Synthesis of 4',8-dihydroxyisoflavon-7-yl α-D-arabinofuranoside

Article abstract of DOI:10.1016/S0957-4166(99)00135-4

4',8-Dihydroxyisoflavon-7-yl α-D-arabinofuranoside 1 (A-76202), which is a strong α-glucosidase I and II inhibitor, was synthesized by the glycosylation of 2,3,5-tri-O-benzyl-α-D-arabinofuranosyl bromide 2 and the lithium salt of 4',8-diallyloxy-7-hydroxy

Full text of DOI:10.1016/S0957-4166(99)00135-4

Pergamon
Tetrahedron: Asymmetry 10 (1999) 1477–1482
Synthesis of 40,8-dihydroxyisoflavon-7-yl α-D-arabinofuranoside
Masao Shiozaki ∗
Exploratory Chemistry Research Laboratories, Sankyo Co. Ltd, Hiromachi 1-2-58, Shinagawa-ku, Tokyo 140-8710, Japan
Received 1 March 1999; accepted 1 April 1999
Abstract
40,8-Dihydroxyisoflavon-7-yl α-D-arabinofuranoside 1 (A-76202), which is a strong α-glucosidase I and II
inhibitor, was synthesized by the glycosylation of 2,3,5-tri-O-benzyl-α-D-arabinofuranosyl bromide 2 and the
lithium salt of 40,8-diallyloxy-7-hydroxyisoflavone 4, and successive deprotection of allyl groups and benzoyl
esters of the glycosylated product 5. © 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction
40,8-Dihydroxyisoflavon-7-yl α-D-arabinofuranoside 1 (A-76202) was isolated from Rhodococcus
sp. SANK61694. It strongly inhibits α-glucosidases I and II, which exist in endoplasmic reticulum,
and participates in the processing of secretory-, cell membrane- and virus surface-glycoproteins. The
structure of A-762021 has been elucidated as a D-arabinose analogue 1 which contains a phenolic
isoflavone as the aglycon. The synthesis of A-76202 is interesting because of its biological activity and
also for developing the synthetic route for related compounds and is described here.
2. Results and discussion
The 40 and 8 positions of 40,7,8-trihydroxyisoflavone 32 was protected by allyl bromide and 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU) in MeOH to give mainly 40,8-diallyloxy-7-hydroxyisoflavone 3
(mp 125–126°C) in 41% yield after chromatographic purification from accompanying 8-allyloxy-40,7-
dihydroxyisoflavone, 7,8-diallyloxy-40-hydroxyisoflavone and 40,7,8-triallyloxyisoflavone. Treatment of
4 with butyl lithium in tetrahydrofuran (THF) generated the corresponding lithium salt, which was further
reacted with 2,3,5-tri-O-benzoyl-α-D-arabinofuranosyl bromide 23 in THF under reflux temperature to
give 5 (26% from 4, and 53% from 4 allowing for recovery of unreacted 4), 2,3,5-tri-O-benzoyl-D-
arabinofuranose (30% from 2), and recovered 4 (51%). From this reaction, the product was only the
∗
E-mail: shioza@shina.sankyo.co.jp
0957-4166/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(99)00135-4

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M. Shiozaki / Tetrahedron: Asymmetry 10 (1999) 1477–1482
α-anomer without any β-anomer.4 When the reaction of 4 with 2 in dichloromethane in the presence of
silver carbonate (1 equiv.)–silver triflate (1 equiv.)–4Å molecular sieves (excess) according to the reported
conditions5 gave an inseparable mixture which contained 5 and three other compounds (from FAB MS
data). The other procedure, which was employed for the coupling of the isoflavones and pyranosyl
bromides by Wagner et al.,6 was applied for this reaction. Treatment of a mixture of 2 and 4 in pyridine
with silver carbonate led to recovery of the starting 4 without any coupling products (Scheme 1).
Scheme 1. Reagents and conditions: (a) allyl bromide, DBU, MeOH, rt then reflux 3 h, 41%; (b) n-BuLi, 2 THF rt, then reflux
6 h, 26% from 4 (recovery 4, 51%; revised 53%); (c) RhCl3 hydrate, EtOH, under N2, reflux, 3 h, 78%; (d) 0.2 M NaOH aq. in
MeOH, rt, 3 h, 76%
Deprotection of the allyl groups from 5 was performed by the treatment of RhCl3 hydrate in EtOH at
reflux to afford 6. Saponification of the three benzoyl esters of 6 with 0.2 M NaOH aq. in MeOH gave
1 {([α]D24 +115 (c 0.12, DMSO); natural A-76202: [α]D24 +125 (c 0.12, DMSO)}, which was identical
with natural A-76202 in the 1H and 13C NMR, IR, and FAB MS data.
A few alternative stereoselective routes to synthesize the equivalent compounds 50 of the α-anomer
5 were attempted (Scheme 2). Treatment of 2,3,5-tri-O-acetyl-D-arabinofuranosyl fluoride 77,12 and 4
using 1.2 equivalents of bis(cyclopentadienyl)hafnium dichloride, 1.2 equivalents of silver trifluorome-
thanesulfonate and excess 4Å molecular sieves as Lewis acids in dichloromethane at 0–24°C under
nitrogen according to Suzuki’s method8 gave an inseparable 3:1 mixture of α- and β-anomers (40,8-
diallyloxyisoflavon-7-yl 2,3,5-tri-O-acetyl-D-arabinofuranoside 50 (R1=R2=R3=Ac)) in extremely low
yield (<2%). In this coupling reaction, the use of silver perchlorate–bis(cyclopentadienyl)zirconium
dichloride–4Å molecular sieves8 caused the degradation of compound 7.
In another attempt to obtain the correct stereochemical 1,2-trans α-arabinoside, 1,2-anhydro-5-O-
tert-butyldiphenylsilyl-3-O-trimethylsilyl-β-D-arabinofuranose 89 was treated with 4 according to the re-
ported procedure,9b however, the coupling compound 50 (R1=tert-butyldiphenylsilyl, R2=trimethylsilyl,
R3=H) was not obtained. Also a solution of both 2,3-di-O-acetyl-1,5-anhydro-β-D-arabinofuranose 910
and 4 in dichloromethane was treated with boron trifluoride diethyl etherate, trimethylsilyl trifluorome-
thanesulfonate, or titanium(IV) chloride under various conditions, however, no coupling compounds were
detected only degradation of 9. The other compound 2,3,4-tri-O-acetyl-5-O-(tert-butyldiphenylsilyl)-D-

M. Shiozaki / Tetrahedron: Asymmetry 10 (1999) 1477–1482
1479
Scheme 2.
arabinose diethyldithioacetal 10, which was obtained easily by acetic anhydride–pyridine acetylation
of 5-O-(tert-butyldiphenylsilyl)-D-arabinose diethyldithioacetal,11 was treated with 4 using boron tri-
fluoride diethyl etherate, mercury(II) chloride, or mercury(II) chloride–mercury(II) oxide under various
conditions. These attempts did not yield any coupling compounds 11.
Thus, in these attempts, the only successful coupling procedure was the reaction between the lithium
salt of 40,8-diallyloxy-7-hydroxyisoflavone and 2,3,5-tri-O-benzoyl-α-D-arabinofuranosyl bromide.
3. Experimental
3.1. General
Melting points were determined on a Yanagimoto micro melting point apparatus and are uncorrected.
1H NMR (270 and 400 MHz) spectra were recorded with JEOL JNM-270 and JEOL JNM-GSX 400
spectrometers using tetramethylsilane as an internal standard. IR absorption spectra were determined
with an IR A-2 spectrophotometer, and mass spectra were obtained with a JMS-700 mass spectrometer.
Elemental analyses were performed by the Institute of Science and Technology, Inc. Separation of the
compounds by column chromatography was carried out with silica gel 60 (230–400 mesh ASTM, E.
Merck) under a slightly elevated pressure (1.2–1.5 atm) for easy elution. The quantity of silica gel used
was 50–100 times the weight charged on the column. Detection involved spraying the chromatogram
with a solution of 17% H2SO4 in water (w/w), containing ammonium molybdate (2.3%) and ceric sulfate
(0.9%) (Hanessian dip), and heating the plate for several minutes at ca. 180°C. Tetrahydrofuran (THF)
was distilled from LiAlH4 and used immediately. Dichloromethane was dried by being passed through
an ICN Alumina B-Super I.

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M. Shiozaki / Tetrahedron: Asymmetry 10 (1999) 1477–1482
3.2. Materials
3.2.1. 40,8-Diallyloxy-7-hydroxyisoflavone 4
To a solution of 40,7,8-trihydroxyisoflavone 3 (270 mg, 1.00 mmol) in MeOH (2 ml) were added
DBU (530 mg, 3.48 mmol) and allyl bromide (223 mg, 1.82 mmol). The mixture was stirred for 16
h at room temperature, acidified with 1 M HCl, diluted with EtOAc, washed with H2O, then with
brine, dried over MgSO4, filtered, and concentrated in vacuo to give a mixture, which was chromato-
graphed on a silica gel column. Elution with cyclohexane:EtOAc (1:1) gave 40,7,8-triallyloxyisoflavone
(25 mg, 6%), 4 (144 mg, 41%) as a semi-solid, 7,8-diallyloxy-40 -hydroxyisoflavone (53 mg, 13%),
and 8-allyloxy-40,7-dihydroxyisoflavone (104 mg, 35%). Physical data of 40,7,8-triallyloxyisoflavone:
Rf=0.877 (cyclohexane:EtOAc=2:3); 270 MHz 1H NMR (CDCl3) δ 4.56–4.74 (6H, m), 5.24–5.51 (6H,
m), 6.01–6.22 (3H, m), 6.99 (2H, d, J=8.7 Hz), 7.04 (1H, d, J=9.1 Hz), 7.49 (2H, d, J=8.7 Hz), 7.99
(1H, s), 8.01 (1H, d, J=9.1 Hz). Physical data of 4: mp 125–126°C (from hexane–EtOAc); Rf=0.745
(cyclohexane:EtOAc=2:3); IR νmax (KBr) 3223, 1628, 1618, 1600, 1573, 1510, 1440 cm−1; 270 MHz
1H NMR (CDCl3) δ 4.58 (2H, dm, J=5.3, ≤1 Hz), 4.74 (2H, d, J=6.1 Hz), 5.28–5.46 (4H, m), 6.01–6.19
(2H, m), 6.27 (1H, brs, OH), 6.99 (2H, d, J=8.9 Hz), 7.05 (1H, d, J=9.1 Hz), 7.49 (2H, d, J=8.9 Hz),
7.97 (1H, s), 7.98 (1H, d, J=9.1 Hz). FAB MS (positive) m/z: 351 [M+H]+. High resolution FAB MS m/z:
calcd for C21H19O5: 351.1232; found: 351.1219. Physical data of 7,8-diallyloxy-40 -hydroxyisoflavone:
Rf=0.667 (cyclohexane:EtOAc=2:3); 270 MHz 1H NMR (CDCl3) δ 4.68–4.75 (4H, m), 5.24–5.51 (4H,
m), 6.03–6.20 (2H, m), 6.89 (2H, d, J=8.5 Hz), 7.05 (1H, d, J=9.2 Hz), 7.44 (2H, d, J=8.5 Hz), 8.00 (1H,
s), 8.01 (1H, d, J=9.2 Hz). FAB MS (positive) m/z: 351 [M+H]+. High resolution FAB MS m/z: calcd for
C21H19O5: 351.1232; found: 351.1209. Physical data of 8-allyloxy-40,7-dihydroxyisoflavone: Rf=0.474
1
(cyclohexane:EtOAc=2:3); 270 MHz H NMR (CDCl3) δ 4.78 (2H, d, J=5.8 Hz), 5.37–5.44 (3H, m,
containing OH), 6.14 (1H, m), 6.92 (2H, d, J=8.6 Hz), 7.10 (1H, d, J=8.7 Hz), 7.45 (2H, d, J=8.6 Hz),
8.00 (1H, s), 8.02 (1H, d, J=8.7 Hz).
Treatment of these compounds with acetic anhydride in pyridine gave the corresponding acetates
which were further confirmed. Physical data of 7-acetoxy-40,8-diallyloxyisoflavone: 270 MHz 1H NMR
(CDCl3) δ 2.37 (3H, s), 4.57–4.60 (2H, m), 4.69 (2H, d, J=5.6 Hz), 5.27–5.48 (4H, m), 6.00–6.16
(2H, m), 7.00 (2H, d, J=8.6 Hz), 7.14 (1H, d, J=8.7 Hz), 7.49 (2H, d, J=8.6 Hz), 8.03 (1H, s), 8.05
(1H, d, J=8.7 Hz). FAB MS (positive) m/z: 393 [M+H]+. High resolution FAB MS m/z: calcd for
C23H21O6: 393.1339; found: 393.1341. Physical data of 40-acetoxy-7,8-diallyloxyisoflavone: 270 MHz
1H NMR (CDCl3) δ 2.32 (3H, s), 4.69–4.78 (4H, m), 5.24–5.51 (4H, m), 6.03–6.20 (2H, m), 7.05 (1H,
d, J=9.1 Hz), 7.17 (2H, d, J=8.6 Hz), 7.59 (1H, d, J=8.6 Hz), 8.01 (1H, d, J=9.1 Hz), 8.02 (1H, s). FAB
MS (positive) m/z: 393 [M+H]+. High resolution FAB MS m/z: calcd for C23H21O6: 393.1339; found:
393.1337. Physical data of 40,7-diacetoxy-8-allyloxyoxyisoflavone: 270 MHz 1H NMR (CDCl3) δ 2.33
(3H, s), 2.38 (3H, s), 4.69 (2H, d, J=5.8 Hz), 5.27–5.45 (2H, m), 6.07 (1H, m), 7.16 (1H, d, J=8.6 Hz),
7.18 (2H, d, J=8.5 Hz), 7.59 (2H, d, J=8.5 Hz), 8.05 (1H, d, J=8.6 Hz), 8.06 (1H, s).
3.2.2. 40,8-Diallyloxyisoflavon-7-yl 2,3,5-tri-O-benzoyl-α-D-arabinofuranoside 5
To a solution of 4 (480 mg, 1.37 mmol) in THF (20 ml) was added n-BuLi (1.53 M n-hexane solution,
1.00 ml) at room temperature. After 5 min stirring, to this solution was added a solution of 2,3,5-tri-O-
benzoyl-α-D-arabinofuranosyl bromide 2 (864 mg, 1.64 mmol, 1.2 equiv.) in THF (10 ml). After 1.5 h
stirring at room temperature, this solution was refluxed for 6 h, and diluted with EtOAc. The solution was
washed with aq. NaHCO3, then with brine, dried over MgSO4, filtered, and concentrated in vacuo to give
a mixture, which was chromatographed on a silica gel column. Elution with cyclohexane:EtOAc (3:1)
gave 5 [285 mg, 26% and 22% from 4 and 2, respectively; and 53% and 31% from 4 and 2 allowing for

M. Shiozaki / Tetrahedron: Asymmetry 10 (1999) 1477–1482
1481
recovery of unreacted 4 and 2, respectively; Rf=0.56 (cyclohexane:EtOAc=2:1)] as a powder, 2,3,5-tri-
O-benzoyl-D-arabinofuranose [224 mg, 30% from 2, Rf=0.49 (cyclohexane:EtOAc=2:1)] and recovery
24
4 [245 mg, 51%, Rf=0.38 (cyclohexane:EtOAc=2:1)]. Physical data of 5: [α]D +40 (c 0.22, CHCl3);
1
IR νmax (KBr) 1726, 1646, 1604, 1568 cm−1; 400 MHz H NMR (CDCl3) δ 4.58 (2H, d, J=5.4 Hz),
4.70–4.87 (5H, m), 5.18 (1H, d, J=10.3 Hz), 5.29–5.46 (3H, m), 5.73 (1H, d), 5.90 (1H, s), 6.04–6.12
(2H, m), 6.13 (1H, s), 6.99 (2H, d, J=8.5 Hz), 7.26–7.65 (12H, m), 8.01–8.16 (8H, m). FAB MS (positive)
m/z: 795 [M+H]+. High resolution FAB MS m/z: calcd for C47H39O12: 795.2441; found: 795.2444. Anal.
calcd for C47H38O12: C, 71.03; H, 4.82; found: C, 70.86; H, 4.99.
3.2.3. 40,8-Dihydroxyisoflavon-7-yl 2,3,5-tri-O-benzoyl-α-D-arabinofuranoside 6
A solution of 5 (40 mg, 0.05 mmol) in anhydrous EtOH (4 ml) containing RhCl3 hydrate (4 mg) was
refluxed for 3 h under nitrogen. The resulting dark solution was concentrated in vacuo to give a residue,
which was chromatographed on a silica gel column. Elution with cyclohexane:EtOAc (1:1) gave 6 (28
mg, 78%) as a white powder. [α]D24 +86 (c 0.05, CHCl3); IR νmax (KBr) 3410 (broad), 1726, 1603, 1575
cm−1; 270 MHz 1H NMR (CDCl3) δ 4.70–4.92 (3H, m), 5.79 (1H, m), 5.93 (1H, d, J=1.0 Hz), 6.02 (1H,
s), 6.63 (1H, bs, OH), 6.85 (2H, d, J=8.4 Hz), 7.26–7.64 (12H, m), 7.79 (1H, d, J=9.2 Hz), 7.98–8.11
(7H, m). FAB MS (positive) m/z: 715 [M+H]+. High resolution FAB MS m/z: calcd for C41H31O12:
715.1815; found: 715.1811. Anal. calcd for C41H30O12: C, 68.89; H, 4.23; found: C, 68.76; H, 4.51.
3.2.4. 40,8-Dihydroxyisoflavon-7-yl α-D-arabinofuranoside 1
Compound 6 (124 mg, 0.17 mmol) in 0.2 M NaOH methanol solution (6.1 ml, 1.21 mmol, 7.0 equiv.)
was stirred for 16 h at 24°C. The solution was concentrated in vacuo to one-third of the volume, acidified
with 1 M HCl (1.3 ml), and diluted with EtOAc. The mixture was washed with H2O, then with brine,
dried over MgSO4, filtered, and evaporated in vacuo to give a residue, which was chromatographed on a
silica gel short column (silica gel, 6 g). Elution with EtOAc, and then EtOAc:MeOH (19:1) gave 1 (53
24
24
mg, 76%). [α]D +115 (c 0.12, DMSO); [cf. natural A-76202: [α]D +125 (c 0.12, DMSO)]; IR νmax
(KBr) 3329 (broad), 2930, 1626, 1600, 1573, 1515, 1451, 1396, 1336, 1283, 1211, 1176 cm−1; 400 MHz
1H NMR (CDCl3:CD3OD=1:1) δ 3.72 (1H, dd, J=3.2, 12.1 Hz), 3.80 (1H, dd, J=3.4, 12.1 Hz), 4.20
(1H, d, J=2.6 Hz), 4.28 (1H, m), 4.34 (1H, s), 5.77 (1H, s), 6.92 (2H, d, J=8.4 Hz), 7.25 (1H, d, J=9.1
Hz), 7.38 (2H, d, J=8.4 Hz), 7.73 (1H, d, J=9.1 Hz), 8.05 (1H, s); 100 MHz 13C NMR (DMSO-d6) δ
61.0, 76.5, 81.3, 85.8, 107.1, 114.4, 114.9, 115.0, 115.1, 119.6, 122.4, 123.2, 130.1, 136.0, 146.0, 147.5,
147.6, 153.1, 157.2, 175.1. FAB MS (positive) m/z: 403 [M+H]+. High resolution FAB MS m/z: calcd for
C20H19O9: 403.1029; found: 403.1026. Anal. calcd for C20H18O9: C, 59.69; H, 4.51; found: C, 59.60;
1
H, 4.55. H and 13C NMR, IR, and FAB MS data of synthetic 1 were identical with those of natural
A-76202.
References
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cation (A).

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M. Shiozaki / Tetrahedron: Asymmetry 10 (1999) 1477–1482
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room temperature, and should be stored in a refrigerator. IR νmax (film) 1748, 1373, 1226, 1064 cm−1; 270 MHz 1H NMR
(CDCl3) δ 2.11–2.15 (9H, m), 4.12–4.58 (3H, m), 5.00–5.44 (2H, m), 5.65–6.00 (1H, m). FAB MS (positive) m/z: 279
[M+H]+, 301 [M+Na]+. High resolution FAB MS m/z: calcd for C11H15O7FNa: 301.0696; found: 301.0700. Anal. calcd
for C11H15O7F: C, 47.49; H, 5.43; F, 6.83; found: C, 47.39; H, 5.44; F, 6.39.
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