) to those reported for the natural (-)-steviamine (1),
and the optical rotation of 2 [[R]20D ) +34.0 (c 1.0, MeOH)]
of cancer by the protection of macrophage activating factor.22
A synthetic iminosugar analogue of GalNAc has recently
been reported as a potent inhibitor of GalNAcases.23 The
specific inhibition GalNAcases by a natural product, and
particularly one that does not contain a NAc or any amide
group, is remarkable.
In conclusion, the first synthesis of (+)-steviamine (2),
the enantiomer of the novel natural indolizidine iminosugar
(-)-steviamine (1), has been accomplished starting from the
readily available D-ribose-derived cyclic nitrone 10. (-)-
Steviamine was found to be a weak inhibitor of an R-ga-
lactosaminidase (GalNAcase), which is a remarkable finding
and might become a starting point for the design and
synthesis of more potent inhibitors of R-galactosaminidase.
was opposite to that of the natural (-)-steviamine [[R]20
-22.0 (c 1.0, MeOH)].
)
D
(-)-Steviamine (1), (+)-steviamine (2), and 5-epi-(+)-
steviamine (3) were assayed as potential glycosidase inhibi-
tors of a range of enzymes (see the Supporting Information).
(-)-Steviamine showed weak inhibition of ꢀ-glucosidases
(IC50 ) 454 µM against ꢀ-glucosidases from almond; IC50
) 739 µM against ꢀ-glucosidases from C. saccharolyticum)
but was a good inhibitor of ꢀ-galactosidase (rat intestinal
lactase, IC50 ) 35 µM). In spite of its structural similarity
to swainsonine (6) and the D-iminolyxitol 9, it showed no
significant inhibition of either R-mannosidase or R-galac-
tosidase, respectively. The enantiomer of (-)-steviamine (2)
and its C-5 epimer 3 were weak inhibitors of R-rhamnosidase
(IC50 ) 484 and 342 µM, respectively), several orders of
magnitude weaker than shown by L-swainsonine (7) and the
C-methyl analogue 8. The N-benzyl derivatives of both of
the enantiomers of the iminolyxitol 9 were more potent
inhibitors of R-rhamnosidase. However (-)-steviamine shows
weak inhibition of an R-galactosaminidase (GalNAcase, IC50
) 814 µM); there has been no prior report of any natural
product inhibiting any GalNAcase. GalNAcase inhibition
may allow the design of chaperones for the treatment of
Schindler-Kanzaki disease21 and a strategy for the treatment
Acknowledgment. This work is supported by Summit
PLC (UK), The National Natural Science Foundation of
China (No. 20672117), The National Basic Research Pro-
gram of China (No. 2009CB526511), The Ministry of
Science and Technology and the Ministry of Health of the
P.R. China (No. 2009ZX09501-006), and The Chinese
Academy of Sciences.
Supporting Information Available: Experimental pro-
1
cedures, characterization data, and H NMR and 13C NMR
spectra for all new compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
OL1007718
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