A β-lactam-azasugar hybrid as a competitive potent galactosidase inhibitor

Article abstract of DOI:10.1016/j.tetlet.2006.09.005

A β-lactam-azasugar hybrid (polyhydroxylated carbacephem) has been designed and synthesized as a potent glycosidase inhibitor.

Full text of DOI:10.1016/j.tetlet.2006.09.005

Tetrahedron Letters 47 (2006) 7923–7926
A b-lactam-azasugar hybrid as a competitive potent
galactosidase inhibitor
Ganesh Pandey,a,* Shrinivas G. Dumbre,a M. Islam Khan,b M. Shababb and
Vedavati G. Puranikc
aDivision of Organic Chemistry (Synthesis), National Chemical Laboratory, Pashan Road, Pune 411008, Maharashtra, India
bDivision of Biochemical Science, National Chemical Laboratory, Pashan Road, Pune 411008, Maharashtra, India
cDivision of Material Science, National Chemical Laboratory, Pashan Road, Pune 411008, Maharashtra, India
Received 27 July 2006; revised 20 August 2006; accepted 1 September 2006
Available online 25 September 2006
Abstract—A b-lactam-azasugar hybrid (polyhydroxylated carbacephem) has been designed and synthesized as a potent glycosidase
inhibitor.
Ó 2006 Elsevier Ltd. All rights reserved.
The design and synthesis of hybrid molecules, that is,
structural motifs developed through domain assimila-
tion of two or more different classes of biologically ac-
tive compounds of natural and/or synthetic origin has
attracted the attention of synthetic chemists in the past
few years owing to the enhanced possibility of discover-
ing new biologically active therapeutic agents.1 In this
context, several hybrid molecules of natural products
such as steroids, taxoids, carbohydrates and peptides
with counterparts such as b-lactams, C60-fullerenes,
anthraquinones, enediyne and porphyrin have been
synthesized and their properties evaluated.2
OH
OH
OH
HO
R
HO
HO
OH
HO
HO
OH
OH
OH
X
N
N
NH
H
NH2
2 X=CH2, R=H
3 X=CH2, R=OH
4 X=NH, R=H
1
5
OH
OH
HO
H
H
HO
HO
HO
HO
O
HO
HO
O
NH
N
N
O
O
O
OH
8
7
6
Azasugar inhibitors of glycosidases and related enzymes
are the subject of intense current research interest due to
their potential clinical applications as anti-diabetic,3
anti-cancer,4 anti-HIV5 and anti-influenza6 agents.
These low molecular weight entities are believed to exhi-
bit their inhibitory activities due to their binding with
glycosidases by mimicking the shape and charge of the
postulated oxo-carbenium ion intermediate for the gly-
cosidic bond cleavage reaction.7 Some of the potent aza-
sugar based glycosidase inhibitors (Fig. 1), such as 1,8
2,9 3,10 4,11 and 512, which become positively charged
on protonation due to the presence of basic amino, ami-
Figure 1. Structures of some potent glycosidase inhibitors.
dine and hydrazine moieties, are suggested to derive
their inhibitory activities either by mimicking the charge
or shape, or both, of the glycosidase transition state.
In contrast, neutral glyconolactams,13 such as 6 (Ki =
85 lM, b-glucosidase), where the glycosidic oxygen is re-
placed by a pseudo sp2 ring nitrogen, was originally be-
lieved to inhibit glycosidases by involving a tautomeric
iminol form. However, Withers and co-workers14 have
suggested that glycosidase inhibition by 6 and similar
other compounds such as 7 and 8 may in fact be caused
by H-bonding of the lactam carbonyl moiety with the
enzyme as the tautomerization energy for the amide–
Keywords: Glycosidase inhibitor; b-Lactam-azasugar; Hybrid mole-
cules; b-Galactosidase.
Corresponding author. Tel.: +91 20 25902324; fax: +91 20
iminol conversion is of the order of 11 kcal molꢀ1
,
15
*
indicating the concentration of the corresponding iminol
form in solution at any given time to be very low.
25902624; e-mail: gp.pandey@ncl.res.in
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.09.005

7924
G. Pandey et al. / Tetrahedron Letters 47 (2006) 7923–7926
Recently, research activity in this field has evolved to
evaluate hybrid molecules as glycosidase inhibitors and
in this context hybrids of D-glucose with several hetero-
cycles,16 D-galactose with 1-deoxynojirimycin and a few
more related structures17 have been synthesized and
evaluated. The possibility of developing new glycosidase
inhibitors through this approach is gaining appreciable
importance for the future developments in this area.
tected cyclic aminoacetal 15 using s-BuLi/TMEDA in
THF at ꢀ78 °C followed by the trimethylsilyl chloride
addition and acidic hydrolysis (Scheme 2). Although,
the cyclization of 16 would have given the correspond-
ing piperidine derivative, our previous experience of a
poor diastereoselectivity in such cyclizations led us to
transform it into cyclic 1,3-oxazine 11 for a better
diastereoselectivity.20b
Based on the aforementioned, we postulated that a b-
lactam-azasugar hybrid molecule of type 9 (Fig. 2),
which can also be referred to as polyhydroxylated car-
bacephem, may function as a potent glycosidase inhibi-
tor due to its conformationally constrained structural
features: (a) b-lactam ring compelling the polyhydroxy-
lated piperidine ring to adopt a nearly half-chair confor-
mation mimicking the shape of the glycosidase
inhibition transition state, (b) the carbonyl group in
the b-lactam ring may provide an additional hydrogen
bonding site for specific enzyme–substrate interactions.
Substrate 11 was cyclized, employing a protocol re-
ported from our group20 by irradiating a dilute solution
of 11 (3 mmol) and 1,4-dicyanonaphthalene (0.4 mmol)
in a mixture of acetonitrile:iso-propanol (3:1, 250 mL)
in a Pyrex vessel using a 450 W Hanovia medium pres-
sure lamp, to give 10 as a single diastereomer in a 60%
yield (Scheme 3). The cyclized product 10 was fully char-
L-(+)Tartaric acid
ref.19
3-aminopropanol
77%
b, c
O
Thus, we have synthesized b-lactam-azasugar hybrid 9,
its enantiomer 23 and another related structure 26 and
have evaluated their glycosidase inhibitory activities.
Herein, we disclose our preliminary results in this letter.
To the best of knowledge, there are no other such stud-
ies in the literature.
O
N
OH
O
Boc
15
14
85%
a
88%
d,e
The synthesis of 9 was pursued through the retrosyn-
thetic analysis shown in Scheme 1. We first synthesized
the key precursor 16 in a 71% yield by coupling 12
and 13 via reductive amination using sodium triacetoxy-
borohydride (2 equiv) as the reducing agent.18 Com-
pound 12 was obtained by IBX oxidation of the
corresponding alcohol 14, prepared from L-(+)-tartaric
acid following the reported procedure.19 Alcohol 13
was obtained by the a-metalation of the N-Boc pro-
O
O
H2N
OH
O
TMS
13
H
12
71%
TMS
f
O
TMS
O
O
g
H
N
OH
O
N
O
95%
11
16
Scheme 2. Synthesis of 11 via reductive amination of 12 and 13:
Reagents and conditions: (a) IBX, EtOAc, reflux, 9 h; (b) (Boc)2O,
TEA, 18 h; (c) CH3CH(OEt)2, PPTS, benzene, reflux, 24 h; (d) s-BuLi,
TMEDA, ꢀ78 °C, 3 h, then TMSCl, ꢀ78 °C to rt, 3 h (e) 2 N HCl,
dioxane, 80 °C, 45 min; (f) NaBH(OAc)3, 1,2-dichloroethane, 12 h,
then 2 N NaOH, 2 h and (g) (CH2O)n, benzene, Dean–Stark, 4 h.
Figure 2. b-Lactam azasugar.
OH
H
H
HO
HO
HO
O
O
HO
H
N
H
O
O
a
O
O
N
N
O
9a
b
11
9a
O
(60%)
O
(90%)
10
N
O
9
10
17
HO
BnO
Reductive
amination
H
N
H
O
O
O
O
O
HO
H2N
10a
O
O
10a
O
10
e
10
c, d
(82%)
9a
9a
O
N
O
(86 %)
N
O
3a
O
3a
TMS
13
H
TMS
18
12
11
19
Scheme 3. Reagents and conditions: (a) hm, 450 W, lamp, CH3CN:
i-PrOH (3:1), 4 h; (b) OsO4, K3Fe(CN)6, K2CO3, py, t-BuOH/H2O
(1:1), rt, 16 h; (c) NaIO4, silica gel, 15 min; (d) NaBH4, MeOH, rt, 4 h
and (e) BnBr, NaH, THF, reflux, 12 h.
L-(+)-tartaric acid
Scheme 1. Retrosynthetic analysis.
3-aminopropanol

G. Pandey et al. / Tetrahedron Letters 47 (2006) 7923–7926
7925
acterized by 1H NMR, 13C NMR, 2D COSY and
NOESY spectral analyzes. The dihydroxylation of 10
using OsO4 produced 17 in a 90% yield. Single crystal
X-ray diffraction analysis21 unequivocally confirmed
the stereochemistry of 10 at H-9a. The diol, upon so-
dium periodate oxidation afforded the corresponding
ketone, which was immediately subjected to sodium
borohydride reduction to afford 18 in a 82% yield as
the exclusive diastereomer. The stereochemistry of 18
ing groups by hydrogenation at 60 psi afforded b-lac-
tam-azasugar hybrid molecule 9 in a 95% yield.23 In
order to correlate the enzyme specific inhibition prop-
erty of 9, we also synthesized its (L-galacto configured)
enantiomer 23 (ent-9) in a similar manner starting from
D-(ꢀ)-tartaric acid.
Since, we had earlier observed24 that 1-N-iminosugar 25
showed better inhibitory activity for the b-glucosidase
(Ki = 30 lM) than 24 (Ki = 90 lM), we thought it would
be interesting to evaluate the enzyme inhibition activity
of 26 as well (Fig. 4). In this context, we synthesized
compound 2625 following an analogous route to that de-
scribed for 9, starting from alcohol 2724 as shown in
Scheme 5. The stereochemistry at C-10 and C-9a of 29
was ascertained by analyzing the coupling constants
for H-3a (d 3.57, ddd, J = 4.1, 7.3, 10.4 Hz), H-10a (d
2.96, dd, J = 8.7, 10.5 Hz) and by 1H–1H NOESY
spectroscopy.
1
was also confirmed from 1D as well as 2D H NMR
spectroscopy of the corresponding benzylated derivative
19. The stereochemistry at C-10 of 19 was ascertained by
analyzing the coupling constants for H-3a (d 4.24, dt,
J = 4.2, 9.8 Hz), H-10a (d 3.40, dd J = 2.2, 9.3 Hz), H-
10 (d 3.84, t, J = 2 Hz) and H-9a (d 2.24, t, J =
3.3 Hz), which suggested orientations H-3a-axial, H-
10a-axial, H-10-equatorial and H-9a-axial (Fig. 3).
This stereochemical analysis was further confirmed by
X-ray crystallography.21
A selective deprotection of the acetonide moiety of 19
and benzyl protection of the resultant diol gave the cor-
responding tribenzylated molecule 20 (Scheme 4). Sub-
sequently, the 1,3-oxazine moiety of 20 was ring
opened by refluxing with 6 N HCl in dioxane–methanol
for 48 h. The resultant secondary amine was re-pro-
tected as its N-Boc derivative prior to PDC oxidation
to the corresponding acid 21. The deprotection of the
N-Boc moiety of 21 by stirring with TFA in DCM at
0 °C for 3 h followed by the treatment with 2-chloro-1-
methylpyridinium iodide (Mukaiyama’s reagent)22 in
the presence of excess triethylamine afforded b-lactam
22 in a 53% yield. The removal of the O-benzyl protect-
The inhibitory activities of 9, 23 and 26 were assessed
against b-galactosidase (Aspergillus oryzae), a-galactosi-
dase (coffee beans), b-glucosidase/b-mannosidase (al-
monds), a-glucosidase (yeast) and a-mannosidase (jack
beans). The results are summarized in Table 1.
From the above results, it is apparent that the D-galacto-
configured b-lactam 9 exhibited competitive and specific
inhibition only against b-galactosidase. It inhibited a-
galactosidase inhibition very poorly and showed no
inhibition against a-/b-glucosidase and a-/b-manno-
sidase. This enzyme specific inhibition of 9 is in good
agreement with its D-galacto-configured structure. Simi-
larly, compound 23, which is L-galacto/L-fuco-config-
ured, showed no inhibition against any of the enzymes
studied suggesting that it might be specific to fucosidase.
Furthermore, b-lactam 26 which lacks the hydroxy func-
HO
OH
H
HO
HO
HO
N
NH
NH
HO
HO
HO
O
25
26
24
Figure 3. ORTEP diagrams of 17 and 19.
Figure 4. b-Lactam-azasugar hybrid 26 and comparative structures.
BnO
BnO
H
O
H
10a
H
O
O
O
O
O
O
10
d
a, b,c
75%
O
9a
BnO
a, b
BnO
BnO
c, d, e
57%
19
N
N
O
OH
60%
3a
78%
N
OH
Boc
N
O
BnO
27
29
28
TMS
20
21
O
OH
BnO
H10
refer to
Scheme 4
H
H
H
f, g
h
HO
BnO
BnO
N
HO
HO
H3a
Similarly,
29
9
53%
95%
H9a
N
N
HO
N
O
O
O
O
O
H10a
23
ent-9)
22
(
26
NOESY cross peaks
Scheme 4. Reagents and conditions: (a) 1 N HCl, MeOH, rt, 4 h; (b)
BnBr, NaH, TBAI, THF, reflux, 24 h; (c) 6 N HCl, dioxane–MeOH,
reflux, 48 h; (d) (Boc)2O, TEA, rt, DCM, 8 h; (e) PDC, DMF, rt, 8 h;
(f) TFA, DCM, 0 °C, 3 h; (g) 2-chloro-1-methylpyridinium iodide,
TEA, CH3CN, 60 °C–rt 32 h and (h) H2, Pd/C, 60 psi, MeOH, 6 h.
Scheme 5. Reagents and conditions: (a) IBX, EtOAc, reflux, 9 h; (b)
13, NaBH(OAc)3, 1,2-dichloroethane, 12 h, then 2 N NaOH, 2 h; (c)
(CH2O)n, benzene, Dean–Stark, 4 h and (d) hm, 450 W lamp,
CH3CN:i-PrOH (3:1), 4 h.

7926
G. Pandey et al. / Tetrahedron Letters 47 (2006) 7923–7926
Table 1. Enzyme inhibition (Ki in lM) data
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9
23
26
b-Galactosidase
a-Galactosidase
b-Glucosidase
a-Glucosidase
b-Mannosidase
a-Mannosidase
172
900
n.i.
n.i.
n.i.
n.i.
n.i.
n.i.
n.i.
n.i.
n.i.
n.i.
n.i.
n.i.
n.i.
n.i.
n.i.
n.i.
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n.i = no inhibition up to 1 mM.
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21. CCDC-607587 and CCDC-607588 contain Supplementary
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Cambridge Crystallographic Data Centre via http://
www.ccdc.ac.uk/data_request/cif.
The fairly good and specific glycosidase inhibition
exhibited by neutral b-lactam-azasugar hybrid molecule
9 points towards the possibility of improving its potency
further by incorporating minor structural variations. We
are probing this aspect, currently, by incorporating a
hydroxymethylene functionality at C-7 of the b-lactam
ring with the hope that it may provide an additional
H-bonding site for recognition and would also increase
the polarity of the molecule. Furthermore, we are also
synthesizing other possible stereoisomeric analogues of
9 and the results will be disclosed appropriately in a full
letter.
Acknowledgements
We thank Dr. P. R. Rajmohan and Mrs. U. D. Phalgune
for the special NMR experiments. S.G.D. and M.S.
thanks UGC, CSIR, respectively, New Delhi, for the
award of Research Fellowships. Financial support by
the DBT, New Delhi, is gratefully acknowledged.
Supplementary data
22. Maison, W.; Kosten, M.; Charpy, A.; Kintscher-Langen-
hagen, J.; Schlemminger, I.; Lutzen, A.; Westerhoff, O.;
¨
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2006.09.005.
Martens, J. Eur. J. Org. Chem. 1999, 2433–2441.
27
23. Data for compound 9: ½aꢁD +19.7 (c 0.25, MeOH);
1H NMR (500 MHz, D2O) d 2.82 (app t, 1H, J = 11.2,
11.6), 3.05–3.12 (br m, 2H), 3.76 (dd, 1H, J = 2.3, 9.7),
3.81–3.85 (m, 1H), 3.90–3.96 (m, 1H), 4.70 (dd, 1H,
J = 6.8, 13.0), 4.24 (aap t, 1H, J = 1.9, 2.3); 13C NMR
(125 MHz, CDCl3) d 37.3 (CH2), 43.2 (CH2), 50.3 (CH),
64.6 (CH), 68.2 (CH), 73.4 (CH), 169.5 (C); MS: 196
(M+Na+, 100%), 174 (MH+, 20%), 155 (18%).
References and notes
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and references cited therein.
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Biomol. Chem. 2003, 1, 3321–3326.
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Int. Ed. 2003, 42, 3996–4028, and references cited therein.
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C.-H. R.; Liu, P. S. J. Org. Chem. 1989, 54, 2539–2542; (b)
Balfour, J. A.; McTavish, D. Drugs 1993, 46, 1025–1054.
27
25. Data for compound 26: ½aꢁD +15.8 (c 0.18, MeOH); IR (in
CHCl3): 3440, 1750, 1212. cmꢀ1 1H NMR (500 MHz,
;
D2O) d 0.84 (d, 3H, J = 6.6), 1.27–1.36 (m, 1H), 2.46–2.59
(m, 2H), 2.96 [two sets of dd, like ddd, 1H, J = (2.2, 4.4),
(1.6, 4.4) and 14.8], 3.03 (dd, 1H, J = 4.4, 9.9), 3.08 (app t,
1H, J = 9.3, 10.4), 3.30–3.36 (m, 1H), 3.78 (dd, 1H,
J = 6.0, 13.2); 13C NMR (125 MHz, CDCl3) d 12.2 (CH3),
41.7 (CH), 42.2 (CH2), 43.8 (CH2), 52.2 (CH), 70.5 (CH),
76.2 (CH), 169.8 (C); MS: 194 (M+Na+, 100%), 172
(MH+, 15%).
´
4. (a) Zitzmann, N.; Mehta, A. S.; Carrouee, S.; Butters, T.
D.; Platt, F. M.; McCauley, J.; Blumberg, B. S.; Dwek, R.
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P. E.; Baptiste, J.; Fernandes, B.; Baker, M.; Dennis, J. W.
Cancer Res. 1994, 54, 1450–1457.
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R. E. Y.; Tulp, A.; Huisman, H. G.; Miedema, F.; Ploegh,