Inhibition of nonmammalian glycosidases by azetidine iminosugars derived from stable 3,5-Di-O-triflates of pentoses

Article abstract of DOI:10.1021/ol2024482

Efficient ring closure of stable crystalline 3,5-di-O-triflates of pentofuranosides with amines to form azetidines allowed preliminary evaluation of four-ring iminosugars as glycosidase inhibitors; significant and specific inhibition of nonmammalian α-glucosidases is shown by l-xylo- and l-arabino-iminosugar azetidines.

Full text of DOI:10.1021/ol2024482

ORGANIC
LETTERS
2011
Vol. 13, No. 21
5834–5837
Inhibition of Nonmammalian Glycosidases
by Azetidine Iminosugars Derived from
Stable 3,5-Di-O-triflates of Pentoses
Gabriel M. J. Lenagh-Snow,† Noelia Araujo,† Sarah F. Jenkinson,†,‡
Catherine Rutherford,† Shinpei Nakagawa,§ Atsushi Kato,§ Chu-Yi Yu,
Alexander C. Weymouth-Wilson,^ and George W. J. Fleet*,†,‡
Chemistry Research Laboratory, Department of Chemistry, University of Oxford,
Mansfield Road, Oxford, OX1 3TA, U.K., Oxford Glycobiology Institute, University of
Oxford, South Parks Road, Oxford, OX1 3QU, U.K., Department of Hospital
Pharmacy, University of Toyama, 2630 Sugitani, Toyama 930-0194, Japan, Beijing
National Laboratory for Molecular Science, CAS Key Laboratory of Molecular
Recognition and Function, Institute of Chemistry, Chinese Academy of Sciences, Beijing
100190, China, and Dextra Laboratories Limited, The Science and Technology Centre,
Whiteknights Road, Reading RG6 6BZ, U.K.
george.fleet@chem.ox.ac.uk
Received September 9, 2011
ABSTRACT
Efficient ring closure of stable crystalline 3,5-di-O-triflates of pentofuranosides with amines to form azetidines allowed preliminary evaluation of
four-ring iminosugars as glycosidase inhibitors; significant and specific inhibition of nonmammalian R-glucosidases is shown by L-xylo- and L-
arabino-iminosugar azetidines.
There are over 200 naturally occurring iminosugars1 in
which the oxygen of the sugar ring is replaced by nitrogen.
For example, the pyrrolidine DAB 1D [Scheme 1] is a 1,4-
dideoxy-1,4-imino-pentitol formally derived from a reduc-
tive amination from a nitrogen at C4 with the C1 aldehyde
of D-arabinose. DAB 1D, first isolated from Angylocaly
boutiqueanus and Arachnoides standishii,2 has been found
in a wide range of plants and is a potent competitive
inhibitor of R-D-glucosidases and of glycogen phos-
phorylase;3 its enantiomer LAB 1L is a more potent and
† Chemistry Research Laboratory, University of Oxford.
‡ Oxford Glycobiology Institute, University of Oxford.
§ University of Toyama.
(2) (a) Jones, D. W. C.; Nash, R. J.; Bell, E. A.; Williams, J. M.
Tetrahedron Lett. 1985, 26, 3125–3126. (b) Nash, R. J.; Bell, E. A.;
Williams, J. M. Phytochemistry 1985, 24, 1620–1622.
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Biochem. J. 1999, 342, 545–550. (b) Fosgerau, K.; Westergaard, N.;
Quistorff, B.; Grunnet, N.; Kristiansen, M.; Lundgren, K. Arch. Bio-
chem. Biophys. 2000, 15, 274–284. (c) Minami, Y.; Kuriyama, C.; Ikeda,
K.; Kato, A.; Takebayashi, K.; Adachi, I.; Fleet, G. W. J.; Kettawan, Q.;
Okamoto, T.; Asano, N. Bioorg. Med. Chem. 2008, 16, 2734–2740.
Chinese Academy of Sciences.
^ Dextra Laboratories Limited.
(1) (a) Asano, N.; Nash, R. J.; Molyneux, R. J.; Fleet, G. W. J.
Tetrahedron: Asymmetry 2000, 11, 1645–1680. (b) Watson, A. A.; Fleet,
G. W. J.; Asano, N.; Molyneux, R. J.; Nash, R. J. Phytochemistry 2001,
56, 265–295.
r
10.1021/ol2024482
Published on Web 10/10/2011
2011 American Chemical Society

specific noncompetitive inhibitor of R-D-glucosidases4
and an equally potent competitive inhibitor of an R-L-
arabinofuranosidase.5 The D-iminoxylitol 2D, isolated
from Angylocalyx pynaertii, shows no significant glycosi-
dase inhibition;6 there are many syntheses of both xylitol
enantiomers 2D and 2L.7 Because of the biological proper-
ties of iminosugars and their promise as chemotherapeutic
agents,8 many pyrrolidine, piperidine, and azepane carbo-
hydrate mimics have been studied. In contrast, there are
few examples of carbohydrate azetidines;9 there is only one
report of any glycosidase inhibition studies on azetidines in
which 1,3-dideoxy-1,3-imino-L-xylitol 4L [Scheme 2] was
shown to be a specific inhibitor of amyloglucosidases.10
similarsequenceon protectedderivativesofD- and L-ribose
gavethe L-iminoxylitol azetidine4Land itsenantiomer4D,
respectively.
Scheme 2. Strategy for the Formation of Azetidines from Pro-
tected Pentoses
Scheme 1. Azetidine and Pyrrolidine Iminosugars
For the synthesis of azet-DAB 3D, D-arabinose was
converted to the diol 5D11 in three steps in an overall yield
of 33% as previously described [Scheme 3].12 Esterification
of 5D with triflic anhydride in dichloromethane in the
presence of pyridine gave the stable crystalline ditriflate 6D
25
[mp 67ꢀ68 ꢀC; [R]D þ7.2 (c 1.1, CHCl3)] in 95% yield.
Reaction of 6D with benzylamine in acetonitrile in the
presence of diisopropylethylamine (DIPEA) afforded the
25
azetidine 7Da [mp 60ꢀ63 ꢀC; [R]D ꢀ74.6 (c 1.0, CHCl3)]
in 86% yield. On selective triflation at the primary alcohol,
an oxetane cannot be formed as it would be trans-fused to
the THF ring. Successful cyclizations of ditriflates by
double displacements by amines have been reported
for the formation of azetidine,13 pyrrolidine,14 and
piperidine15 rings. Removal of the acetonide protecting
group in 7Da with aqueous trifluoroacetic acid gave the
lactol which on reduction by sodium borohydride gave the
This paper reports the syntheses of both enantiomers of
the azetidine analogues 3 and 4 of the arabinose 1 and
xylose 2 pyrrolidines. Formation of the azetidine ring by
nucleophilic displacement by nitrogen of triflate leaving
groups at C5 and C3 (with inversion) of a protected D-
arabinose gave azet-DAB 3D, whereas a corresponding
derivative of L-arabinose gave azet-LAB 3L [Scheme 2]. A
25
azetidine triol 8Da [mp 96ꢀ97 ꢀC; [R]D ꢀ66.2 (c 0.39
(4) (a) Fleet, G. W. J.; Nicholas, S. J.; Smith, P. W.; Evans, S. V.;
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Medich, J.; Farid, P.; Fleet, G. W. J. Tetrahedron 1993, 49, 3359–3366.
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S. K.; Sinnott, M. L. Biochem. J. 1990, 266, 245–249.
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Chem. 2011, 76, 2001–2009. (e) Buchanan, J. G.; Lumbard, K. W.;
Sturgeon, R. J.; Thompson, D. K.; Wightman, R. H. J. Chem. Soc.,
Perkin Trans. 1 1990, 699–706.
in H2O)]. Hydrogenolysis of the benzyl group in 8Da
with 10% Pd/C and ammonium formate in anhydrous
methanol16 gave the parent azet-DAB 3D17 [for the HCl
25
salt [R]D ꢀ32.4 (c 0.37 in H2O)] in 99% yield. A similar
sequence of reactions on the enantiomeric diol 5L derived
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Supporting Information.
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3988–4035.
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Kato, A.; Fleet, G. W. J. Tetrahedron Lett. 2010, 51, 2222–2224.
(16) Ram, S.; Spicer, L. D. Synth. Commun. 1987, 17, 415–418.
(17) Selected data for azet-DAB 3D [as the HCl salt]: δH (D2O, 500
MHz): 3.70ꢀ3.73 (1H, dd, H5, J5,4 5.0 Hz, Jgem 12.0 Hz), 3.73ꢀ3.76 (1H,
(9) (a) Michaud, T.; Chanet-Ray, J.; Chou, S.; Gelas, J. Carbohydr.
Res. 1997, 299, 253–269. (b) Michaud, T.; Chanet-Ray, J.; Chou, S.;
Gelas, J. Carbohydr. Res. 1997, 303, 123–127. (c) Soengas, R. G.; Segade,
dd, H50, J5 ,4 5.4 Hz, Jgem 12.0 Hz), 3.87ꢀ3.90 (1H, dd, H1, J1,2 4.4 Hz,
ꢀ
Y.; Jimenez, C.; Rodrı
´
guez, J. Tetrahedron 2011, 67, 2617–2622.
0
(d) Dekaris, V.; Reissig, H. U. Synlett 2010, 1882–1882. (e) Eniade,
A.; Martin, O. R. Carbohydr. Res. 2002, 337, 273–277.
0
Jgem 11.6 Hz), 4.28ꢀ4.31 (1H, a-dt, H4, J4,5/J4,5 5.2 Hz, J4,3 7.2 Hz),
4.34ꢀ4.37 (1H, dd, H10, J1 ,2 6.8 Hz, Jgem 11.6 Hz), 4.58ꢀ4.61 (1H, a-t,
0
€
€
(10) Kramer, B.; Franz, T.; Picasso, S.; Pruschek, P.; Jager, V. Synlett
J3,4/J3,2 6.9 Hz), 4.81ꢀ4.85 (1H, m, H2); δC (D2O, 125 MHz): 53.9 (C1),
1997, 295–297.
62.5 (C5), 65.1 (C2), 65.4 (C3), 68.3 (C4).
Org. Lett., Vol. 13, No. 21, 2011
5835

from L-arabinose gave azet-LAB 3L [for the hydrochloride
25
was subjected to oxidation by sodium periodate, followed
by borohydride reduction, to afford the 1,2-acetonide 12L
salt [R]D þ31.7 (c 0.31 in H2O)].
25
[mp 88ꢀ89 ꢀC; [R]D ꢀ60.2 (c 0.5, MeOH), 82% over
three steps; 75% from 9L]. The protected enantiomer 12D
was prepared by a similar sequence of reactions from
diacetone glucose 9D.20 Reaction of benzylamine in the
presence of DIPEA with the stable crystalline ditriflate 13L
Scheme 3. Synthesis of azet-DAB 3D and azet-LAB 3L
25
[mp 41ꢀ42 ꢀC; [R]D ꢀ93.2 (c 0.55, CHCl3)], from ester-
25
ification of the diol 12L by triflic anhydride in 93% yield,
afforded the protected azetidine 14La [oil, [R]D ꢀ52.7
(c 0.51, CHCl3), 95%]. Hydrolysis of the isopropylidene
protecting group with aqueous trifluoroacetic acid, fol-
lowed by reduction of the resulting lactol with sodium
borohydride in water, produced the N-benzyl-azetidine
25
15Da [mp 182ꢀ183 ꢀC; [R]D þ106.8 (c 0.23, MeOH)] in
25
87% yield. The structure of the enantiomeric L-iminoxyli-
tol 15La [mp 179ꢀ180 ꢀC; [R]D ꢀ103.0 (c 0.23, MeOH)]
was firmly established by X-ray crystallographic analysis.21
Removal of the benzyl group in 15Da by transfer hydro-
genation with ammonium formate and 10% Pd/C in
methanol gave the D-iminolyxitol 4D22 [for the hydro-
25
chloride salt [R]D ꢀ6.9 (c 0.29, H2O)] in 82% yield; the
overall yield of 4D from the L-ribose diol 12L was 63%.
The enantiomeric D-iminolyxitol 4D [for the hydrochlor-
25
ide salt [R]D þ3.2 (c 0.50, H2O)] was similarly prepared
N-Alkylation of iminosugars significantly modifies the
glycosidase inhibition profile of the parent iminosugar,18
so a series of N-alkyl azetidines was prepared. Reaction of
the ditriflate 6D with methylamine, butylamine, nonyla-
mine, and p-methoxybenzylamine gave the corresponding
azetidines 7Db [65%], 7Dc [54%], 7Dd [69%], and 7De
[88%] which on subsequent hydrolysis and reduction
afforded the N-methyl- 8Db [92%], N-butyl- 8Dc [76%],
N-nonyl- 8Dd [84%], and N-p-methoxybenzyl- 8De [88%]
azetidines, respectively, for biological assays. The corre-
sponding enantiomers were prepared from the L-arabinose
ditriflate 6L.
from 12D.
Both enantiomers of the N-methyl- 14b, N-butyl- 14c,
and N-p-methoxybenzyl- 14d azetidines were prepared by
efficient cyclizations of the ditriflates 13 with methylamine,
butylamine, and p-methoxybenzylamine(14Lb78%, 14Db
67%, 14Lc 91%, 14Dc 82%, 14Ld 86%, 14Dd 96%). Acid
hydrolysis and subsequent borohydride reduction gave
the corresponding unprotected iminopentitols 15b, 15c,
and 15d (15Db 79%, 15Lb 85%, 15Dc 65%, 15Lc 98%,
15Dd 80%, 15Ld 89%) allowing preliminary biological
evaluation.
The inhibition profiles of the parent iminopentitols 3
and 4 against a range of glycosidases were investigated
[Table 1]; the effect of N-alkylation is shown in the tables
in the Supporting Information. The specific inhibition
reported by Jager10 by L-iminoxylitol azetidine 4L was
confirmed, showing good inhibition against Rhizopus sp.
and A. niger amyloglucosidase [IC50 25 and 414 μM,
respectively]; 4L, which also inhibited bacterial (A. niger)
R-glucosidase, did not inhibit mammalian or plant glyco-
sidases and thus was highly specific for bacterial enzymes.
The D-xylo-azetidine enantiomer 4D did not inhibit any
glycosidases. azet-LAB 3L was a good inhibitor of A. niger
R-glucosidase, rat intestinal lactase, and Rhizopus sp.
The protected ribofuranoses 12L and 12D were the key
intermediates for the respective syntheses of the azetidine
xylitols 4D and 4L [Scheme 4]. For the synthesis of 4D, 12L
was prepared from diacetone L-glucose 9L, readily avail-
able from D-glucoheptonolactone.19 Oxidation of 9L with
Dess-Martin periodinane gave the corresponding ketone
which, on reduction with sodium borohydride, formed
diacetone L-allose 10L [91% yield]. Selective hydrolysis
of the terminal isopropylidene group with methanol and
aqueous sulfuric acid gave the monoacetonide 11L which
(18) (a) Rawlings, A. J.; Lomas, H.; Pilling, A. W.; Lee, M. J.-R.;
Alonzi, D. S.; Rountree, J. S. S.; Jenkinson, S. F.; Fleet, G. W. J.; Dwek,
R. A.; Jones, J. H.; Butters, T. D. ChemBioChem 2009, 10, 1101–1105.
(b) Mellor, H. R.; Nolan, J.; Pickering, L.; Wormald, M. R.; Platt, F. M.;
Dwek, R. A.; Fleet, G. W. J.; Butters, T. D. Biochem. J. 2002, 366, 225–
233. (c) Asano, N.; Nishida, M.; Kato, A.; Kizu, H.; Matsui, K.;
Shimada, Y.; Itoh, T.; Baba, M.; Watson, A. A.; Nash, R. J.; Lilley,
P. M.; de, Q.; Watkin, D. J.; Fleet, G. W. J. J. Med. Chem. 1998, 41,
2565–2571. (d) Fleet, G. W. J.; Daher, S. A.; Namgoong, S. K.;
Winchester, B. Biochem. J. 1989, 258, 613. (e) Mercer, T. B.; Jenkinson,
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Asymmetry 2009, 20, 2368–2373.
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5300–5309. (b) Estevez, J. C.; Fairbanks, A. J.; Fleet, G. W. J. Tetra-
hedron 1998, 54, 13591–13620. (c) Baker, S. J.; Young, D. W. J. Labelled
Compd. Radiopharm. 2000, 43, 1023–1032.
(21) Jenkinson, S. F.; Lenagh-Snow, G. M. J.; Fleet, G. W. J.;
Thompson, A. L. Acta Crystallogr. 2011, E67, o2452.
(22) Selected data for iminolyxitol 4D [as the HCl salt]: δH (D2O, 500
MHz): 3.59ꢀ3.62 (1H, dd, H5, J5,4 5.2 Hz, Jgem 12.1 Hz), 3.40ꢀ3.74 (1H,
m, H50), 3.92ꢀ3.95 (1H, dd, H1, J1,2 4.5 Hz, Jgem 11.6 Hz), 4.30ꢀ4.31
0
(19) Weymouth-Wilson, A. C.;Clarkson, R.; Best, D.; Pino-Gonzalez,
M.-S.; Wilson, F. X.; Fleet, G. W. J. Tetrahedron Lett. 2009, 50,
6307–6310.
(1H, m, H4), 4.34ꢀ4.38 (1H, dd, H10, J1 ,2 7.3 Hz, Jgem 11.3 Hz),
4.58ꢀ4.61 (1H, a-t, H3, J3,2/J3,4 6.7 Hz), 4.75 (H2, under HDO peak);
δ
C (D2O, 125 MHz): 54.1 (C1), 63.2 (C5), 64.0 (C2), 67.0 (C3), 67.2 (C4).
5836
Org. Lett., Vol. 13, No. 21, 2011

Scheme 4. Synthesis of Azetidine-xylitols 4D and 4L
weaker, inhibition of a number of enzymes. In particular,
3D was a weak inhibitor of R-D-mannosidase in contrast to
the weak inhibition of R-L-rhamnosidase by the enantio-
mer 3L; enantiomers of inhibitors of R-D-mannosidase are
usually equally potent inhibitors of R-L-rhamnosidase.23
N-Alkylation of 3D preserved the inhibition of R-D-man-
nosidase but significantly increased the specificity.
Table 1. Concentration of Iminosugars Giving 50% Inhibition
of Various Glycosidases
In summary, the reaction of amines with stable ditri-
flatesallowed the efficient formation of1,3-iminopentitols;
the scope and limitation of the formation of iminosugar
azetidines by this method has yet to be determined. While
other biological properties are under investigation, it is
clear that azetidine iminosugars may be easily accessible
and are worthy of further investigation.
Acknowledgment. This work was supported by EPSRC
(G.L.-S.), Junta de Extremadura (N.A.), and a Grant-in-
Aid for Scientific Research (C) (No. 23590127) (A.K.)
from the Japanese Society for the Promotion of Science
(JSPS).
Supporting Information Available. Experimental de-
tails. This material is available free of charge via the
Internet at http://pubs.acs.org.
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amyloglucosidase, with IC50 values of 39, 70, and 19 μM,
respectively. The inhibition profile of 3L wasnot asspecific
as that of 4L. azet-DAB 3D also showed broad, but
Org. Lett., Vol. 13, No. 21, 2011
5837