SYNTHESIS AND ALLYLIC OXIDATION OF 2,3-DEHYDRO-9β-BENZOYLOXY-β-AGAROFURAN

Article abstract of DOI:10.1135/cccc19950521

2,3-Dehydro-9β-benzoyloxy-β-agarofuran (VI), possible precursor of (+/-)-triptogelin G-2 (I), has been synthesized through a twelve-step procedure, allylic oxidation of which with various oxidative reagents has also been investigated, and three unusual ox

Full text of DOI:10.1135/cccc19950521

Synthesis of β-Agarofuran Derivative
521
SYNTHESIS AND ALLYLIC OXIDATION
OF 2,3-DEHYDRO-9β-BENZOYLOXY-β-AGAROFURAN
Xin CHEN, Fajun NAN, Zhaoming XIONG, Sichang SHAO*, Tongshuang LI
and Yulin LI**
State Key Laboratory of Applied Organic Chemistry and Institute of Organic Chemistry,
Lanzhou University, Lanzhou 730000, People’s Republic of China
Received November 2, 1994
Accepted December 27, 1994
2,3-Dehydro-9β-benzoyloxy-β-agarofuran (VI), possible precursor of (±)-triptogelin G-2 (I), has been
synthesized through a twelve-step procedure, allylic oxidation of which with various oxidative rea-
gents has also been investigated, and three unusual oxidized products, ketoaldehyde X, peroxide XI, and
ketone XII, were isolated.
A large number of β-dihydroagarofuran polyol esters have been isolated1–4 from the
plants of family Celastraceae. Some of them exhibit insecticidal properties5,6, insect
antifeedant effect7,8 and antitumor activity9. Recently, Takaishi et al. isolated10 trip-
togelin G-2 (I) from the achenes of Tripterygium wilfordii J. D. HOOK.
Although triptogelin G-2 (I) is one of the least structurally complex β-dihydroagaro-
furan polyol esters, it still presents a considerable synthetic challenge, due primarily to
the presence of five axial substituents appended to a trans-decahydronaphthalene skele-
ton. In the present paper, we wish to describe the synthesis of 2,3-dehydro-9β-benzoyl-
oxy-β-agarofuran (VI), the possible precursor of compound I. We also investigated the
allylic oxidation of compound VI with various oxidative reagents and obtained three
unusual oxidized products X – XII.
Our synthetic design (Scheme 1) was to employ 9-oxo-α-agarofuran (II), which was
easily prepared in seven steps from carvone11, as starting material. According to the
published method12,13, compound II was converted to 3β-hydroxy-9-oxo-β-agarofuran
(III) with an overall yield of 50%. Dehydration of alcohol III to 2,3-dehydro-9-oxo-β-
agarofuran (IV) could be effected by using phosphorus oxychloride/pyridine system;
* Visiting scholar from the Department of Chemistry, Fuyang Normal College.
**The author to whom correspondence should be addressed.
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522
Chen, Nan, Xiong, Shao, Li T., Li Y.:
however, a better yield (87%) with no trace of by-products was obtained by using
anhydrous copper sulfate dispersed on silica gel14.
The carbonyl group of IV was stereoselectively reduced with lithium aluminum hy-
dride at −50 °C to give exclusively the required 9β-hydroxy group12,13. Since the 9β-
hydroxy group was sterically hindered and strongly hydrogen-bonded, the alcohol V
was completely resistant to esterification under normal conditions. In the presence of
the strong base butyl lithium, the hydroxy group could be transformed to benzoate ester VI.
In the course of alternative synthetic work, we required 1,2-dehydro-9β-benzoyloxy-
α-agarofuran (VII) as the key intermediate. Quite unexpectedly, dehydration of alcohol
IX, prepared from 2-oxo-9β-benzoyloxy-α-agarofuran15 (VIII), under identical reaction
conditions as used for dehydration of III afforded β-agarofuran VI as the exclusive
product (Scheme 2). This result also confirmed the stereostructure of VI described in
Scheme 1, because the desired stereochemistry of the 9-benzoyloxy group in compound
VIII was assigned by Huffman15.
The initial strategy for the introduction of an oxygen substituent at C-1 was the
allylic oxidation of conjugated diene VI with 10% chromium trioxide supported on
silica gel, but the only product which could be isolated was the rearranged unsaturated
dicarbonyl compound X (Scheme 3). With a catalytic amount of chromium trioxide and
tert-butyl hydroperoxide, the product was the unsaturated peroxide XI (77% yield).
Selenium dioxide oxidation of VI resulted in a complex mixture of products. Employing
the chromium trioxide/pyridine complex generated in situ16 as the oxidative agent, the
exocyclic double bond of diene VI was cleaved to produce 4-nor-agarofuranone XII in
SCHEME 1
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Synthesis of β-Agarofuran Derivative
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SCHEME 2
SCHEME 3
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Chen, Nan, Xiong, Shao, Li T., Li Y.:
40% yield. Using the chromium trioxide/3,5-dimethylpyrazole complex12,13 the isolated
products were X and XII (Scheme 3). The structures of compounds X, XI and XII were
determined by their IR, 1H NMR, 13C NMR (DEPT) and high-resolution mass spectra.
It was apparent that allylic oxidation of VI was not a viable approach to I, but might
be convenient for preparation of 12-oxo-β-dihydroagarofuran polyol esters.
EXPERIMENTAL
Melting points are uncorrected. IR spectra were recorded on a Nicolet FT-170SX spectrophotometer as
1
liquid films (wavenumbers in cm−1). H NMR and 13C NMR spectra were measured on a Bruker AM-400
1
spectrometer (400 MHz for H and 100 MHz for 13C) in deuteriochloroform with tetramethyl- silane
as internal standard. Chemical shifts are given in ppm (δ-scale) and coupling constants (J) in Hz. Mass
spectra were determined on a VG ZAB-HS spectrometer (energy of ionizing electrons 70 eV). For
column chromatography, silica gel (200 – 300 mesh) and petroleum ether (b.p. 60 – 90 °C) were used.
2,3-Dehydro-9-oxo-β-agarofuran (IV)
A mixture of III (0.75 g, 3.0 mmol) dissolved in dry carbon tetrachloride (30 ml) and anhydrous
CuSO4 absorbed on silica gel (3.0 g, 3.9 mmol CuSO4) was refluxed under stirring for 3 h. After
cooling, the catalyst was removed by filtration, washed with acetone (10 ml) and the filtrate was
evaporated. The crude product was purified by chromatography eluting with petroleum ether–ether
(4 : 1) to give 0.61 g, (87%) of IV as colorless needles, m.p. 98 – 100 °C. IR spectrum: 2 975, 1 702
(C=O), 1 639, 1 603. 1H NMR spectrum: 0.95 s, 3 H (CH3-10); 1.19 s, 3 H (CH3-11); 1.24 s, 3 H
(CH3-11); 5.04 and 5.25 2 × bs, 2 H (H-12); 5.82 m, 1 H (H-2); 6.19 dd, 1 H, J = 9.7, J′ = 2.8 (H-3).
Mass spectrum, m/z (%): 232 (M+, 40), 217 (56), 214 (100), 199 (72). For C15H20O2 (232.3) calcu-
lated: 77.55% C, 8.68% H; found: 77.56% C, 8.66% H.
2,3-Dehydro-9β-hydroxy-β-agarofuran (V)
To a stirred suspension of LiAlH4 (0.14 g, 3.7 mmol) in ether (20 ml) at −50 °C under argon was
added dropwise a solution of IV (0.66 g, 2.8 mmol) in ether (20 ml). The mixture was stirred at the
same temperature for 5.5 h, then warmed to room temperature and stirred for additional 18 h. The
resulting suspension was cooled to −10 °C, and to this water (1 ml) and 10% aqueous NaOH (1 ml)
were added cautiously to destroy the excess reagent. The reaction mixture was filtered and the white
residue was washed with hot tetrahydrofuran (15 ml), the combined filtrates were dried with anhy-
drous MgSO4. Removal of the solvents under reduced pressure and purification by chromatography
(petroleum ether–ether, 1 : 2) afforded 0.63 g (95%) of V as colorless needles, m.p. 114 – 116 °C.
1
IR spectrum: 3 460 (OH), 2 924, 1 642, 1 601, 1 454. H NMR spectrum: 0.90 s, 3 H (CH3-10); 1.30 s,
3 H (CH3-11); 1.53 s, 3 H (CH3-11); 3.60 m, 1 H (H-9); 5.10 and 5.28 2 × bs, 2 H (2 × H-12); 5.83 m,
1 H (H-2); 6.16 dd, 1H, J = 9.8, J′ = 3.1 (H-3). Mass spectrum, m/z (%): 234 (M+, 54), 219 (42),
216 (100). For C15H22O2 (234.3) calculated: 76.88% C, 9.46% H; found: 76.70% C, 9.52% H.
2,3-Dehydro-9β-benzoyloxy-β-agarofuran (VI)
A) To a stirred solution of V (1.4 g, 6.0 mmol) and a few crystals of 2,2′-bipyridyl in tetrahydro-
furan (20 ml) at room temperature 1.5 M solution of butyl lithium in ether (ca 4 ml) was added drop-
wise until excess base appeared (red color). After 10 min a solution of benzoyl chloride (0.93 g, 6.6 mmol)
in tetrahydrofuran (4 ml) was added. The mixture was stirred at room temperature for 15 min and
then was refluxed for 1 h, resulting in a yellow solution. The cooled reaction mixture was poured
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into saturated aqueous NaHCO3 (15 ml), and extracted with ether (3 × 20 ml). The combined ether
extracts were washed with saturated aqueous NaHCO3 (2 × 10 ml) and brine (2 × 10 ml), and dried
over anhydrous Na2SO4. Chromatographic purification using petroleum ether–ether (8 : 1) as eluent
gave 1.95 g (96%) of VI as colorless needles, m.p. 129 – 130 °C. IR spectrum: 2 972, 1 712
(C6H5COO), 1 641, 1 602, 1 452. 1H NMR spectrum: 1.02 s, 3 H (CH3-10); 1.30 s, 3 H (CH3-11);
1.53 s, 3 H (CH3-11); 2.70 m, 1 H (H-7); 5.05 and 5.27 2 × s, 2 H (2 × H-12); 5.21 d, 1 H, J = 7.8
(H-9), 5.72 m, 1 H (H-2); 6.17 dd, 1 H, J = 9.9, J′ = 2.7 (H-3); 7.44 – 8.10 m, 5 H (5 × Ar-H).
13C NMR spectrum: 24.67 (C-15), 25.09 (C-13), 30.41 (C-14), 31.73 (C-8), 32.81 (C-6), 33.15 (C-1),
42.94 (C-7), 44.25 (C-10), 73.91 (C-9), 82.78 (C-11), 83.35 (C-5), 112.74 (C-12), 127.50 (C-2),
128.05 (C-3), 114.36 (C-4), 166.10 (C=O), 128.47, 129.77, 130.51, 132.92 (6 × Ar-C). High resolu-
tion mass spectrum, m/z: for C22H26O3 (M+) calculated 338.1882, found 338.1890.
B) To an ice-cooled mixture of VIII (0.30 g, 0.85 mmol) and cerium(III) chloride heptahydrate (0.45 g,
1.0 mmol) dissolved in methanol–ether (5 : 1, 24 ml) sodium borohydride (0.090 g, 2.4 mmol) was
added in several portions. The reaction mixture was stirred at room temperature for 3 h. Then the
excess hydride was destroyed by addition of 5% aqueous HCl (2 ml) at 0 °C and stirring continued
for additional 10 min. The solvents were removed in vacuo and the aqueous layer was extracted with
dichloromethane (2 × 10 ml). After washing with brine and drying over anhydrous MgSO4, the solvent
was evaporated. The crude product was purified by chromatography (petroleum ether–ether, 2 : 1) to
give 0.26 g (88%) of IX as a white solid. IR spectrum: 3 540 (OH); 1 709 (C6H5COO). 1H NMR
spectrum: 1.08 s, 3 H (CH3-10); 1.33 s, 3 H (CH3-11); 1.48 s, 3 H (CH3-11); 1.83 d, 3 H, J = 1.5
(CH3-4); 4.21 br, 1 H (H-2); 5.08 d, 1 H, J = 5.4 (H-9); 5.73 bs, 1 H (H-3); 7.31 – 8.09 m, 5 H (5 × Ar-H).
A stirred mixture of IX (0.24 g, 0.70 mmol) in carbon tetrachloride (10 ml) and anhydrous CuSO4
supported on silica gel (1.0 g, 1.3 mmol CuSO4) was refluxed for 10 min. After usual working up
0.21 g (91%) of compound VI was obtained, identical with the product prepared by procedure A).
Allylic Oxidation of 2,3-Dehydro-9β-benzoyloxy-β-agarofuran (VI)
A) With 10% CrO3 dispersed on silica gel. Chromium trioxide (1.0 g) was dissolved in minimum
quantity of water (ca 2 ml), and diluted by addition of methanol (10 ml). This solution was mixed
with silica gel (10 g, 200 – 300 mesh), resulting in a fine slurry. Evaporation of the solvents under
reduced pressure afforded a free-flowing powder of reagent.
A mixture of VI (0.2 g, 0.60 mmol) in dry dichloromethane (25 ml) and freshly prepared reagent
(1.8 g, 1.8 mmol) was stirred at room temperature for 1 h. The reaction mixture was filtered and the
solid was washed with ether (20 ml). Evaporation of the combined filtrates followed by chromato-
graphy (petroleum ether–ether, 6 : 1) gave 0.15 g (68%) of dicarbonyl compound X as colorless
needles, m.p. 190 – 192 °C. IR spectrum: 1 712 (C6H5COO), 1 686 (C=O), 1 660 (CHO). 1H NMR
spectrum: 1.19 s, 3 H (CH3-10); 1.26 s, 3 H (CH3-11); 1.47 s, 3 H (CH3-11); 5.17 d, 1 H, J = 7.8
(H-9); 6.56 s, 1 H (H-3); 7.45 – 8.07 m, 5 H (5 × Ar-H); 9.86 s, 1 H (CHO). 13C NMR spectrum:
24.38 (C-15), 25.12 (C-13), 30.02 (C-14), 30.59 (C-8), 32.94 (C-6), 43.86 (C-7), 44.67 (C-1), 45.85
(C-10), 73.16 (C-9), 81.18 (C-11), 84.87 (C-5), 141.99 (C-3), 148.06 (C-4), 165.72 (C=O), 194.21
(C-12), 199.84 (C-2), 128.58, 129.74, 133.31 (6 × Ar-C). High-resolution mass spectrum, m/z: for
C21H21O5 (M+ − CH3) calculated 353.1389, found 353.1389.
B) With CrO3/tert-butyl hydroperoxide. tert-Butyl hydroperoxide (75%, 1 ml) was added dropwise
at room temperature to a stirred solution of CrO3 (0.03 g, 0.30 mmol) in dichloromethane (15 ml),
yielding a brown solution. Then, compound VI (0.34 g, 1.0 mmol) was introduced. Stirring was con-
tinued for 2 h, during which time the brown reaction solution turned gray. The resulting solution was
diluted with ether (50 ml), and washed successively with saturated aqueous NaHCO3 (2 × 20 ml),
H2O (2 × 20 ml) and brine (20 ml) prior to drying over anhydrous Na2SO4. Chromatography using
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Chen, Nan, Xiong, Shao, Li T., Li Y.:
petroleum ether–ether (6 : 1) as eluent afforded 0.34 g (77%) of peroxide XI as a white solid, m.p.
1
161 – 163 °C. IR spectrum: 1 715 (C6H5COO); 1 679 (C=O). H NMR spectrum: 1.21 s, 3 H (CH3-10);
1.26 s, 9 H ((CH3)3CO); 1.38 s, 3 H (CH3-11); 1.57 s, 3 H (CH3-11); 4.63 and 4.74 AB system,
J(AB) = 14.3 (2 × H-12); 5.15 d, 1 H, J = 6.9 (H-9); 6.18 s, 1 H (H-3); 7.44 – 8.07 m, 5 H (5 × Ar-H).
13C NMR spectrum: 24.25 (C-15), 25.29 (C-13), 26.36 ((CH3)3C), 30.32 (C-14), 30.89 (C-8), 32.99
(C-6), 43.57 (C-7), 44.20 (C-1), 45.71 (C-10), 73.09 (C-12), 73.24 (C-9), 80.79 ((CH3)3C), 82.50
(C-11), 83.49 (C-5), 128.99 (C-3), 151.99 (C-4), 165.81 (C=O), 199.10 (C-2), 128.53, 129.78, 133.20
(6 × Ar-C). High-resolution mass spectrum, m/z: for C26H34O6 (M+) calculated 442.2355, found 442.2323.
C) With CrO3/pyridine complex. To an ice-cooled stirred solution of pyridine (5 ml) in dry
dichloromethane (30 ml) CrO3 (2.6 g, 26 mmol) was added over a period of 5 min. After 15 min a
dark red slurry was formed. This mixture was combined with a solution of compound VI (0.10 g,
0.30 mmol) in dichloromethane (10 ml). The mixture was warmed to room temperature, and stirring
was continued for additional 4 h. The reaction mixture was decanted from the brown tarry residue
which was washed with ether (60 ml). The combined organic phases were washed with saturated
aqueous NaHCO3 (2 × 20 ml), H2O (2 × 20 ml) and brine (2 × 20 ml), and dried over anhydrous
Na2SO4. The products were separated by chromatography (petroleum ether–ether, 6 : 1) to afford
0.04 g (40%) of ketone XII as a white solid: m.p. 155 – 157 °C. IR spectrum: 1 714 (C6H5COO), 1 683
1
(C=O). H NMR spectrum: 1.16 s, 3 H (CH3-10); 1.28 s, 3 H, (CH3-11); 1.57 s, 3 H (CH3-11); 5.20 d,
1 H, J = 7.5 (H-9); 6.11 dd, 1 H, J = 10.1, J′ = 2.7 (H-3); 6.91 m, 1 H (H-2); 7.47 – 8.11 m, 5
H (5 × Ar-H). 13C NMR spectrum: 23.95 (C-15), 24.31 (C-13), 29.68 (C-14), 30.64 (C-8), 32.41
(C-1, C-6), 43.22 (C-7), 46.49 (C-10), 73.51 (C-9), 84.15 (C-11), 84.13 (C-5), 127.11 (C-3), 149.27
(C-2), 166.01 (C=O), 194.70 (C-4), 128.58, 129.72, 130.50, 133.20 (6 × Ar-C). High-resolution mass
spectrum, m/z: for C20H21O4 (M+ − CH3) 325.1440, found 325.1420.
D) With CrO3/3,5-dimethylpyrazole complex. Following a published procedure15, compound VI
was converted to ketoaldehyde X and ketone XII in yields of 21% and 17%, respectively.
We are grateful for financial support from the National Natural Science Foundation of China.
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