Synthesis of a furanosyl-pyranone derivative related to the tri-o-heterocyclic core of herbicidins

Article abstract of DOI:10.1002/jccs.201100744

A new compound that is structurally related to the undecose ring structure of herbicidins has been prepared. The synthesis of this novel furanosyl-pyranone derivative was made possible through the regioselective reductive ring-opening of a 3,5-O-benzylidene-D-xylofuranose and the hetero-Diels-Alder reaction of an aldehyde and a Danishefsky-type diene. The highly functionalized pyranone derivative can be a useful precursor for the synthesis of herbicidins.

Full text of DOI:10.1002/jccs.201100744

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Article
Synthesis of a Furanosyl-pyranone Derivative Related to the Tri-O-heterocyclic Core
of Herbicidins
Ching-Yun Hsu,a,* I-Chi Lee,b,c Larry S. Lico,c Biing-Jiun Uangb,* and Shang-Cheng Hungc,d,*
aDepartment of Chemical and Materials Engineering, Cheng Shiu University, 840, Chengcing Road, Niaosong District,
Kaohsiung City 83347,Taiwan, R.O.C.
bDepartment of Chemistry, National Tsing Hua University, 101, Section 2, Kuang-Fu Road, Hsinchu 30043,
Taiwan, R.O.C.
cGenomics Research Center, Academia Sinica, 128, Section 2, Academia Road, Taipei 11529, Taiwan, R.O.C.
dDepartment of Applied Chemistry, National Chiao Tung University, No. 1001, Ta-Hsueh Road, Hsinchu 300,
Taiwan, R.O.C.
(Received: Jan. 10, 2012; Accepted: Jan. 17, 2012; Published Online: Feb. 6, 2012; DOI: 10.1002/jccs.201100744)
A new compound that is structurally related to the undecose ring structure of herbicidins has been pre-
pared. The synthesis of this novel furanosyl-pyranone derivative was made possible through the
regioselective reductive ring-opening of a 3,5-O-benzylidene-D-xylofuranose and the hetero-Diels–Alder
reaction of an aldehyde and a Danishefsky-type diene. The highly functionalized pyranone derivative can
be a useful precursor for the synthesis of herbicidins.
Keywords: Benzylidene acetal; Cycloaddition; Danishefsky-type diene; Herbicidins;
Regioselectivity.
INTRODUCTION
In the 1970’s, a series of undecose (C11)-based nu-
rice.
Unique structural features make herbicidins a con-
cleoside antibiotics, collectively known as herbicidins (1),
were isolated and characterized from the strains of Strepto-
myces saganonensis (Fig. 1).1 These compounds were
shown to exhibit valuable herbicidal and anti-fungal activ-
ity while being relatively non-toxic to animals. For exam-
ple, herbicidins A (1a) and B (1b) are selectively toxic to
some dicotyledons and efficiently inhibits Xanthomonas
oryzae, the bacteria responsible for leaf blight infections in
spicuous target among synthetic chemists. The core 11-car-
bon structure that is b1‘-linked to adenine is an interesting
furanopyranopyran ring system having an internal hemi-
ketal bridging the C3¢and C7¢ positions and a C-glycoside
linking C5¢ and C6¢. The rigid backbone also forces the
C7¢, C8¢, C9¢, and C10¢ substituents to assume the axial po-
sition. Because the C-glycoside functionality separates the
tri-O-heterocylic structure into two simple sugars, syn-
thetic efforts primarily targeted the difficult generation of
this crucial sugar link as the key step. To date, however,
only Matsuda and co-workers2 have completed the synthe-
sis of a herbicidin congener, that is herbicidin B. Other re-
search groups only accomplished the partial3 or full assem-
bly4 of the undecose backbone. Thus, new avenues that
may improve the synthesis of this important series of com-
pounds are still much desired.
Our synthetic strategy focusing on the formation of
the glycosyl core structure of herbicidins (1) is depicted in
Scheme I. We envisioned the furanosyl-pyranone 2 as a di-
rect precursor to the tricylic ring system and is, thus, the
target material in the current work. The plan is to access
Fig. 1. Structures of herbicidins.
Dedicated to the memory of Professor Yung-Son Hon (1955–2011).
* Corresponding author. Tel: +886-2-2787-1279; Fax: +886-2-27878771; E-mail: erichsu@csu.edu.tw, bjuang@mx.nthu.edu.tw,
schung@gate.sinica.edu.tw
J. Chin. Chem. Soc. 2012, 59, 421-425
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Hsu et al.
Scheme I Retrosynthetic analysis of herbicidin
Scheme II Reductive ring opening of 3,5-O-benzyli-
dene acetal
this compound through the [4 + 2]-cycloaddition of the
Danishefsky-type diene 3 and the aldehyde 4. Here, the
stereoselectivity may be controlled by an appropriately se-
lected Lewis acid catalyst.5 Compound 4, in turn, could be
generated by transformations of the commercially abun-
dant 1,2-O-isopropylidene-a-D-xylofuranose (5).
Scheme III Synthesis of the aldehyde 4
RESULTS AND DISCUSSION
The first encountered challenge is the installation of a
benzyl ether at the O3 position of the diol 5. Presumably,
this transformation may be directly achieved by regiose-
lective reductive ring opening of an installed 3,5-O-ben-
zylidene acetal. Nonetheless, although the 1,3-dioxane-
type benzylidene acetals in pyranoses (e.g. 4,6-O-benzyli-
dene) could be readily opened in either direction by a suit-
able set of reagents,6 literature reports on the ring opening
of the corresponding acetal in furanosyl sugars only re-
sulted in the formation of the primary benzyl ether and a
secondary hydroxyl group as the major isomer.7 To test the
viability of this synthetic route, compound 5 was converted
to the desired acetal 6 according to a known procedure7a
(Scheme II). Treatment of 6 with LiAlH4 and AlCl3 only
produced the unwanted 3-OH-5-OBn regioisomer 7 in 43%
yield without any trace of the target 5-alcohol 8. Fortu-
nately, our recently reported BH3×THF/Cu(OTf)2 combina-
tion6a,b delivered the primary alcohol 8 from acetal 6 to-
gether with the diol 5 in 52% and 30% yields, respectively.
The corresponding 3-alcohol 7 was not isolated from the
reaction mixture. To our knowledge, this is the first report
of a regioselective reductive ring opening at the O5 posi-
tion of a 3,5-O-benzylidene acetal in a furanoside.
Oxidation of compound 8 with NaHCO3 and NaIO4 in
phase transfer conditions led to the aldehyde 98 in an excel-
lent yield. The terminal alkene 10, generated from 9
through a typical Wittig reaction (62%), was sequentially
treated with 9-borabicyclo[3.3.1]nonane (9-BBN) and
H2O2/NaOH to form the desired primary alcohol 11 to-
gether with its isomer 12 in 67% and 21% yields, respec-
tively. Compound 11 was, then, subjected to Swern oxida-
tion to afford the target aldehyde 4 in 92% yield.
The formation of the furanosyl-pyranone 2 was facili-
tated through the hetero-Diels–Alder reaction of the Dan-
ishefsky-type diene 3 and the aldehyde dienophile 4 (Table
With the 5-alcohol 8 in hand, we continued the trans-
formations toward the required aldehyde 4 (Scheme III).
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Toward the Synthesis of Herbicidins
Table 1. Hetero-Diels–Alder cycloaddition of the Danishefsky-
feasibility of this precursor in accessing the herbicidin
congeners.
type diene 3 and the aldehyde 4
H
O
O
O
O
OBn
O
EXPERIMENTAL
O
OBn
R
O
General remarks
O
2a: R = H
MeO
O
All solvents (CH2Cl2, THF, and Et2O) were purified
and dried from a safe purification system filled with anhy-
drous Al2O3. All other reagents obtained from commercial
sources were used without further purification. Flash col-
umn chromatography was carried out on Silica Gel 60
(230-400 mesh, E. Merck). TLC was performed on pre-
coated plates of Silica Gel 60 F254 (0.25 mm, E. Merck); de-
tection was executed by spraying with a solution of
Ce(NH4)2(NO3)6, (NH4)6Mo7O24, and H2SO4 in water and
subsequent heating on a hotplate. 1H and 13C NMR spectra
were recorded using a 400 MHz spectrometer. Chemical
shifts are in ppm from Me4Si calibrated using the resonance
of the residual proton and carbon of the deuterated solvent.
3-O-Benzyl-1,2-O-isopropylidene-a-D-xylofuranose (8)
BH3×THF (1 M solution in THF, 0.36 mL, 0.36 mmol)
was slowly added to the acetal 6 (20.0 mg, 0.0719 mmol) at
room temperature under nitrogen atmosphere. After stir-
ring for 10 min, freshly dried Cu(OTf)2 (3.91 mg, 10.8
mmol) was added to the solution, and the mixture was kept
stirring for 2 h. The reaction was quenched by sequential
additions of Et3N and methanol, and the resulting mixture
was concentrated under reduced pressure. The residue was
purified by flash column chromatography (ethyl acetate/
hexanes = 1/2) to afford the 5-alcohol 8 (10.5 mg, 0.0375
mmol) and the diol 5 (4.1 mg, 0.022 mmol). [a]D29 –33.4 (c
0.3, CHCl3); IR (thin film) n 3396, 1216, 1161, 1074 cm-1;
1H NMR (400 MHz, CDCl3) d 7.36-7.30 (m, 5H, ArH),
5.97 (d, J = 3.8 Hz, 1H, H-1), 4.70 (d, J = 11.9 Hz, 1H,
CH2Ph), 4.62 (d, J = 3.8 Hz, 1H, H-2), 4.55 (d, J = 11.9 Hz,
1H, CH2Ph), 4.26 (q, J = 4.5 Hz, 1H, H-4), 4.00 (d, J = 4.5
Hz, 1H, H-3), 3.93 (dd, J = 12.1, 4.5 Hz, 1H, H-5a), 3.84
(m, 1H, H-5b), 2.13 (bs, 1H, 5-OH), 1.47 (s, 3H, CH3), 1.31
(s, 3H, CH3); 13C NMR (100 MHz, CDCl3) d 137.0 (C),
128.7 (CH), 128.2 (CH), 127.7 (CH), 111.8 (C), 105.1
(CH), 82.8 (CH), 82.5 (CH), 80.1 (CH), 71.9 (CH2), 61.0
(CH2), 26.8 (CH3), 26.3 (CH3); HRMS (FAB, [M+H]+)
calcd for C15H21O5 281.1389, found 281.1388.
2b: R = OBz
4
+
R
conditions
OTES
H
O
O
O
O
3a: R = H
3b: R = OBz
O
OBn
R
2c: R = H
2d: R = OBz
Product Yield
Entry Diene
Condition
(ratio)
(%)
1
2
3
3a
TiCl4, CH2Cl2, –78 °C, 1 h;
TFA, rt, 1 h
2a/2c
56
(3/1)
3a
3b
ZnCl2, THF, 0 °C to rt, 24 h; 2a/2c
TFA, rt, 1 h (4/3)
88
77
ZnCl2, THF, 0 °C to rt, 24 h; 2b/2d
TFA, rt, 1 h
1). The reaction requires a Lewis acid catalyst to promote
the cycloaddition and subsequent treatment with trifluoro-
acetic acid (TFA) leads to the elimination the triethylsilyl
(TES) and methoxy groups.5a In the present case, the endo-
adduct is more favored, but the Lewis acid catalyst exerts
an effect on the stereoselectivity. For example, TiCl4 al-
lowed for an endo/exo addition (2a/2c) ratio of 3/1 (entry
1), whereas ZnCl2 gave a corresponding 4/3 ratio (entry 2).
The yield of the reaction using ZnCl2, however, was much
higher (88%). When the benzoate 3b (the E/Z ratio was not
determined) was utilized as the diene, compounds 2b and
2d were furnished in a combined yield of 77% (entry 3).
Collectively, pyranone 2 possesses the proper set of func-
tional groups that may allow entry to the herbicidin tricylic
core.
CONCLUSION
The reactions used in this study provided a novel ap-
proach towards herbicidins. Herein, we successfully car-
ried out the first case of regioselective reductive ring open-
ing of 3,5-O-benzylidine acetal in a furanoside that favored
the primary alcohol formation. This product is a useful in-
termediate for the synthesis of an aldehyde dienophile that
enabled access to a suitable precursor to the undecose ring
system of herbicidins via hetero-Diels–Alder reaction. Fu-
ture undertakings will focus on further examinations on the
3-O-Benzyl-1,2-O-isopropylidene-a-D-xylo-dialdose (9)
Compound 8 (0.244 g, 0.872 mmol) was dissolved in
CH2Cl2 (1.78 mL). Water (1.33 mL) was added, followed
by NaHCO3 (22.0 mg, 1.74 mmol) and Bu4NHSO4 (29.6
mg, 0.087 mmol). The resulting mixture was cooled to 0
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°C. NaIO4 (0.373 g, 1.744 mmol) was added in portions for
over 30 min. After another 30 min, the mixture was allowed
to warm to room temperature and stirred for another 3 h.
The crude target material was extracted with CH2Cl2, and
the organic layer was concentrated and dissolved in Et2O.
The resulting solution was washed with water, dried over
MgSO4, filtered, and concentrated. The residue was puri-
fied by flash column chromatography (ethyl acetate/hex-
anes = 1/3) to give the aldehyde 9 (95% yield). [a]D25 –34.5
(c 1.02, CHCl3); IR (thin film) n 1735, 1450, 1372 cm–1; 1H
NMR (400 MHz, CDCl3) d 9.64 (d, J = 1.5 Hz, 1H, CHO),
7.30-7.24 (m, 5H, Ph-H), 6.11(d, J = 4.0 Hz, 1H, H-1), 4.64
(d, J = 4.0 Hz, 1H, H-2), 4.60 (d, J = 12.0 Hz, 1H, CH2Ph),
4.59 (d, J = 12.0 Hz, 1H, CH2Ph), 4.63-4.50 (m, 1H, H-4),
4.33 (d, J = 4.0 Hz, 1H, H-3), 1.45 (s, 3H, CH3), 1.30 (s,
3H, CH3).
lowed to stir for 2 h at 40 °C. Water was added and the
crude target material was extracted using ethyl acetate,
dried over MgSO4, filtered, and concentrated under re-
duced pressure. Purification of the residue by flash column
chromatography (ethyl acetate/hexanes = 1/4) gave the
6-alcohol 11 (5.79 g, 67%). [a]D20 –39.4 (c 2.0, CHCl3); IR
(thin film) n 3600-3200, 1640, 1450, 1380, 1210, 1050
1
cm–1; H NMR (400 MHz, CDCl3) d 7.31-7.26 (m, 5H),
5.89 (d, J = 4.0 Hz, 1H), 4.66, 4.46 (ABq, J = 12.4 Hz, 2H),
4.59 (d, J = 4.0 Hz, 1H), 4.32-4.28 (m, 1H), 3.79 (d, J = 2.8
Hz, 1H), 3.70 (t, J = 6.0 Hz, 2H), 2.46 (bs, 1H, OH), 2.07-
1.98 (m, 1H), 1.85-1.77 (m, 1H), 1.45 (s, 3H), 1.28 (s, 3H);
13C NMR (100 MHz, CDCl3) d 137.4 (C), 128.6 (CH),
128.0 (CH), 127.7 (CH), 111.5 (C), 104.8 (CH), 82.6 (CH),
82.2 (CH), 78.5 (CH), 71.8 (CH2), 60.3 (CH2), 31.0 (CH2),
26.7 (CH3), 26.2 (CH3); HRMS (EI, [M]+) calcd for
C16H22O5 294.1545, found 294.1549.
5,6-Dideoxy-3-O-benzyl-1,2-O-isopropylidene-a-D-xylo-
hex-5-eno-furanose (10)
5-C-Formyl-5-deoxy-3-O-benzyl-1,2-O-isopropylidene-
a-D-glucofuranose (4)
The Wittig reagent was generated by dropwise addi-
tion of n-BuLi (2.5 M solution in hexane, 0.5 mL, 1.25
mmol) to an ice-cold solution of CH3BrPPh3 (0.63 g, 1.56
mmol) in THF (6 mL). After 30 min, the Wittig reagent was
added dropwise to a solution of compound 9 (0.289 g, 1.04
mmol) in THF (3 mL) under nitrogen atmosphere. The re-
action was stirred at 0 °C for 3 h, then, saturated NH4Cl(aq)
was added to the reaction mixture. The crude target mate-
rial was extracted using CH2Cl2, dried over MgSO4, fil-
tered, and concentrated under reduced pressure. Purifica-
tion by flash column chromatography (ethyl acetate/hex-
DMSO (3.8 mL, 53.6 mmol) was added dropwise to a
solution of (COCl)2 (3.2 mL, 36.3 mmol) in CH2Cl2 (25
mL) at –78 °C. After 5 minutes, compound 11 (5.25 g, 17.9
mmol) dissolved in CH2Cl2 (15.0 mL) was added, and the
resulting mixture was stirred at the same temperature for 12
min. Subsequently, Et3N (12.5 mL, 89.1 mmol) was added
followed by warming to room temperature for a 1-h period.
Et2O was added to the reaction mixture and any solids that
formed were filtered through paper. The filtrate was con-
centrated to give a residue which was purified using flash
column chromatography (ethyl acetate/hexanes = 1/8) to
obtain the aldehyde 4 (4.81 g, 92%). [a]D20 +56.4 (c 5.0,
CHCl3); IR (thin film) n 2960, 2920, 1720, 1500, 1450,
1400, 1220 cm–1; 1H NMR (400 MHz, CDCl3) d 9.72 (d, J
=1.6 Hz, 1H), 7.33-7.23 (m, 5H), 5.88 (d, J = 2.8 Hz, 1H),
4.62, 4.40 (ABq, J = 12.0 Hz, 2H), 4.60-4.55 (m, 2H), 3.96
(d, J = 3.6 Hz, 1H), 2.84-2.73 (m, 2H), 1.46 (s, 3H), 1.29 (s,
3H); 13C NMR (100 MHz, CDCl3) d 199.8 (CH), 137.0 (C),
128.3 (CH), 127.7 (CH), 127.6 (CH), 111.5 (C), 104.5
(CH), 82.1 (CH), 82.0 (CH), 75.3 (CH), 71.8 (CH2), 42.4
(CH2), 26.6 (CH3), 26.0 (CH3); HRMS (EI, [M]+) calcd for
C16H20O5 292.1311, found 292.1287.
20
anes = 1/1) provided the alkene 10 (178 mg, 62%). [a]D
+200.4 (c 5.0, CHCl3); IR (thin film) n 2954, 1453, 1376,
1
1215, 1078, 1026 cm–1; H NMR (400 MHz, CDCl3) d
7.32-7.27 (m, 5H), 6.04-5.97 (m, 1H), 5.95 (d, J = 3.6 Hz,
1H), 5.42 (dd, J = 18.0, 2.8 Hz, 1H), 5.31 (dd, J = 10.4, 1.2
Hz, 1H), 4.70-4.48 (m, 4H), 3.87 (d, J = 3.2 Hz, 1H), 1.48
(s, 3H), 1.31 (s, 3H); 13C NMR (100 MHz, CDCl3) d 137.3
(C), 132.1 (CH), 128.1 (CH), 127.5 (CH), 127.2 (CH),
118.5 (CH2), 111.1 (C), 104.5 (CH), 83.1 (CH), 82.5 (CH),
81.2 (CH), 71.7 (CH2), 26.5 (CH3), 25.9 (CH3); HRMS (EI,
[M–CH3]+) calcd for C15H17O4 261.1361, found 261.1343.
5-Deoxy-3-O-benzyl-1,2-O-isopropylidene-a-D-gluco-
furanose (11)
[2-(3-O-Benzyl-1,2-O-isopropylidene-5-deoxy-a-D-xylo-
1,4-furanosyl)-2,3-dihydro-4H-pyran-4-one] (2a and
2c)
A 1 M solution of 9-BBN in THF (30 mmol, 30 mL)
was added dropwise to a solution of compound 10 (8.12 g,
29.4 mmol) in THF (80 mL) under nitrogen atmosphere.
After stirring at 0 °C for 6 h, a co-solution of H2O2 (35%,
7.6 mL) and NaOH (5.92 g, 148 mmol) was added and al-
The Danishefsky-type diene 3a (1.49 g, 8.66 mmol)
was added to the solution of the aldehyde 4 (1.15 g, 3.94
mmol) in THF (10 mL) at room temperature under nitrogen
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Toward the Synthesis of Herbicidins
atmosphere. The mixture was cooled to 0 °C, and a suspen-
sion of ZnCl2 (0.59 g, 4.34 mmol) in THF (15 mL) was
added. After stirring for 24 h at room temperature, TFA(0.3
mL, 3.93 mmol) was added to the reaction flask and was
stirred for another hour. Saturated NaHCO3(aq) was added
to neutralize the excess acid. The crude target material was
extracted with ethyl acetate, dried over MgSO4, filtered,
and concentrated under reduced pressure. Purification of
the residue using flash column chromatography (ethyl ace-
tate/hexanes = 1/8) yielded the pyranones 2a and 2c (1.25
g, 88%). IR (thin film) n 2942, 1732, 1677, 1595, 1269,
1075 cm–1; 1H NMR (400 MHz, CDCl3) d 7.35-7.24 (m,
6H), 5.88 (d, J = 3.6 Hz, 1H), 5.36 (d, J = 6.0 Hz, 1H), 4.69
(dd, J = 12.0, 5.2 Hz, 1H), 4.63-4.18 (m, 5H), 3.80 (d, J =
3.2 Hz, 1H), 2.52-2.21 (m, 2H), 2.05-1.80 (m, 2H), 1.46 (s,
3H), 1.23 (s, 3H); HRMS (EI, [M]+) calcd for C20H24O6
360.1338, found 360.1329.
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for 2a/2c furnished the products 2b and 2d (1.97 g, 77%
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ACKNOWLEDGEMENTS
This work was supported by the National Science
Council (NSC 97-2113-M-001-033-MY3, NSC 98-2119-
M-001-008-MY2, NSC 98-3112-B-007-005, NSC 98-
2627-B-007-008) and Academia Sinica.
J. Chin. Chem. Soc. 2012, 59, 421-425
© 2012 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.jccs.wiley-vch.de
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