Synthesis of bromo-conduritol-B and bromo-conduritol-C as glycosidase inhibitors

Article abstract of DOI:10.1016/j.carres.2008.12.005

For the synthesis of bromo-conduritol-B skeleton, bromo-1,4-benzoquinone was subjected to bromination followed by the reduction of the carbonyl groups with NaBH₄. Substitution of bromides bonded to sp³-hybridized carbon atoms with AgOAc gave the bromo-conduritol-B tetraacetate in high yield. For the construction of bromo-conduritol-C skeleton, 2,2-dimethyl-3a,7a-dihydro-1,3-benzodioxole was used as the starting material. Photooxygenation of the diene unit gave an unsaturated bicyclic endoperoxide. Bromine was incorporated into the molecule by the addition of bromine to the double bond. Opening of the peroxide linkage followed by HBr elimination and reduction of the carbonyl group provided the conduritol-C structure in good yield. Bromo-conduritol-B exhibited strong enzyme-specific inhibition against α-glycosidase.

Full text of DOI:10.1016/j.carres.2008.12.005

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Carbohydrate Research 344 (2009) 426–431
Contents lists available at ScienceDirect
Carbohydrate Research
journal homepage: www.elsevier.com/locate/carres
Synthesis of bromo-conduritol-B and bromo-conduritol-C
as glycosidase inhibitors
Seda Cantekin a, Arif Baran a,b, Raßsit Çalısßkan a,c, Metin Balci a,
*
a Department of Chemistry, Middle East Technical University, 06531 Ankara, Turkey
b Department of Chemistry, Sakarya University, 54100 Sakarya, Turkey
c Department of Chemistry, Süleyman Demirel University, 32260 Isparta, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
For the synthesis of bromo-conduritol-B skeleton, bromo-1,4-benzoquinone was subjected to bromina-
tion followed by the reduction of the carbonyl groups with NaBH4. Substitution of bromides bonded to
sp3-hybridized carbon atoms with AgOAc gave the bromo-conduritol-B tetraacetate in high yield. For
the construction of bromo-conduritol-C skeleton, 2,2-dimethyl-3a,7a-dihydro-1,3-benzodioxole was
used as the starting material. Photooxygenation of the diene unit gave an unsaturated bicyclic endoper-
oxide. Bromine was incorporated into the molecule by the addition of bromine to the double bond. Open-
ing of the peroxide linkage followed by HBr elimination and reduction of the carbonyl group provided the
conduritol-C structure in good yield. Bromo-conduritol-B exhibited strong enzyme-specific inhibition
Received 19 September 2008
Received in revised form 5 November 2008
Accepted 2 December 2008
Available online 13 December 2008
Keywords:
Quinones
Endoperoxides
Conduritol-B
Conduritol-C
Haloconduritol
Enzyme inhibition
against a-glycosidase.
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
2. Results and discussion
Conduritols (1–6) are cyclohexene tetrols. The presence of four
stereogenic C-atoms allows them to exist in six different configura-
tions.1–4 Conduritols and their derivatives have been found to pos-
sess antibiotic, antileukemic and tumor-inhibitory properties, and
glycosidase inhibitory activity (Chart 1).1,5
Conduritols A–F are useful intermediates in organic synthesis,
and some derivatives such as epoxides can act as irreversible
glycosidase inhibitors.4 In connection with the conduritols, halo-
conduritols and double bond-substituted conduritols have also
gained importance in the last decade. For instance, bromo-con-
duritols are interesting molecules in AIDS research because they
Commercially available hydroquinone (7) was used as the start-
ing material for the synthesis of bromo-conduritol-B 14. Bromina-
tion of hydroquinone (7) as described in the literature14,15 followed
by oxidation of bromohydroquinone (8) with Ce(NH4)2(NO3)6
(CAN) gave the key compound bromo-quinone (9)16 in 85% yield
(Scheme 1).
The quinone 9 was brominated at À20 °C to give only the trans-
dibromide 10 in 94% yield. The NMR spectral studies indicated the
regiospecific addition of bromine to the unsubstituted double
bond.17 The regiospecific addition of bromine to the unsubstituted
double bond can be attributed to the steric effect caused by the
bromine atom as well as by reduced electron density. It is known
that bromine decreases the reactivity of the double bond toward
electrophiles. Reduction of the carbonyl groups in 10 with NaBH4
in ether at À10 °C followed by acetylation of the hydroxyl groups
in 11 with Ac2O and pyridine gave the diacetate 12. The NMR spec-
tral studies clearly established the formation of only one isomer of
the diacetate 12.18 The coupling constant between the protons
close to bromine atoms is measured to be J5,6 = 10.9 Hz. This value
is consistent with a typical axial/axial coupling constant in the
cyclohexene ring, indicating the trans-configuration as well as
the equatorial/equatorial position of the bromine atoms. Further-
more, the observed coupling constants between the protons H-1
are active site directed, covalent inhibitors of
a-glucosidases.
Various groups have reported the synthesis of halo-substituted
conduritols starting from cis-cyclohexa-3,5-diene-1,2-diols,
which are available by microbial oxidation of aromatic com-
pounds.6–13 We, herein, report two novel, simple, and efficient
methodologies for the synthesis of bromo-conduritol-B (14)
and bromo-conduritol-C (24) and their enzyme-specific inhibi-
tion against
a-glycosidase.
* Corresponding author. Tel.: +90 312 210 5140; fax: +90 312 210 3200.
E-mail address: mbalci@metu.edu.tr (M. Balci).
0008-6215/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carres.2008.12.005

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S. Cantekin et al. / Carbohydrate Research 344 (2009) 426–431
427
Diacetate 12 was then treated with AgOAc and Ac2O in acetic
acid in order to substitute the bromine atoms with acetate groups.
The NMR spectral studies showed the formation of a single tetra-
acetate, 13. The exact configuration of tetraacetate was determined
on the basis of the 1H and 13C NMR spectra in conjunction with 2D-
NMR (DEPT, COSY, HMQC, and HMBC) experiments. In particular,
the measured coupling constants J1,6 = 7.1 Hz, J4,5 = 6.6 Hz, and
J5,6 = 10.4 Hz indicate that the relative configuration of the acetate
groups is trans/trans/trans. The stereoselective formation of 13 can
be explained by the neighboring group participation.19 Removal of
the acetate groups in 13 with ammonia in methanol resulted in the
formation of bromo-conduritol-B 14 in 95% yield.
For the synthesis of the bromo-conduritol-C 24, we started from
2,2-dimethyl-3a,7a-dihydro-1,3-benzodioxole (15).20,21 Photooxy-
genation of 15 in methylene chloride (500 W, projection lamp) at
room temperature using tetraphenylporphyrine as the sensitizer
afforded bicyclic endoperoxide 16 in a yield of 95% (Scheme 2).22
In order to introduce the bromine atom into the molecule, the dou-
ble bond in 16 was subjected to bromination. Treatment of the
endoperoxide 16 with bromine in methylene chloride at 0 °C affor-
ded the dibromo adduct 17 in 87% yield. The structure of the ad-
duct was elucidated on the basis of 1H and 13C NMR data. The
symmetrical NMR spectrum clearly indicates the cis-addition23 of
bromine atoms. The endo (relative to the peroxide linkage) stereo-
chemical assignment for the bromine atoms is supported by the
absence of a measurable coupling between CHBr protons and
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
1
2
3
Conduritol-A
Conduritol-B
Conduritol-C
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
4
5
6
Conduritol-D
Conduritol-E
Conduritol-F
Chart 1.
and H-6 (J1,6 = 6.6 Hz), and H-4 and H-5 (J4,5 = 6.8 Hz) confirm the
trans-configuration of the acetate groups and bromine atoms.
OH
OH
O
Br
Br
Br2, ether
CAN, CH3CN
0 oC, 87%
rt, 85%
OH
OH
O
7
9
8
Br2
94% -20 oC
CH2Cl2
OAc
Br
OH
O
Br
Br
Br
Br
Br
Br
1
4
NaBH4
ether, -10 oC
82%
2
3
Ac2O, Pyridine
-5 oC, 80%
6
5
Br
Br
O
OAc
OH
11
12
10
AgOAc
CH3COOH
74%
Ac2O, reflux
OH
OAc
OH
OH
OAc
OAc
Br
Br
1
NH3, MeOH
rt, 95%
2
3
6
5
4
OH
OAc
14
13
Bromo-conduritol-B
Scheme 1.

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428
S. Cantekin et al. / Carbohydrate Research 344 (2009) 426–431
Br
Br
O
O
O
O
O
O
O2, TPP, h
Br2, CH2Cl2
0 oC, 87%
O
O
O
O
CH2Cl2, rt
95%
17
15
16
DMSO
rt, quant.
O
O
Br
Br
Br
O
O
O
O
-HBr
OH
OH
19
18
Scheme 2.
bridgehead protons. AM1 calculations also support this finding. In
the case of endo-configuration, the dihedral angle between the rel-
evant protons is 72°; however, in the exo-configuration the corre-
sponding angle is 55°.
For the synthesis of the target compound 24, bromoenone 19
was subjected to a NaBH4 reduction in THF at 0 °C followed by
acetylation with acetic anhydride in the presence of acid to give
two reduced products, 22 and 23 (Scheme 3). Spectral analysis
indicated the formation of a conduritol-C derivative. Because of
the unsymmetrical environment of the carbonyl group, reduction
can proceed in two ways to give a either conduritol-A and/or con-
duritol-C derivative. To determine the configuration of the formed
product, we measured the coupling constant between the protons
H-1 and H-6. The coupling constant J1,6 = 3.8 Hz of 22, which is
consistent with the typical axial/equatorial coupling constant in a
cyclohexene ring, indicates the cis-configuration of the acetate
groups. The configuration of the remaining three acetate groups
should not be changed during the reduction reaction. Furthermore,
the extracted coupling constants were in agreement with the pro-
posed cis/cis/trans configuration of the acetate groups. Having
After successful isolation and characterization of the endoper-
oxide 17, we turned our attention to the opening of the peroxide
linkage in 17. Unsaturated bicyclic endoperoxides can be easily
converted into the corresponding hydroxy enones upon treatment
with base or acids.24,25 However, we noted that endoperoxide 17
rearranges quantitatively into the corresponding bromoenone 19
upon dissolving in DMSO. We assume that the endoperoxide first
undergoes a rearrangement to form the saturated hydroxyketone
18 followed by HBr elimination to give the bromoketone 19. Since
the bond cleavage takes place at the peroxide linkage, the configu-
ration of the hydroxyl group in 18 as well as in 19 will be
preserved.
OH
OH
O
Br
Br
Br
O
O
O
O
O
O
NaBH4, THF
0 oC
+
OH
OH
OH
21
20
19
H+/Ac2O
H+/Ac2O
Pyridine
Pyridine
rt
rt
OH
OAc
OAc
Br
OH
OH
Br
Br
OAc
OH
NH3, MeOH
rt, 97%
OAc
OAc
1
4
2
3
6
5
OH
OAc
23
OAc
22 (60%)
24
(13%)
Bromo-conduritol-C
Scheme 3.

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S. Cantekin et al. / Carbohydrate Research 344 (2009) 426–431
429
ascertained the configuration of 22, we suggest the following
mechanism. The stereoselective formation of 22 can be attributed
to the fact that NaBH4 approaches the carbonyl group from the
less-crowded side of the carbonyl group to form 20 exclusively.
As a side product, 23 was isolated in 13% yield, arising from the
reduction of the double bond followed by reduction of the carbonyl
group. The exact constitution and configuration of 23 were eluci-
dated with the help of the 1H and 13C NMR spectra in conjunction
with 2D-NMR (DEPT, HMQC, HMBC, and COSY) experiments.
Removal of the acetate groups in 22 by ammonia in methanol re-
sulted in the formation of bromo-conduritol-C13 24 in high yield.
spectrometer. Infrared spectra were recorded on a Perkin Elmer
1600 Series FT-IR spectrometer. Column chromatographic separa-
tions were performed using Fluka Silica Gel 60 plates with
0.063–0.200 mm particle size. Thin layer chromatography (TLC)
was effected using precoated 0.25 mm silica gel plates purchased
from Fluka.
3.2. (rel-5S,6S)-2,5,6-Tribromocyclohex-2-ene-1,4-dione (10)
To a solution of 916 (6.0 g, 32 mmol) in CH2Cl2 was added drop-
wise a solution of bromine (7.5 g, 47 mmol) in CH2Cl2 at À20 °C
over 2 h. The mixture was stirred at the same temperature for an
additional 2.5 h, and then it was allowed to warm up to room tem-
perature. The solvent was removed under reduced pressure to af-
ford 10 (10.5 g, 94%) from EtOAc–hexane as yellow crystals, mp
63–65 °C. 1H NMR (400 MHz, CDCl3/CCl4) d: 7.24 (d, J = 1.8 Hz,
1H, H-3), 4.97 (d, J = 2.7 Hz, 1H, H-6), 4.81 (dd, J = 2.7 and 1.8 Hz,
1H, H5); 13C NMR (100 MHz, CDCl3/CCl4) d: 184.6, 180.6, 137.6,
137.2, 44.7, 43.4; IR (KBr, cmÀ1) 3048 (w), 3000 (w), 1685 (s),
1577 (m), 1327 (m), 1303 (m), 1269 (s), 1224 (s), 1166 (w), 1126
(w), 1023 (m), 951 (s), 906 (m), 791 (w), 768 (w), 708 (w), 632
(m), 617 (m); MS (m/z, relative intensity): 350/348/346/344, (M+,
2, 6, 6, 2), 269/267/265 (M+ÀBr, 60, 100, 63), 186/188 (M+À2Br,
24, 24), 135 (40) 133 (45), 103 (20), 82 (30), 79 (45). Anal. Calcd
for C6H3Br3O2: C, 20.78; H, 0.87. Found: C, 20.69; H, 0.98.
2.1.
a-Glycosidase inhibition assay
The inhibitory activities of 14 and 24 were screened against
glycosidase. The results are summarized in Table 1. The isomer 14
showed -glycosidase inhibition, and the inhibition rate was
84 6.9% for 40 M concentration. The other isomer 24 also exhib-
ited an 8.75 6.9% inhibition rate for 5 M concentration. When
a-
a
l
l
the concentration of 24 was increased to 200 lM, the inhibition
rate did not change.
In summary, with relatively little synthetic effort, we achieved
the stereoselective synthesis of two isomeric bromo-conduritols
14 and 24 using easily available starting materials. One of the syn-
thesized molecules 14 exhibited enzyme-specific inhibition
against a-glycosidase.
3.3. (rel-1S,4S,5R,6R)-2,5,6-Tribromocyclohex-2-ene-1,4-diol
(11)
3. Experimental
3.1. General
Tribromo quinone 10 (8.0 g, 23 mmol) was dissolved in 25 mL
of ether and cooled down to À10 °C. To this mixture was added
dropwise an aqueous solution of NaBH4 at 0 °C (2.2 g, 58 mmol).
The reaction was monitored by TLC. After the completion of the
reaction, the organic phase was separated, and the aqueous phase
was extracted with ether (3 Â 50 mL). The combined organic ex-
tracts were dried over Na2SO4. Removal of the solvent gave the
crude product, which was crystallized from MeOH–hexane (4:1)
to give white solid 11 (6.6 g, 82%), mp 160–162 °C. 1H NMR
(400 MHz, MeOH-d4) d: 6.17 (br s, 1H, H-3), 4.82 (br s, 2H, –OH),
4.44 (br d, J1,6 = 7.4 Hz, 1H, H-1), 4.38 (d, J4,5 = 7.8 Hz, 1H, H-4),
4.21 (dd, A part of AB-system J5,6 = 10.9 and J1,6 = 7.4 Hz, 1H, H-
6), 4.16 (dd, B part of AB-system, J5,6 = 10.9 and J5,4 = 7.8 Hz, 1H,
H-5); 13C NMR (100 MHz, MeOH-d) d 134.5, 126.7, 76.8, 74.4,
60.0, 59.2; IR (KBr, cmÀ1) 3357 (br), 2887 (w), 1648 (w), 1588
(w), 1448 (m), 1299 (m), 1260 (m), 1230 (s), 1194 (m), 1058 (s),
892 (m), 819 (m); MS (m/z, relative intensity): 353/351/349/347
(M+, 1), 273/271/269 (M+ÀHBr, 15/30/15), 255/253/251 (M+ÀHBr,
ÀH2O, 35/60/35), 174/172 (M+À2HBr, ÀH2O, 75/80), 110
(M+À2HBr, 100). Anal. Calcd for C6H7Br3O2: C, 20.54; H, 2.01.
Found: C, 20.73; H, 2.05.
Nuclear magnetic resonance (1H, 13C, 2D) spectra were recorded
on a Bruker Instrument, Avance Series-Spectrospin DPX-400 Bru-
ker, Ultra Shield (400 MHz), High Performance digital FT-NMR
Table 1
Inhibition of
a
-glycosidases 14 and 24
d
Compound
Inhibitiona (%)
84 6.9b
IC50
30
(lM)
OH
Br
OH
OH
OH
14
8.75 0.1c
NTe
OH
3.4. (rel-1S,4S,5R,6R)-4-(Acetyloxy)-2,5,6-tribromocyclohex-2-
en-1-yl acetate (12)
Br
OH
OH
To a stirred solution of 11 (6.0 g, 17.1 mmol) in 10 mL of pyri-
dine was added acetic anhydride (5.2 g, 51.0 mmol) dropwise at
À5 °C. The reaction mixture was stirred at room temperature for
8 h. The mixture was poured into 40 mL of HCl solution in ice
and extracted with ether (3 Â 50 mL). The combined organic ex-
tracts were washed with NaHCO3 and water, and then dried over
MgSO4. The removal of the solvent under reduced pressure fol-
lowed by crystallization from EtOH afforded pure white solid prod-
uct 12 (6.0 g, 80%), mp 110–112 °C. 1H NMR (400 MHz, CDCl3/CCl4)
d: 6.18 (t, J3,4 = 2.3 Hz, 1H, H-3), 5.88 (d, J1,6 = 6.6 Hz, 1H, H-1), 5.58
(dd, J4,5 = 6.8 Hz and J3,4 = 2.3 Hz, 1H, H-4), 4.31 (dd, A part of AB-
system, J5,6 = 10.9 Hz and J1,6 = 6.6 Hz, 1H, H-6), 4.29 (dd, B part
OH
24
a
Four experiments are performed for all compounds and in duplicate in each
experiment.
b
Inhibition by 40
l
l
M compound.
M compound.
Concentration required for 50% inhibition of the enzyme activity under the
c
Inhibition by 5
d
assay conditions.
e
NT: Not tested.