Concise Syntheses of (-)- and (+)-Syringolide 1 and (-)-Δ7-Syringolide 1

Article abstract of DOI:10.1021/jo970461b

(-)- and (+)-Syringolide 1 have been synthesized from 2,3-O-isopropylidene-β-threo-pentulofuranose (1), which was readily prepared from D- or L-xylose, respectively. Condensation of 1 with 3-oxooctanoic acid and treatment of the ester with TFA:H₂O (9:1) produced syringolide 1 in 6.3% yield for the two steps. According to the same synthetic route by replacing 3-oxooctanoic acid with 3-oxo-7-octenoic acid, (-)-Δ⁷-syringolide 1 was prepared in 5.8% yield. Primary β-keto esters of arabinulose and fructose were also prepared to test the selectivity of the biomimetic cyclization to form syringolide analogs.

Full text of DOI:10.1021/jo970461b

4780
J . Org. Chem. 1997, 62, 4780-4784
Con cise Syn th eses of (-)- a n d (+)-Syr in golid e 1 a n d
(-)-∆7-Syr in golid e 1
Chun-min Zeng, Sharon L. Midland, Noel T. Keen, and J ames J . Sims*
Plant Pathology Department, University of California, Riverside, California 92521
Received March 13, 1997X
(-)- and (+)-Syringolide 1 have been synthesized from 2,3-O-isopropylidene-â-threo-pentulofuranose
(1), which was readily prepared from D- or L-xylose, respectively. Condensation of 1 with
3-oxooctanoic acid and treatment of the ester with TFA:H2O (9:1) produced syringolide 1 in 6.3%
yield for the two steps. According to the same synthetic route by replacing 3-oxooctanoic acid with
3-oxo-7-octenoic acid, (-)-∆7-syringolide 1 was prepared in 5.8% yield. Primary â-keto esters of
arabinulose and fructose were also prepared to test the selectivity of the biomimetic cyclization to
form syringolide analogs.
Sch em e 1
Communication in the microbial world is often carried
out by natural products. Signal molecules have been
commonly classified as toxins, antibiotics, and hormones.
The syringolides are compounds produced by the plant
pathogenic bacterium Pseudomonas syringae pv. tomato,
which are recognized by certain cultivars of soybean
plants.1 Recognition of the presence of a syringolide by
the plant is the first step of a complex active defense
reaction, the hypersensitive response. A single aviru-
lence gene, avrD, in the bacterium and a single resistance
gene, Rpg4, in the plant are both necessary and sufficient
for this response which leads to the inability of the
bacterium to successfully parasitize the plant.2 We
reported the isolation and structure determination of the
syringolides in 1993.3 Since then, these compounds have
been synthesized in low yield from tartaric acid deriva-
tives,4 D-xylose5 and substituted 2-butenolides.6
We proposed a biosynthesis which combined interme-
diates from the fatty acid biosynthetic pathway with the
well known bacterial metabolite D-xylulose.3 The bio-
synthesis was based on previous work by Rickards7a and
Yamada7b indicating that the Streptomyces autoregula-
tors were biosynthesized from esters formed from dihy-
droxyacetone and â-keto fatty acids and, furthermore,
that internal Knoevenagel condensation of a â-keto acyl
derivative of dihydroxyacetone took place readily. This
provides a pathway for the first two steps of a biomimetic
synthesis of syringolides. Thus, one has only to synthe-
size the 1-â-keto acyl ester of xylulose to test the
biomimetic chemistry. We report here synthesis of such
an ester and its conversion into syringolide 1 (4). We
also have extended this chemistry to prepare ∆7-syrin-
golide 1, permitting tritium-labeling for receptor studies.
Finally, we have explored the stereochemical require-
ments of the biomimetic syringolide cyclization with 1-â-
keto esters of arabinulose and fructose.
The synthesis is outlined in Scheme 1. Starting with
the inexpensive D-xylose, isomerization in pyridine fol-
lowed by ion exchange chromatography and reaction with
acetone gave the 2,3-acetonide of D-xylulose (1) in 20 g
batches.8 Esterification of 1 with hexanoyl Meldrum’s
acid9 in refluxing toluene gave primary ester 2, secondary
ester, and diester in 39.8%, 29.3%, and 28.9% yield,
respectively. The selectivity between the primary hy-
droxyl group and the secondary hydroxyl group was 1.4:
1. Acylation of 1 with 3-oxooctanoic acid in the presence
of dicyclohexylcarbodiimide improved the selectivity be-
tween the primary and secondary hydroxyls groups to
3.6:1 and afforded the primary ester 2, secondary ester,
and diester in 48.8%, 13.6% and 27.6% yield, respectively.
Treatment of 2 with aqueous trifluoroacetic acid gave (-)-
X Abstract published in Advance ACS Abstracts, J une 15, 1997.
(1) Keen, N. T.; Kobayashi, D.; Tamaki, S.; Shen, H.; Stayton, M.;
Lawrence, D.; Sharma, A.; Midland, S.; Smith, M.; Sims, J . J . In
Advances in Molecular Genetics of Plant-Microbe Interactions; Hen-
necke, H., Verma, D. P. S., Eds.; Kluwer: Dordrecht, Netherlands,
1991; Vol. 1, p 37.
(2) Keen, N. T. Annu. Rev. Genet. 1990, 24, 447.
(3) (a) Midland, S. L.; Keen N. T.; Sims, J . J .; Midland, M. M.;
Burton, V.; Smith, M. J .; Mazzola, E. P.; Graham, K. J .; Clardy, J . J .
Org. Chem. 1993, 58, 2940. (b) Smith, M. J .; Mazzola, E. P.; Sims, J .
J .; Midland, S. L.; Keen, N. T.; Burton, V.; Stayton, M. M. Tetrahedron
Lett. 1993, 34, 223.
(4) (a) Wood, J . L.; J eong, S.; Salcedo, A.; J enkins, J . J . Org. Chem.
1995, 60, 286. (b) Kuwahara, S.; Moriguchi, M.; Miyakawa, K.; Konno,
M.; Kodama, O. Tetrahedron Lett. 1995, 36, 3201. (c) Kuwahara, S.;
Moriguchi, M.; Miyakawa, K.; Konno, M.; Kodama, O. Tetrahedron
1995, 51, 8809.
(5) Henschke, J . P.; Rickards, R. W. Tetrahedron Lett. 1996, 37,
3557.
(6) (a) Ishihara, J .; Sugimoto, T.; and Murai, A. Synlett 1996, 335.
(b) Honda, T.; Mizutani, H.; and Kanai, K. J . Org. Chem. 1996, 61,
9374.
(7) (a) Rickards, R. W. Proc. Asian Symp. Med. Plants Spices, 6th
1989, 91. (b) Sakuda, S.; Higashi, A.; Tanaka, S.; Nihira, T.; Yamada,
Y. J . Am. Chem. Soc. 1992, 114, 663.
(8) Tipson, R. S.; Brady, R. F., J r. Carbohyd. Res. 1969, 10, 549.
(9) Oikawa, Y.; Sugano, K.; Yonemitsu, O. J . Org. Chem. 1978, 43,
2087.
S0022-3263(97)00461-1 CCC: $14.00 © 1997 American Chemical Society

Concise Syntheses of Syringolides
J . Org. Chem., Vol. 62, No. 14, 1997 4781
Sch em e 2
Sch em e 3
H2O (9:1), TFA-CH2Cl2, Py-DMF for the cyclization,
finally, we found that when 9 was treated with 4% TFA
in CH3OH, the cyclized product, 4′-O-benzyl syringolide
1 (10) was obtained in 23.3% yield. It was hydrogeno-
lyzed in the presence of 10% Pd/C catalyst to give (-)-
syringolide 1 in quantitative yield. Alternatively, treat-
ment of 8 with TiCl4 at -30 °C cleaved both the acetonide
and the benzyl ether to produce the ester 3 in 80.4% yield.
Because of the easy availability of D-arabinose and
D-fructose, we explored the behavior of the arabinose-
derived ester 15, which has opposite configuration from
3 at C-3, and fructose-derived ester 19, which could form
a larger ring with the same configuration as 3 at C-3 to
test the stereochemical requirements for syringolide
formation and activity. Ester 15 was prepared (Scheme
3) from 11, the diacetonide of D-arabinulose.8 Treatment
of 11 with benzyl alcohol in the presence of catalytic
amounts of trifluoromethanesulfonic acid selectively
opened the 1,2-acetonide to give 12,11 which reacted with
hexanoyl Meldrum’s acid (7) in refluxing toluene to afford
primary â-keto ester 13. Hydrogenolysis of 13 to form
14, followed by treatment with aqueous trifluoroacetic
acid produced ester 15. Following subjection of 15 to
water of varying acidity from pH 4 to 9, only one cyclized
product, 16, was found. Ketal 16 was isomeric with 5.
Apparently, the stereochemistry at C-3 of xylulose is very
important for the formation of syringolides.
â-Keto ester 19 was prepared from 17, the diacetonide
of D-fructose.12 Esterification of 17 with 7 gave 18, which
was deprotected with TiCl4 at -30 °C to give ester 19.
Unfortunately, no cyclization occurred when ester 19 was
treated with water at different pH values. From this
preliminary work we can conclude that cyclization of 19,
derived from a six-carbon sugar, is more difficult than
cyclization of 3, derived from a five-carbon sugar.
In order to prepare tritium-labeled syringolides for the
identification and purification of the syringolide receptor,
we also synthesized (-)-∆7-syringolide 1 (21), a double
bond analogue of 4, according to the synthetic route
shown in Scheme 1. Condensation of D-xylulose ac-
etonide (1) with 3-oxo-7-octenoic acid in the presence of
dicyclohexylcarbodiimide afforded ester 20 in 47.9% yield.
Deprotection of 20 with aqueous TFA and cyclization
with water gave 21 in 12.2% yield. Compound 21 was
active in soybean leaves. It was quantitatively converted
to syringolide 1 (4) by hydrogenation in the presence of
syringolide 1 (4) in 13.0% yield, which was identical to
previously isolated material by comparison of NMR
spectral data and biological activity. In our attempts to
improve the yield of syringolide, we found that treatment
of 2 with titanium tetrachloride at low temperature
selectively removed the acetonide protecting group yield-
ing the ester 3. This ester had no biological activity;
NMR spectra of 3 showed it to be a mixture of cyclic sugar
forms. To simulate biosynthetic conditions of condensa-
tion and cyclization, we treated 3 with water of varying
acidity from pH 4 to 9. The best result from these
experiments provided the naturally occurring (-)-4 after
treatment at pH 6 for a week in 11.6% yield.
By replacing D-xylose with L-xylose, unnatural (+)-
syringolide 1 (4) was similarly synthesized. It was
1
identical to (-)-4 by H and 13C NMR. The rotations of
the synthetic (+) and (-) enantiomers were +74.7 and
-70.0, respectively. But unexpectedly, both enantiomers
of 4 showed the same specific activity as elicitors of the
hypersensitive response on soybean plants harboring the
Rpg4 gene.
Cyclic ketal 5 was a byproduct found in the acidic
syringolide cyclization reactions. We also have isolated
5 from cultured syringolide preparations which were
acidified to pH 2 before extraction. And we have ob-
served steady conversion of 4 to 5 in 10% MeOH/CHCl3.
The structure of 5 was fully elucidated by 1H and 13C
NMR, assigned by 1H-1H and 1H-13C COSY, INAPT
1
(selective long range H-13C spectra), and the secondary
deuterium isotope shift of the 13C peak which correlated
to its allylic alcohol methine.
To improve the preparation of primary ester, D-xylulose
acetonide (1) was treated with trityl chloride to selectively
afford the 1-trityl derivative, which was converted to its
secondary benzyl ether and then detritylated with acid
to give 6 in 77.0% yield10 (Scheme 2). Acylation of 6 with
hexanoyl Meldrum’s acid (7) in refluxing toluene gave
primary â-keto ester 8 in 97.8% yield.9 Treatment of 8
with TFA-H2O (9:1) at room temperature yielded the
deacetonized product 9 in 80.0% yield. After trying
various reagents including SnCl4-CH2Cl2, SiO2, AcOH-
(11) Nakajima, N.; Matsumoto, M.; Kirihara, M.; Hashimoto, M.;
Katoh, T.; Terashima, S. Tetrahedron 1996, 52, 1177.
(12) Brady, R. F., J r. Carbohyd. Res. 1970, 15, 35.
(10) Bernotas, R. C.; Pezzone, M. A.; Ganem, B. Carbohyd. Res.
1987, 167, 305.

4782 J . Org. Chem., Vol. 62, No. 14, 1997
Zeng et al.
) 7.4 Hz), 1.57∼1.62 (2H, m), 1.50 (3H, s), 1.36 (3H, s), 1.26-
1.32 (4H, b), 0.89 (3H, t, J ) 7.0 Hz); 13C NMR (CDCl3) δ 203.4,
166.4, 112.3, 112.2, 85.8, 75.5, 74.1, 65.0, 48.9, 43.5, 31.1, 27.2,
26.3, 23.1, 22.4, 13.9; DCIMS (NH3) m/ z (intensity, %), 348
(M + NH4+, 69.4), 331 (M + H+, 17.0), 315 (23.7), 273 (100);
HRMS calcd for C16H27O7 331.1757, found 331.1749. Secon d -
a r y ester : 1H NMR (CDCl3) δ 5.13 (1H, d, J ) 2.8 Hz), 4.51
(1H, s), 4.21 (1H, dd, J ) 10.2, 2.8 Hz), 3.98 (1H, d, J ) 10.2
Hz), 3.66 (2H, b), 3.42 (2H, s), 2.44 (2H, t, J ) 7.4 Hz),
1.50∼1.54 (2H, m), 1.45 (3H, s), 1.31 (3H, s), 1.22∼1.23 (4H,
b), 0.82 (3H, t, J ) 6.7 Hz); 13C NMR (CDCl3) δ 202.8, 166.3,
114.7, 112.4, 82.8, 77.9, 71.0, 63.1, 48.8, 43.2, 31.0, 27.3, 26.6,
23.0, 22.3, 13.8.
1-(3-Oxoocta n oyl)-2,3-O-isop r op ylid en e-â-D-th r eo-p en -
tu lofu r a n ose (2). To a stirred solution of 3-oxooctanoic acid13
(44.0 mg, 0.28 mmol) in 5 mL of anhydrous CH2Cl2 at 0 °C
were added DMAP (6.0 mg), DCC (108.0 mg, 0.52 mmol), and
1 (53.1 mg, 0.28 mmol), and the mixture was then stirred at
0 °C for 3 h. Precipitated urea was filtered off and the filtrate
evaporated in vacuo. The residue was taken up in ethyl
acetate and, if necessary, filtered free of any further precipi-
tated urea. The solvent was removed by evaporation, and the
residue was separated by vacuum flash chromatography
washed with 5% acetone in chloroform to yield primary ester
2 (29.5 mg, 48.8%), secondary ester (8.2 mg, 13.6%), diester
(23.8 mg, 27.6%), and recovered 1 (18.3 mg).
Pd/C catalyst; thus, it should be readily reduced with
tritium by the same method to afford a biologically active
tritiated syringolide for receptor studies.
1-(3-Oxoocta n oyl)-D-th r eo-p en tu lose (3). To a solution
of 50.3 mg (0.15 mmol) of 2 in 5 mL of anhydrous CH2Cl2 was
added 0.045 mL (2.5 equiv) of TiCl4 at -30 °C under argon.
The reaction mixture was stirred for 5 h and quenched by
adding saturated aqueous NH4Cl solution. The product was
extracted with ethyl acetate and purified by passing through
a silica Sep-Pak washed with ethyl acetate to afford 3 as a
mixture of R and â cyclized forms (37.1 mg, 84.0%). IR(film)
In conclusion, we have provided a biomimetic method
for the synthesis of syringolides. We have applied this
method to synthesis of (-)- and (+)-syringolide 1 and (-)-
∆7-syringolide 1 and found that each of these compounds
is active as an elicitor of the hypersensitive response. We
have also tested the stereochemical requirements of the
syringolide cyclization reaction and found that the threo
five-carbon sugar ester is favored for biomimetic cycliza-
tion.
νmax 3350, 2950, 1760, 1730, 1070 cm-1 1H NMR (CDCl3) δ
;
5.0-5.2 (m), 4.73 (1H, d, J ) 11.8 Hz), 4.54 (1H, d, J ) 11.8
Hz), 4.3-4.4 (m), 4.15-4.25 (m), 4.0-4.1 (m), 3.72 (dd, J )
8.6, 3.0 Hz), 3.53 (d, J ) 16.6 Hz), 3.47 (d, J ) 16.6 Hz), 3.6
(s), 2.8 (bs), 2.55 (t, J ) 7.4 Hz), 1.6 (bm), 1.3 (bs), 0.9 (bt); 13
C
Exp er im en ta l Section
NMR (CDCl3) δ 205.0, 167.5, 167.0, 105.5, 101.4, 79.7, 785,
76.7, 76.2, 74.8, 70.8, 66.5, 65.2, 49.0, 48.9, 43.4, 43.3, 31.1,
23.1, 22.4, 13.9; DCIMS (NH3) m/ z (intensity, %), 308 (M +
NH4+, 60.9), 290 (M+, 67.2), 273 (100), 255 (21.9), 230 (77.5);
HRMS calcd for C13H26NO7 308.1709, found 308.1715.
IR spectra were measured as films for liquids or KBr discs
1
for solids. NMR experiments were conducted at 300 MHz H
or 75.5 MHz 13C, and chemical shifts are relative to residual
protonated solvent shifts. Pyridine was dried by distillation
from KOH, and CH2Cl2 was distilled from CaH2. All solvents
were distilled before use for chromatography. Merck Kieselgel
60H No. 7736 silica gel was used for vacuum flash chroma-
tography. Small samples were prepared using Waters silica
gel Sep-Paks and, when required, HPLC on a Phenomenex
Maxsil 5 (250 × 4.6 mm) silica column. Larger HPLC samples
were purified using a Rainin Dynamax (25 × 2.54 cm) silica
column.
(-)-Syr in golid e 1 (4). The ester 2 (46.5 mg, 0.14 mmol)
was stirred in 1 mL of TFA-H2O (9:1) solution at room
temperature for 2 h. Then, the reaction mixture was neutral-
ized with saturated aqueous NaHCO3 and extracted with ethyl
acetate. The extracts were dried over Na2SO4, filtered, and
concentrated. After passing through a silica Sep-Pak washed
with ethyl acetate, the residues were purified by HPLC using
40% ethyl acetate in hexane on a Maxsil 5 silica gel column to
yield (-)-4 (5.0 mg, 13.0%). [R]25 ) -70.0 (c 0.25, CHCl3),
D
2,3-O-Isop r op ylid en e-â-D-th r eo-p en t u lofu r a n ose (1).
Compound 1 was prepared by isomerization of D-xylose in
pyridine followed by chromatography using 75% n-PrOH as
eluant on Dowex 1-X8 in bisulfite form and reaction of the
dried xylulose with acetone.8 1: 1H NMR (DMSO-d6) δ 5.08
(2H, m), 4.24 (1H, s), 4.03 (1H, ddd, J ) 4.3, 2,8, 1.0 Hz), 3.95
(1H, dd, J ) 9.5, 2.8 Hz), 3.67 (1H, dd, J ) 9.5, 1.0 Hz), 3.51
(1H, dd, J ) 11.8, 5.7 Hz), 3.43 (1H, dd, J ) 11.8, 6.4 Hz),
1.34 (3H, s), 1.26 (3H, s); 13C NMR (DMSO-d6) δ 115.4, 111.3,
84.8, 74.4, 73.6, 62.3, 28.0, 27.1.
{lit. [R]24D ) -83.66 (c 0.15, CHCl3)}; 1H and 13C NMR spectral
data were identical with those of isolated (-)-syringolide 1.3a
As an alternate preparation of 4, ester 3 (22.9 mg, 0.08
mmol) was mixed with 20 mL water (pH ) 6). The mixture
was stirred at room temperature for one week and extracted
with ethyl acetate. The extracts were dried over Na2SO4,
filtered, and evaporated. The residues were purified by HPLC
to give (-)-4 (2.5 mg, 11.6%).
(1R ,7R ,8R )-7-H y d r o x y -4,10,11-t r io x a -1-p e n t y lt r i-
cyclo[6.2.1.02,6]u n d ec-2(6)-en -3-on e (5). Tricycloketal 5 was
also formed as a byproduct of acid treatment of syringolide
preparations. It was purified from culture fluid or synthetic
preparations by HPLC. With hexane-ethyl acetate-2-pro-
panol (45.3:45.3:9.4) as eluant on the Dynamax silica column,
1-(3-Oxoocta n oyl)-2,3-O-isop r op ylid en e-â-D-th r eo-p en -
tu lofu r a n ose (2). Hexanoyl Meldrum’s acid (7) was prepared
by stirring equimolar amounts of n-hexanoyl chloride and 2,2-
dimethyl-1,3-dioxane-4,6-dione in CH2Cl2 at 0 °C overnight.9
Acetonide 1 (79.8 mg, 0.42 mmol) was esterified by stirring
with 7 (116.0 mg, 0.48 mmol) in refluxing toluene (10 mL) for
2.5 h. Then the solvent was removed in vacuo, and the residue
was purified by vacuum flash chromatography with 5% acetone
in chloroform to yield primary ester 2 (43.0 mg, 39.8%),
secondary ester (31.6 mg, 29.3%), diester (44.5 mg, 28.9%), and
5 had a retention time of 35 min (µ ) 6 mL/min). [R]25
)
D
-22.31 (c 1.34, CH2Cl2); IR(film) νmax 3450, 2950, 2880, 1750,
1670, 1440, 1340, 1250, 1180, 1090, 1030, 995, 940, 910, 780,
1
740 cm-1; H NMR (CDCl3) δ 5.1 (1H, d, J ) 4 Hz), 5.0 (1H,
dd, J ) 18, 0.6 Hz), 4.8 (1H, dd, J ) 18, 1.5 Hz), 4.6 (1H, ddd,
J ) 5.8, 4.5, 2.5 Hz), 4.1 (1H, m, J ) 8.8, 2.5 Hz), 4.05 (1H, m,
recovered 1 (17.6 mg). 2: [R]25 ) +3.0 (c 1.09, CH2Cl2);
D
IR(film) νmax 3500, 2950, 1760, 1730, 1200, 1070 cm-1; 1H NMR
(CDCl3) δ 4.46 (1H, s), 4.44 (2H, m), 4.26 (1H, d, J ) 2.8 Hz),
4.18 (1H, dd, J ) 10.0, 2.8 Hz), 3.98 (1H, d, J ) 10.0 Hz), 3.56
(1H, d, J ) 15.7 Hz), 3.49 (1H, d, J ) 15.7 Hz), 2.54 (2H, t, J
(13) 3-Oxooctanoic acid was prepared by refluxing hexanoyl Mel-
drum’s acid (7) with methanol and saponification of the methyl ester
with KOH.

Concise Syntheses of Syringolides
J . Org. Chem., Vol. 62, No. 14, 1997 4783
J ) 8.8, 5.9, 1.3 Hz), 2.35 (1H, m), 2.1 (1H, m), 1.4 (2H, m),
1.3 (4H, m), 0.8 (3H, t, J ) 7 Hz); 13C NMR (CDCl3) δ 169.9,
163.0, 128.9, 104.2, 75.4, 69.0, 66.7, 63.9, 31.8, 31.1, 22.6, 22.5,
13.9; DCIMS (NH3) m/ z (intensity, %), 272(MNH4+, 31), 256
(15), 255 (MH+, 100), 254 (11), 237 (5), 236 (4), 207 (5); HRMS
calcd for C13H19O5 255.123249, found 255.1227.
mixing with 3 mL of TFA-H2O (9:1) and stirring at rt for 2 h.
Then the reaction mixture was neutralized with saturated
aqueous NaHCO3 and extracted with ethyl acetate. The
extracts were dried over Na2SO4, filtered, and evaporated. The
residue was purified by vacuum flash chromatography using
40% ethyl acetate in hexane to elute 9 as a mixture of R and
â cyclized forms (42.7 mg, 80.0%). IR(KBr) νmax 3300, 2950,
(+)-Syr in golid e 1. Using L-xylose to replace D-xylose, (+)-
1760, 1720, 1070 cm-1 1H NMR (CDCl3) δ 7.3-7.4 (m), 5.1
;
syringolide 1 was also synthesized. [R]25 ) +74.7 (c 0.20,
D
CHCl3), 1H and 13C NMR spectral data were identical with
those of isolated (-)-syringolide 1.
(m), 4.9 (m), 4.72 (d, J ) 11.7 Hz), 4.64 (s), 4.57 (d, J ) 11.7
Hz), 4.2 (m), 4.1 (m), 4.04 (d, J ) 11.4 Hz), 3.84 (dd, J ) 8.6,
2.9 Hz), 3.5-3.7 (m), 2.49 (t, J ) 7.3 Hz), 1.55 (m), 1.27 (m),
0.9 (t, J ) 6.9 Hz); 13C NMR (CDCl3) δ 204.7, 167.6, 166.9,
137.6, 128.6, 128.5, 127.9, 127.8, 105.6, 101.6, 83.5, 83.3, 77.6,
2,3-O-Isopr opyliden e-4-O-ben zyl-â-D-th r eo-pen tu lofu r a-
n ose (6). A solution of 1 (2.1435 g, 11.3 mmol) in 70 mL of
anhydrous pyridine was dried by distilling off 40% of the
solvent. After cooling the solution to room temperature, trityl
chloride (3.1561 g, 11.3 mmol) was added, and the mixture
was stirred at room temperature for three days based on TLC
monitoring and then diluted with saturated aqueous NaHCO3
solution (20 mL). Then the pyridine was removed at 2 mmHg,
and the aqueous layer was extracted with ethyl acetate (3 ×
20 mL). The combined extracts were dried over anhydrous
sodium sulfate and evaporated in vacuo to afford the oily
monotrityl derivative, which was filtered through silica gel,
washed with 10% and 50% ethyl acetate in hexane to give
fraction one containing the primary trityl ether, and fraction
two containing the recovered starting material (306.9 mg),
respectively. Fraction one was dissolved in DMF (15 mL) and
added over 15 min to a solution of NaH (874.0 mg, 36.4 mmol)
in 50 mL of DMF at 0 °C. After 1 h, benzyl chloride (2.6 mL,
22.6 mmol) was added. One hour later, the mixture was
warmed to room temperature and stirred overnight and then
treated with sufficient acetic acid to decompose the excess of
NaH, diluted with 150 mL of water, and extracted with ethyl
acetate (3 × 30 mL). The extracts were combined, dried, and
evaporated to give the 4-O-benzyl-1-O-trityl derivative, which
was dissolved in a mixture of 40 mL of CHCl3, 20 mL of MeOH,
and 5 mL of 2 N HCl. After stirring at room temperature for
3 h, the mixture was basified with saturated aqueous NaHCO3,
organic solvents were evaporated, and the aqueous residues
were extracted with ethyl acetate (3 × 20 mL). The extracts
were combined, dried over Na2SO4, evaporated in vacuo, and
purified by vacuum flash chromatography with 5% ethyl
77.3, 71.9, 71.7, 69.6, 66.6, 65.1, 49.1, 49.0, 43.3, 31.1, 23.1,
+
22.4. 13.9; DCIMS (NH3) m/ z (intensity, %), 398 (M + NH4
,
30.1), 380 (M + NH4+ - H2O, 34.6), 363 (40.9), 240 (100), 132
(44.3), 108 (81.1), 91 (53.4); HRMS calcd for C20H30NO6
380.2073, found 380.2073.
4′-O-Ben zylsyr in golid e 1 (10). The cyclization of ester 9
(23.9 mg, 0.06 mmol) was accomplished by stirring with 5 mL
of absolute CH3OH and 0.2 mL of TFA at room temperature
for three days. Then, the reaction mixture was neutralized
with saturated aqueous NaHCO3 solution and extracted with
ethyl acetate. The extracts were dried over Na2SO4, filtered,
and evaporated. The residues were purified by HPLC using
12% ethyl acetate in hexane on a Maxsil 5 silica gel column to
give 10 (5.3 mg, 23.3%). [R]25 ) -52.1 (c 1.06, CH2Cl2);
D
IR(KBr) νmax 3350, 3010, 2950, 1760, 1070 cm-1
;
1H NMR
(CDCl3) δ 7.30-7.39 (5H, m), 4.69 (1H, d, J ) 10.3 Hz), 4.67
(1H, s), 4.58 (2H, s), 4.45 (1H, d, J ) 10.3 Hz), 4.19 (1H, d, J
) 10.5 Hz), 4.02 (1H, d, J ) 2.9 Hz), 3.76 (1H, dd, J ) 10.5,
2.9 Hz), 3.09 (1H, s), 1.91 (2H, t, J ) 6.9 Hz), 1.40-1.55 (2H,
m), 1.26∼1.34 (4H, bm), 0.90 (3H, t, J ) 6.8 Hz); 13C NMR
(CDCl3) δ 172.3, 137.6, 128.6, 128.0, 127.6, 108.3, 97.7, 89.3,
81.3, 74.8, 71.8, 71.2, 59.2, 38.9, 31.6, 23.2, 22.5, 14.0; FABMS
(NBA) m/ z (intensity, %), 363 (M + H+, 20.9), 345 (100), 255
(19.9); HRMS calcd for C20H27O6 363.1808, found 363.1787.
(-)-Syr in golid e 1 (4). To a solution of 10 (5.3 mg) in 2
mL of ethyl acetate was added 10% Pd/C catalyst (2.0 mg).
The mixture was stirred under H2 atmosphere at rt for 3 h.
Then, the catalyst was removed by filtration, and the solvent
was evaporated to afford 4 in almost quantative yield. Syn-
thetic and isolated (-)-syringolide 1 samples had identical 1H
and 13C NMR spectra.3a
1-(3-Oxoocta n oyl)-D-th r eo-p en tu lose (3). To a solution
of 8 (95.5 mg, 0.23 mmol) in 5 mL of anhyd CH2Cl2 at -30 °C
was added TiCl4 (63 mL, 2.5 equiv) under argon. The reaction
was stirred 5 h at this temperature and then quenched with
saturated aqueous NH4Cl. The product was extracted with
ethyl acetate, concentrated, and passed through a silica Sep-
Pak using ethyl acetate as eluant to give deprotected primary
ester 3 (53.0 mg, 80.4%).
acetate in hexane to afford 6 (2.0833 g, 77.0% from 1). [R]25
D
) -13.4 (c 1.79, CH2Cl2); IR(film) νmax 3500, 2900, 1500, 1450,
1350, 1200, 1070 cm-1; 1H NMR (CDCl3) δ 7.31-7.38 (5H, b),
4.62 (1H, d, J ) 11.9 Hz), 4.62 (1H, s), 4.55 (1H, d, J ) 11.9
Hz), 4.15 (1H, dd, J ) 10.1, 2.6 Hz), 4.10 (1H, d, J ) 10.1 Hz),
4.06 (1H, d, J ) 2.6 Hz), 3.82 (1H, d, J ) 11.9 Hz), 3.76 (1H,
d, J ) 11.9 Hz), 2.10 (1H, bs), 1.51 (3H, s), 1.39 (3H, s); 13C
NMR (CDCl3) δ 137.3, 128.5, 127.9, 127.7, 114.7, 112.1, 82.6,
81.8, 71.4, 71.1, 63.5, 27.5, 26.7. DCIMS (NH3) m/ z (intensity,
%), 298 (M + NH4+, 10.5), 281 (M + H+, 33.6), 266 (9.9), 223
(17.8), 91 (100); HRMS calcd for C15H24NO5 298.1654 , found
298.1668.
Ben zyl 3, 4-Isop r op ylid en e-â-D-er yth r o-p en tu lofu r a n o-
sid e (12). Diacetonide 11 was prepared by pyridine isomer-
ization of D-arabinose followed by bisulfite ion exchange
chromatography and acetonization.8 A solution of 11 (798.5
mg, 3.47 mmol) and TfOH (50 mL, 0.57 mmol) in benzyl alcohol
(5.5 mL) was stirred at 0 °C for 5 h. The reaction was made
basic with saturated aqueous NaHCO3 at the same tempera-
ture and extracted with ethyl acetate. The extracts were
concentrated and evacuated to 2 mmHg to remove unreacted
benzyl alcohol. The residue was then purified by vacuum flash
chromatography using 10% ethyl acetate in hexane to afford
1-(3-Oxoocta n oyl)-2,3-O-isop r op ylid en e-4-O-ben zyl-â-
D-th r eo-p en tu lofu r a n ose (8). Hexanoyl Meldrum’s acid (7)
was prepared by stirring equimolar amounts of n-hexanoyl
chloride and 2,2-dimethyl-1,3-dioxane-4,6-dione in CH2Cl2 at
0 °C overnight.9 The protected alcohol 6 (203.4 mg, 0.73 mmol)
was refluxed with 7 (248.9 mg, 1.03 mmol) in 10 mL of toluene
for 2 h. Then the solvent was removed, and the residue was
purified by vacuum flash chromatography using 5% ethyl
acetate in hexane to give ester 8 (297.8 mg, 97.6%). [R]25
)
D
-15.6 (c 1.69, CH2Cl2); IR(film) νmax 3010, 2950, 1760, 1720,
1450, 1350, 1200, 1070 cm-1 1H NMR (CDCl3) δ 7.31-7.38
;
benzyl ketal 12 (590.1 mg, 60.7%). [R]25 ) -53.9 (c 1.86,
D
CHCl3); IR(film) νmax 3500, 3100, 1490, 1380, 1080 cm-1; H
1
(5H, m), 4.59 (1H, s), 4.56 (2H, m), 4.50 (1H, d, J ) 11.9 Hz),
4.31 (1H, d, J ) 11.9 Hz), 4.14 (1H, dd, J ) 10.1, 2.6 Hz), 4.10
(1H, d, J ) 10.1 Hz), 4.05 (1H, d, J ) 2.6 Hz), 3.38 (1H, d, J
) 15.7 Hz), 3.31 (1H, d, J ) 15.7 Hz), 2.45 (2H, t, J ) 7.4 Hz),
1.52∼1.60 (2H, m), 1.50 (3H, s), 1.37 (3H, s), 1.23∼1.35 (4H,
b), 0.89 (3H, t, J ) 7.1 Hz); 13C NMR (CDCl3) δ 202.6, 166.7,
137.4, 128.5, 128.0, 127.8, 112.8, 112.3, 83.0, 82.0, 71.6, 71.3,
64.2, 48.9, 43.0, 31.2, 27.5, 26.5, 23.1, 22.4, 14.0; DCIMS (NH3)
m/ z (intensity, %), 438 (M + NH4+, 100), 405 (4.7), 363 (27.7),
108 (9.5), 91 (20.2); HRMS calcd for C23H36NO7 438.2492, found
438.2519.
NMR (CDCl3) δ 7.37-7.33 (5H, m), 4.87 (1H, dd, J ) 5.9, 3.7
Hz), 4.62 (1H, d, J ) 5.9 Hz), 4.57 (1H, d, J ) 13.0 Hz), 4.56
(1H, d, J ) 13.0 Hz), 4.06 (1H, d, J ) 10.4 Hz), 3.95 (1H, d, J
) 12.3 Hz), 3.88 (1H, d, J ) 12.3 Hz), 3.87 (1H, dd, J ) 10.4,
3.7 Hz), 1.52 (3H, s), 1.34 (3H, s); 13C NMR (CDCl3) δ 138.1,
128.5, 127.7, 126.9, 112.7, 109.2, 84.8, 80.3, 71.7, 63.2, 59.5,
+
26.2, 24.6; DCIMS (NH3) m/ z (intensity, %), 298 (M + NH4
,
+
10.7), 280 (M + NH4 - H2O, 3.0), 190 (100), 173 (26.2), 131
(20.9), 91 (30.4); HRMS calcd for C15H24NO5 298.1654, found
298.1656.
1-(3-Oxooctan oyl)-4-O-ben zyl-D-th r eo-pen tu lose (9). The
acetonide of ester 8 (58.0 mg , 0.14 mmol) was hydrolyzed by
Ben zyl 1-(3-Oxoocta n oyl)-3,4-isop r op ylid en e-â-D-er yth -
r o-p en tu lofu r a n osid e (13). A mixture of 12 (427.4 mg, 1.53

4784 J . Org. Chem., Vol. 62, No. 14, 1997
Zeng et al.
mmol) and hexanoyl Meldrum’s acid (7, 480.8 mg, 1.99 mmol)
was refluxed in 15 mL of toluene overnight, and then the
solvent was removed and the residue was purified by vacuum
flash chromatography using 20% ethyl acetate in hexane as
1.53 (3H, s), 1.47 (3H, s), 1.37 (3H, s), 1.33 (3H, s), 1.29 (4H,
m), 0.88 (3H, t, J ) 7.1 Hz); 13C NMR (CDCl3) δ 202.4, 166.6,
109.1, 108.8, 101.3, 70.7, 70.5, 70.0, 66.0, 61.3, 49.1, 42.9, 31.1,
26.5, 25.9, 25.2, 24.1, 23.1, 22.4, 13.9; DCIMS (NH3) m/ z
(intensity, %), 418 (M + NH4+, 100), 401 (M + H+, 13.8), 385
(31.8), 360 (8.6), 343 (26.4), 278 (10.4); HRMS calcd for
C20H33O8 401.2175, found 401.2185.
eluant to give ester 13 (505.6 mg, 78.9%). [R]25 ) -47.1 (c
D
1.22, CHCl3); IR(KBr) νmax 3100, 2900, 1760, 1720, 1490, 1080
cm-1; 1H NMR (CDCl3) δ 7.37-7.28 (5H, m), 4.87 (1H, dd, J )
3.8, 5.5Hz), 4.75 (1H, d, J ) 11.9 Hz), 4.53 (3H, m), 4.25 (1H,
d, J ) 11.9 Hz), 4.02 (1H, d, J ) 10.3 Hz), 3.88 (1H, dd, J )
3.8, 10.3 Hz), 3.48 (2H, s), 2.53 (2H, t, J ) 7.4 Hz), 1.57 (2H,
m), 1.50 (3H, s), 1.33 (3H, s), 1.27 (4H, m), 0.88 (3H, t, J ) 7.1
Hz); 13C NMR (CDCl3) δ 202.5, 166.8, 137.7, 128.4, 127.8,
127.7, 112.7, 107.6, 84.6, 80.3, 71.9, 63.5, 60.1, 49.1, 42.8, 31.2,
26.3, 24.9, 23.1, 22.4, 13.9; DCIMS (NH3) m/ z (intensity, %),
438 (M + NH4+, 32.6), 313 (100), 271 (13.8), 91 (20.5); HRMS
calcd for C23H36NO7 438.2492, found 438.2500.
1-(3-Oxo-7-oct en oyl)-2,3-O-isop r op ylid en e-â-D-th r eo-
p en tu lofu r a n ose (20). To a solution of 1 (219.0 mg, 1.15
mmol), 3-oxooctenoic acid (152.2 mg, 0.98 mmol),14 and DMAP
(31.5 mg, 0.26 mmol) in 50 mL of anhyd CH2Cl2 was added
DCC (305.2 mg, 1.48 mmol) at 0 °C. The reaction mixture
was stirred at 0 °C for 3 h. Precipitated urea was filtered off,
and the filtrate was evaporated to dryness. The residue was
taken up in ethyl acetate and filtered free of any further
precipitated urea. The solvent was removed by evaporation,
and the residue was purified by vacuum flash chromatography
using 5% acetone in chloroform to yield the desired primary
monoester 20 (104.2 mg , 47.9%), secondary monoester (35.0
mg, 16.1%), diester (66.7 mg, 21.6%), and recovered 1 (93.0
1-(3-Oxoocta n oyl)-3,4-isop r op ylid en e-D-er yth r o-p en tu -
lofu r a n ose (14). To a solution of 13 (111.9 mg) in 5 mL of
ethyl acetate was added 10% Pd/C catalyst (40.3 mg). The
mixture was stirred under H2 atmosphere at rt overnight.
Then the catalyst was removed by filtration, and the solvent
was evaporated to give 14 in almost quantative yield. IR(KBr)
mg). 20: [R]25 ) +2.2 (c 1.11, CH2Cl2); IR(film) νmax 3500,
D
2950, 1780, 1730, 1200, 1070 cm-1; H NMR (CDCl3) δ 5.74
1
νmax 3500, 2950, 1760, 1720, 1080 cm-1 1H NMR (CDCl3) δ
;
(1H, m), 5.01 (2H, m), 4.46 (1H, s), 4.46 (1H, d, J ) 11.8 Hz),
4.41 (1H, d, J ) 11.8 Hz), 4.26 (1H, d, J ) 2.7 Hz), 4.19 (1H,
dd, J ) 9.4, 2.7 Hz), 3.98 (1H, d, J ) 9.4 Hz), 3.55 (1H, d, J )
15.6 Hz), 3.49 (1H, d, J ) 15.6 Hz), 2.56 (2H, t, J ) 7.3 Hz),
2.07 (2H, bq, J ) 7.0 Hz), 1.71 (2H, m), 1.51 (3H, s), 1.36 (3H,
s); 13C NMR (CDCl3) δ 203.2, 166.5, 137.6, 115.5, 112.4, 112.1,
85.4, 75.1, 74.0, 64.6, 48.9, 42.3, 32.7, 27.2, 26.3, 22.3; DCIMS
(NH3) m/ z (intensity, %), 346 (M + NH4+, 100), 329 (M + H+,
12.9), 313 (14.0), 288 (8.0), 271 (70.3), 208 (27.0); HRMS calcd
for C16H28NO7 346.1866, found 346.1871.
4.86 (1H, dd, J ) 3.6, 5.9 Hz), 4.59 (1H, d, J ) 11.6 Hz), 4.50
(1H, d, J ) 5.9 Hz), 4.28 (1H, d, J ) 11.6 Hz), 4.05 (1H, dd, J
) 3.6, 10.6 Hz), 3.96 (1H, d, J ) 10.6 Hz), 3.54 (2H, s), 2.50
(2H, t, J ) 7.4 Hz), 1.54 (2H, m), 1.43 (3H, s), 1.28 (3H, s),
1.26 (4H, m), 0.85 (3H, t, J ) 7.0 Hz); 13C NMR (CDCl3) δ
204.3, 166.9, 112.8, 104.2, 84.9, 80.6, 71.6, 65.8, 49.1, 43.1, 31.1,
26.2, 24.8, 23.1, 22.4, 13.9; DCIMS (NH3) m/ z (intensity, %),
348 (M + NH4+, 47.6), 330 (M + NH4+ - H2O, 3.7), 313 (100),
190 (12.4); HRMS calcd for C16H30NO7 348.2022, found
348.2025.
(-)-∆7-Syr in golid e 1 (21). Primary ester 20 (56.6 mg, 0.17
mmol) was stirred with 2 mL of TFA-H2O (9:1) at rt for 2 h.
Then the reaction mixture was neutralized with saturated
aqueous NaHCO3 and extracted with ethyl acetate (3 × 15
mL). The extracts were combined, dried over Na2SO4, filtered,
and evaporated. After prechromatography on a silica Sep-Pak
using ethyl acetate as eluant, the residues were purified by
HPLC with 40% ethyl acetate in hexanes on a Maxsil 5 silica
(1R ,7S ,8R )-7-H y d r o x y -4,10,11-t r io x a -1-p e n t y lt r i-
cyclo[6.2.1.02,6]u n dec-2(6)-en -3-on e (16). The ester 14 (115.2
mg, 0.35 mmol) was treated with 1 mL of TFA-H2O (9:1) for
15 min to deacetonize. Then the mixture was neutralized with
saturated aqueous NaHCO3 and extracted with ethyl acetate.
The crude reaction extracts were dried over Na2SO4, filtered,
and evaporated to give 95.0 mg of oily product 15 which was
then stirred with 90 mL of water (pH ) 6) for one week. Then
the mixture was extracted with ethyl acetate and purified by
HPLC using 2-propanol/EtOAc/hexanes (9.4:45.3:45.3) on a
Dynamax silica gel column (µ ) 6 mL/min, tR ) 78 min) to
yield 16 (16.5 mg, 18.6%). [R]25D ) +26.3 (c 0.43, CH2Cl2); IR-
(KBr) νmax 3450, 2950, 1750, 1620, 1040 cm-1; 1H NMR (CDCl3)
δ 4.97 (1H, d, J ) 17.7 Hz), 4.82 (1H, d, J ) 17.7 Hz), 4.78
(1H, bs), 4.14 (1H, dd, J ) 8.4, 7.2 Hz), 4.08 (1H, s), 3.41 (1H,
dd, J ) 8.4, 2.4 Hz), 2.27 (1H, m), 2.15 (1H, m), 1.50 (2H, m),
1.37 (4H, m), 0.90 (3H, t, J ) 6.8 Hz); 13C NMR (CDCl3) δ
gel column to isolate 21 (5.6 mg, 12.2%). [R]25 ) -69.4 (c
D
0.50, CHCl3); IR(KBr) νmax 3350, 2950, 1760, 1070 cm-1
;
1H
NMR (acetone-d6) δ 5.79 (1H, m), 5.41 (1H, s), 5.00 (1H, d, J
) 17.1 Hz), 4.92 (1H, d, J ) 10.2 Hz), 4.66 (1H, d, J ) 10.3
Hz), 4.47 (1H, s), 4.35 (bs), 4.31 (1H, d, J ) 10.3 Hz), 4.13
(1H, d, J ) 2.8 Hz), 3.94 (1H, d, J ) 10.0 Hz), 3.81 (1H, dd, J
) 10.0, 2.8 Hz), 3.09 (1H, s), 2.05 (m), 1.89 (2H, t, J ) 8.0 Hz),
1.58-1.70 (2H, m); 13C NMR (acetone-d6) δ 171.8, 138.5, 114.2,
107.8, 98.1, 91.4, 74.7, 74.5, 74.0, 58.9, 38.1, 33.5, 22.9; DCIMS
(NH3) m/ z (intensity, %), 288 (M + NH4+, 11.0), 270 (M+, 12.5),
253 (23.1), 228 (100), 184 (99.2), 167 (73.6); HRMS calcd for
C13H22NO6 288.1447, found 288.1452.
(-)-Syr in golid e 1 (4). To a solution of 21 (2 mg) in ethyl
acetate (2 mL) was added 10% Pd/C catalyst (0.5 mg). This
mixture was stirred at rt for 3 h under hydrogen atmosphere
and then filtered. Evaporation of the solvent from the filtrate
afforded 4 in nearly quantitative yield based on 1H NMR
analysis.
168.9, 158.4, 130.4, 104.1, 77.8, 69.1, 66.3, 64.3, 31.8, 30.9, 22.5,
+
22.4, 13.9; DCIMS (NH3) m/ z (intensity, %), 272 (M + NH4
,
66.5), 255 (M + H+, 100), 207 (15.2); HRMS calcd for C13H19O5
255.1232, found 255.1229.
1-(3-Oxoocta n oyl)-2,3:4,5-d i-O-isop r op ylid en e-â-D-fr u c-
top yr a n ose (18). Diacetonide 17 (322.5 mg, 1.24 mmol),
which was prepared from D-fructose by isomerization in
pyridine followed by acetonization,12 was esterified by stirring
with hexanoyl Meldrum’s acid (7, 374.1 mg, 1.55 mmol) in
refluxing toluene (15 mL) for 3 h. Then the solvent was
removed in vacuo, and the residue was purified by vacuum
flash chromatography with 20% ethyl acetate in hexane to give
18 (463.3 mg, 93.4%). [R]25D ) -23.1 (c 2.11, CHCl3); IR(KBr)
Ack n ow led gm en t. Mass spectra were determined
by Ron New of the U.C. Riverside Analytical Instru-
mentation Facility. We thank the U.S. Department of
Agriculture (grant no. 9501093) for financial support.
We thank Professor R. W. Rickards for sharing informa-
tion and insightful discussions.
Su p p or t in g In for m a t ion Ava ila b le: 1H and 13C NMR
spectra of 2, 5, 6, 8, 10, 12-14, 16, 18, 20, and 21 (25 pages).
This material is contained in libraries on microfiche, im-
mediately follows this article in the microfilm version of the
journal, and can be ordered from the ACS; see any current
masthead page for ordering information.
νmax 2950, 1760, 1720, 1380, 1080 cm-1 1H NMR (CDCl3) δ
;
4.59 (1H, dd, J ) 2.5, 7.9 Hz), 4.42 (1H, d, J ) 11.7 Hz), 4.29
(1H, d, J ) 2.5 Hz), 4.22 (1H, dd, J ) 1.8, 7.9 Hz), 4.13 (1H,
d, J ) 11.7 Hz), 3.88 (1H, dd, J ) 1.8, 13.0 Hz), 3.75 (1H, d, J
) 13.0 Hz), 3.47 (2H, s), 2.53 (2H, t, J ) 7.4 Hz), 1.58 (2H, m),
(14) 3-Oxooctenoic acid was prepared using 5-hexenoyl chloride to
replace hexanoyl chloride.13 The 5-hexenoyl chloride was prepared from
5-hexen-1-ol by oxidation using PDC in DMF followed by chlorination
with SOCl2.
J O970461B