Synthesis from d-altrose of (5 R,6 R,7 R,8 S)-5,7-dihydroxy-8- hydroxymethylconidine and 2,4-dideoxy-2,4-imino-d-glucitol, azetidine analogues of swainsonine and 1,4-dideoxy-1,4-imino-d-mannitol

Article abstract of DOI:10.1021/ol301844n

Ring closure of a 3,5-di-O-triflate derived from d-altrose with benzylamine allowed the formation of both monocyclic and bicyclic azetidine analogues of swainsonine.

Full text of DOI:10.1021/ol301844n

ORGANIC
LETTERS
XXXX
Vol. XX, No. XX
000–000
Synthesis from D‑Altrose of
(5R,6R,7R,8S)‑5,7-Dihydroxy-8-
hydroxymethylconidine and 2,4-Dideoxy-
2,4-imino‑D‑glucitol, Azetidine Analogues
of Swainsonine and 1,4-Dideoxy-1,4-
imino‑D‑mannitol
†,‡
†
†,‡
†
ꢀ
´
Noelia Araujo, Sarah F. Jenkinson, R. Fernando Martınez, Andreas F. G. Glawar,
Mark R. Wormald,‡ Terry D. Butters,‡ Shinpei Nakagawa,§ Isao Adachi,§ Atsushi Kato,*,§
Akihide Yoshihara, Kazuya Akimitsu, Ken Izumori, ,^ 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, Rare Sugar
Research Center, Kagawa University, 2393 Ikenobe, Miki-cho, Kita-gun, Kagawa
761-0795, Japan, and Faculty of Agriculture, Kagawa University, 2393 Ikenobe, Miki-cho,
Kita-gun, Kagawa 761-0795, Japan
george.fleet@chem.ox.ac.uk; kato@med.u-toyama.ac.jp
Received July 5, 2012
ABSTRACT
Ring closure of a 3,5-di-O-triflate derived from D-altrose with benzylamine allowed the formation of both monocyclic and bicyclic azetidine ana-
logues of swainsonine.
The biological activity of monocyclic azetidine iminosugars1
is of current interest.2 N-Alkyl hydroxyazetidines are
potent inhibitors of purine nucleoside phosphorylase
with subnanomolar Ki,3 azetidine iminosugar analogues of
pentoses have been found to be specific inhibitors of
nonmammalian glycosidases,4,5 and N-nonyl trihydroxya-
zetidines are specific inhibitors of some ceramide-specific
glucosyl transferases and glucosidases.6 The only reported
† Chemistry Research Laboratory, University of Oxford.
‡ Oxford Glycobiology Institute, University of Oxford.
§ University of Toyama.
(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. (c) Nash, R. J.; Kato, A.; Yu, C.-Y.; Fleet, G. W. J. Future
Med. Chem. 2011, 3, 1513–1521.
Rare Sugar Research Center, Kagawa University.
^ Faculty of Agriculture, Kagawa University.
r
10.1021/ol301844n
XXXX American Chemical Society

bicyclic azetidine iminosugars, designed as glycosidase
inhibitors, have no hydroxyl group in the azetidine ring.7
This paper describes the synthesis of (5R,6R,7R,8S)-5,7-
dihydroxy-8-hydroxymethylconidine 18 and 2,4-dideoxy-
2,4-imino-D-glucitol 2, which are azetidine analogues of
swainsonine 4and DIM [1,4-dideoxy-1,4-imino-D-mannitol]
5 (Figure 1).9 The synthesis requires the formation of the
azetidine ring by a double displacement of leaving groups
at C3 and C5 in a protectedderivative of D-altrose6, to give
the key intermediate aldehyde 3.
potentialchemotherapeutic agent for cancer,12 for its effect
as an immunoregulator,13 and as a probe in the life cycle of
prions.14 StructureÀactivity relationships of swainsonine ana-
logueson R-mannosidaseinhibition have been studied.15,16
This paper also reports assays of glycosidase inhibition by
the azetidine analogues 1 and 2.
Scheme 1. Formation of D-Altrose 6 from D-Fructose 7
Biotechnology, and in particular the isomerization of
common to rare hexoses by Izumoring,17 has greatly in-
creased the number of carbohydrates that may conveni-
ently be used aschirons. D-Altrose6, a hithertoinaccessible
sugar, was formed in a batch reactor from D-fructose 7 by
epimerization of C3 by D-tagatose-3-epimerase (DTE) to
D-psicose 8 which was equilibrated in situ by D-arabinose
isomerase (DAI) to form D-altrose 6 [Scheme 1] without
the need for the isolation of intermediates.18
Figure 1. Swainsonine analogues.
The sterically crowded azetidine 14 is a divergent inter-
mediate for the synthesis of 1 and 2 [Scheme 2]. Protection
of D-altrose 6 with acetone in the presence of sulfuric acid
and anhydrous copper(II) sulfate gave an inseparable
mixture of the pyranose 9 and furanose 10 diacetonides
in 85% yield in a ratio of 2:3.19 Partial hydrolysis of the
mixture of 9 and 10 with aqueous acetic acid resulted in
selective hydrolysis of the side chain acetonide of the
furanose isomer 10 which allowed isolation of the required
Both swainsonine 410 and DIM 511 are specific and po-
tent inhibitors of an R-mannosidase of N-linked glycopro-
tein processing. Swainsonine is currently being studied as a
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Org. Lett., Vol. XX, No. XX, XXXX

monoacetonide 11 (50%) together with pure pyranose
diacetonide 9 (31%).20
in Figure 2A;24 the close fit of the two structures indicates
that the molecular modeling provided a realistic represen-
tation of the molecule. Therefore azetidine 14 was modeled
using the same presets, and its structure is shown in
Figure 2B in comparison to the same crystal structure of
15D. Inclusion of the additional TBDMS protected hydro-
xymethyl side chain in 14 has displaced both the N-benzyl
and isopropylidene protecting groups, reinforcing the high
degree of spatial crowding in azetidine 14. The low yield of
14 may be due to steric crowding developing in the transi-
tion state of the formation of the azetidine ring.
Scheme 2. Synthesis of Key Intermediate 14
Hydrolysis of 14 with aqueous trifluoroacetic acid re-
moved both the silyl ether and acetonide protecting groups
to give 3 which, on reduction by sodium borohydride in
water, gave the N-benzylazetidine 16 (80% yield from 14)
[Scheme 3]. Transfer hydrogenation of 16 by ammonium
formate in the presence of palladium on carbon gave the
azetidine analogue 2 of DIM. Reaction of 3 with the
stabilized ylid, Bu3PdCHCO2Me, in 1,4-dioxane afforded
the Wittig product 17 (48% from 14). Transfer hydrogena-
tion of 17 caused debenzylation and reduction of the CdC
and gave a mixture of products from which the bicyclic
lactam 18 could be isolated in 34% yield. From the IR
spectrum of the crude reaction mixture, it was apparent
that a lactone was also formed, but it was not possible to
obtain a pure sample of any product other than lactam 18.
The low yield of cyclization to 18 may again be due to steric
congestion in the highly substituted azetidine ring. In order
to avoid competing lactone formation, all the hydroxyl
groups in 17 were protected as the corresponding trisilyl
ether 19 by treatment with TBDMS triflate in DMF in the
presence of 2,6-lutidine (75%). Transfer hydrogenation
of 19 in methanol gave a mixture of an ester together
with lactam 20; further heating of the reaction mixture in
acetonitrile allowed the isolation of pure fully protected
lactam 20 but, again, only in poor yield (30%). Reduction
of lactam 20 with borane in THF gave the borane adduct
21 in which four substituents on the azetidine ring are cis.
Treatment of 21 with acidic ion-exchange resin in metha-
nol destroyed the borane complex and removed the silyl
ethers to allow the isolation of the free trihydroxyconidine
1 (92%). It is noteworthy that, though the cyclizations
proceeded in low yield, possibly due to steric crowding in
the bicyclic azetidine, the azetidine swainsonine analogue 1
was stable to the acid treatment in the final step.
Treatment of 11 with TBDMS chloride in DMF in the
presence of imidazole afforded selective protection of the
primary alcohol to give the silyl ether 12 (75%). Esterifica-
tion of the two remaining hydroxyl groups in 12 with triflic
anhydride in dichloromethane in the presence of pyri-
dine formed the ditriflate 13 (97%). Reaction of 13 with
benzylamine in acetonitrile in the presence of diisopropy-
lethylamine (DIPEA) gave the key azetidine 14 in low yield
(21%).
Cyclizations of 3,5-di-O-triflates of protected furano-
sides with amines usually proceed in excellent yield.4,21 The
extent of steric congestion in azetidine 14 was explored
with the gas phase geometries of 15D and 14 by DFT cal-
culations at the B3LYP/6-31G* level of theory.22 The
overlay of the calculated molecular model of 15D with its
corresponding X-ray structure23 (shown in green) is shown
(20) A crystal structure of compound 11 was obtained. Data were
collected at low temp [Cosier, J.; Glazer, A. M. J. Appl. Crystallogr.
1986, 19, 105107], and the structure was solved using SIR92[Altomare,
A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Burla, M. C.;
Polidori, G.; Carnalli, M. J. Appl. Crystallogr. 1994, 27, 435] and refined
using the CRYSTALS software suite[Betteridge, P. W.; Carruthers,
J. R.; Cooper, R. I.; Prout, K.; Watkin, D. J. J. Appl. Crystallogr. 2003,
36, 1487]; full refinement details are given in the Supporting Informa-
tion. Crystallographic data (excluding structure factors) have been
deposited with the Cambridge Crystallographic Data Centre (CCDC
893177), and copies of the data can be obtained free of charge via www.
ccdc.cam.ac.uk/data_request/cif.
Inhibition by the azetidines 1, 2, 16, and 18 of the fol-
lowing glycosidases was studied:25 R-glucosidases (rice,
yeast, rat intestinal maltase, A. niger), β-glucosidases
(almond, bovine liver), R-galactosidase (coffee beans),
β-galactosidase (bovine liver), R-mannosidase (Jack bean),
β-mannosidase (snail), R-L-rhamnosidase (P. decumbens),
ꢀ
(21) Lenagh-Snow, G. M. J.; Araujo, N.; Jenkinson, S. F.; Martınez,
´
R. F.; Shimada, Y.; Yu, C.-Y.; Kato, A.; Fleet, G. W. J. Org. Lett. 2012,
14, 2142–2145.
€
(24) Figures were generated using Maestro 9.2 of the Schrodinger
Software Suite 2011.
(22) For full details of the performed calculations, please see the
Supporting Information.
(25) For details of assays, see: (a) Mercer, T. B.; Jenkinson, S. F.;
Nash, R. J.; Miyauchi, S.; Kato, A.; Fleet, G. W. J. Tetrahedron:
Asymmetry 2009, 20, 2368–2373. (b) Best, D.; Wang, C.; Weymouth-
Wilson, A. C.; Clarkson, R. A.; Wilson, F. X.; Nash, R. J.; Miyauchi, S.;
Kato, A.; Fleet, G. W. J. Tetrahedron: Asymmetry 2010, 21, 311–319. A
table of the glycosidase inhibition by the azetidines is shown in the
Supporting Information.
(23) Edgeley, D. S.; Jenkinson, S. F.; Lenagh-Snow, G.; Rutherford,
C.; Fleet, G. W. J.; Thompson, A. L. Acta Crystallogr. 2012, E68, o2410
(CCDC 893176). To generate the crystal structure of 15D the coordi-
nates of the X-ray structure of the enantiomer 15L were inverted using
Crystals 14.11.
Org. Lett., Vol. XX, No. XX, XXXX
C

Figure 2. (A) Overlay of calculated and X-ray crystal structure (shown in green) of 15D, validating the applicability of the DFT
calculations to azetidine ring systems. (B) Overlay of calculated structure for 14 with X-ray crystal structure (shown in green) of 15D,
indicating the high degree of spatial crowding in azetidine 14.
hibitor of almond β-glucosidase [IC50 545 μM]. Both the
conidine analogue of swainsonine 1 and the corresponding
lactam 18 were weak inhibitors of β-galactosidase [IC50
Scheme 3. Formation of Swainsonine Analogues 1, 2, and 18
492 and 341 μM, respectively]. Azetidine swainsonine
1 also showed weak inhibition of R-mannosidase
[IC50 640 μM].
In summary this paper describes the synthesis of azeti-
dine analogues of the mannosidase inhibitors swainsonine
and DIM via a highly hindered bicyclic azetidine. The
sterically congested and functionalized conidine 1 was
remarkably stable to acid, in contrast to the ready acid
catalyzed polymerization of conidine itself.26 None of the
azetidines were potent inhibitors of any glycosidase.
Acknowledgment. This work was supported by Junta
ꢀ
ꢀ
de Extremadura (N.A.), Fundacion Ramon Areces
(R.F.M.), Cancer Research UK (A.F.W.G.), and Grant-
in-Aid for Scientific Research (C) (No: 23590127) (A.K.)
from the Japanese Society for the Promotion of Science
(JSPS). The Research & Technological Innovation and
ꢀ
Supercomputing Center of Extremadura (CenitS) is grate-
fully acknowledged for permitting the use of the super-
computer LUSITANIA.
R-L-fucosidase(bovinekidney), trehalase(porcine kidney),
and amyloglucosidase (A. niger). The monocyclic azetidine
analogue of DIM 2 showed no inhibition of any glycosi-
dase; its N-benzyl derivative 16 was a specific weak in-
Supporting Information Available. Experimental pro-
cedures, full spectroscopic data, full details of the biolo-
gical assays and computational data. This material is
available free of charge via the Internet at http://pubs.
acs.org.
(26) (a) Cigdem, Y.; Irem, K.; Nuket, O. Chin. J. Polym. Sci. 2010, 28,
39–44. (b) Wang, X.; Yue, Q.; Gao, B.; Si, X.; Sun, X.; Zhang, S.
J. Polym. Res. 2011, 18, 1067–1072. (c) Lavagnino, E. R.; Chauvette,
R. R.; Cannon, W. N.; Kornfeld, E. C. J. Am. Chem. Soc. 1960, 82,
2609–2613.
The authors declare no competing financial interest.
D
Org. Lett., Vol. XX, No. XX, XXXX