Enantioselective synthesis of syringolide 2 from D-arabinose

Article abstract of DOI:10.1139/v99-252

Syringolide 2, a bacterial signal molecule which elicits active defence in resistant plants, has been synthesized in 9 steps from 2,3:4,5-di-O- isopropylidene-D-arabinose, a known derivative of D-arabinose.

Full text of DOI:10.1139/v99-252

275
Enantioselective synthesis of syringolide 2 from
D-arabinose
Robert Chênevert and Mohammed Dasser
Abstract: Syringolide 2, a bacterial signal molecule which elicits active defence in resistant plants, has been synthe-
sized in 9 steps from 2,3:4,5-di-O-isopropylidene-D-arabinose, a known derivative of D-arabinose.
Key words: syringolide, arabinose, synthesis.
Résumé : Le syringolide 2, un signal moléculaire bactérien induisant une défense active chez les plantes résistantes, a
été préparé en 9 étapes à partir du 2,3:4,5-di-O-isopropylidène-D-arabinose, un dérivé connu du D-arabinose.
Mots clés : syringolide, arabinose, synthèse. Chênevert and Dasser 279
Introduction
Results and discussion
Syringolides 1 and 2, the first non-proteinaceous elicitors,
were isolated from the plant pathogen bacterium Pseudomo-
nas syringea pv. tomato by Sims and co-workers in 1993
(1). These elicitors trigger a hypersensitive defence response
in soybean plants carrying a resistance gene (2). The hyper-
sensitive defence phenomenon involves the localized cell
death in the infected tissues and production of phytoalexins.
Syringolides attract considerable interest since they have
features in common with antigens recognized by the immune
systems of vertebrates. Research on this defence mechanism
could lead to cloning of disease-resistance genes and plant
immunization through molecular genetic manipulation.
Four different strategies have been proposed for the syn-
thesis of these C-glycosides possessing a new oxygen rich
tricyclic structure. Syringolides have been synthesized from
xylose (3–5), tartaric acid derivatives (6, 7), butenolides (8,
9), or via the Sharpless catalytic asymmetric dihydroxylation
(10). In this paper, we report the enantioselective synthesis
of syringolide 2 from D-arabinose.
D-Arabinose (1) was transformed into di-O-isopropyli-
dene-D-arabinose (2) in a three step sequence according to
known procedures (11) (Scheme 1). Aldehyde 2 was reduced
with NaBH4 in ethanol to afford 3. Alcohol 3 was protected
by reaction with p-methoxybenzyl chloride (MPM-Cl) in the
presence of NaH to give 4. Regioselective deprotection of 4
was achieved by treatment with methanol under acidic catal-
ysis (p-toluenesulfonic acid). The primary alcohol of 5 was
selectively acylated with acetyl chloride in the presence of
2,4,6-collidine at –78°C. The Swern oxidation of 6 provided
ketone 7. The acetate group of 7 was selectively removed by
transesterification with ethanol in diisopropyl ether in the
presence of Candida antarctica lipase.
Acylation of 8 with the Meldrum’s acid derivative 9 (12)
in refluxing THF gave the unstable β−keto ester 10 (Scheme
2). Any attempt to purify 10 by chromatography on silica
gel gave 11, the product of the intramolecular Knoevenagel
condensation; thus, 10 was stirred with silica gel in ethyl
acetate – hexanes to give 11 in 71% yield for the two last
steps. The p-methoxy-phenylmethyl (MPM) protecting group
was removed by treatment with dichlorodicyanoquinone. All
previous syntheses of syringolides invariably involved acid
treatment of analogs of compound 12 to give syringolides in
very low yields. Recently, Wong et al. (8) reported that the
use of p-toluenesulfonic acid in large excess improved the
yield of this rearrangement. Thus, treatment of 12 with a
large excess of p-toluenesulfonic acid in acetone–water pro-
vided syringolide 2 (13) in 54% yield after recrystallization
25
24
([α]D –75.1 (CHCl3, c 0.06); litt. (1) [α ]D –75.91
(CHCl3, c 0.22)). The spectroscopic data of the synthetic
syringolide 2 (13) were identical to those reported in the
literature.
Received October 14, 1999.
R. Chênevert1 and M. Dasser. Département de chimie, Faculté des sciences et de génie, Université Laval, Québec QC G1K 7P4,
Canada.
1Author to whom correspondence may be addressed. Telephone: (418) 656-3283. Fax: (418) 656-7916.
e-mail: robert.chenevert@chm.ulaval.ca
Can. J. Chem. 78: 275–279 (2000)
© 2000 NRC Canada

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Can. J. Chem. Vol. 78, 2000
Scheme 1.
Scheme 2.
In conclusion, syringolide 2 has been synthesized in 9 steps
and 15.3% overall yield from diisopropylidene-D-arabinose.
Previous syntheses of syringolides from the sugar chiral pool
involved the tedious and inefficient isomerization (-7%
yield) of xylose to xylulose isolated as its acetonide. Both
enantiomers of the starting material, arabinose, are readily
© 2000 NRC Canada

Chênevert and Dasser
277
available at a reasonable cost. The use of p-toluenesulfonic
acid in large excess greatly improved the yield of the last
step (rearrangement of 12 to syringolide 2).
Triethylamine (1 mL) was added and the solvent was evapo-
rated. Flash chromatography of the crude product (gradient
from ether:hexane 1:1 to 4:1 and then pure MeOH) provided
5 (2.6 g, 73%) as a colorless oil along with starting material
25
4 (200 mg) and the corresponding tetraol (182 mg). [α]D
Experimental section
–15.52 (CHCl3, c 1.15); IR (neat) νmax: 3600–3100, 3000–
1
2050, 2950–2850, 1380, 1370, 1300–1100 cm–1; H NMR
Candida antarctica lipase, fraction B (chirazyme L-2, car-
rier-fixed, lyo., 500 U/mg dry carrier) was purchased from
Boehringer Mannheim. Melting points are uncorrected. NMR
spectra were recorded at 300 MHz (1H) and 75.44 MHz (13C).
(CDCl3) δ: 7.17 (2H, d, J = 8.5 Hz), 6.80 (2H, d, J = 8.5
Hz), 4.45 (2H, s), 4.00 (1H, m), 3.72 (3H, s), 3.70–3.53
(5H, m), 3.46 (1H, dd, J1 = 9.1 Hz, J2 = 7.4 Hz), 1.30
(6H, s); 13C NMR (CDCl3) δ: 159.45, 129.56, 128.74,
113.85, 109.23, 80.51, 77.89, 73.48, 72.17, 70.20, 63.80,
55.13, 26.66; HRMS (CI, NH3) m/z: calcd. for C16H24O6
312.1573 (M+), found 312.1581 ± 0.0009.
1,2:3,4-Di-O-isopropylidene-D-lyxitol (3)
Sodium borohydride (566 mg, 15.28 mmol) was added to
a solution of aldehyde 2 (3.52 g, 15.29 mmol) in ethanol
95%. The mixture was stirred at room temperature for 6 h.
The mixture was cooled to 0°C and acidified with 1 N HCl.
The solvent was evaporated and the residue was dissolved in
ether (200 mL), washed with 1 N HCl (30 mL), saturated
aqueous NaHCO3 (30 mL) and brine (2 × 20 mL). The or-
ganic layer was dried (MgSO4) and evaporated. Flash chro-
matography on silica gel (EtOAc:hexane, 3:7) gave 3 (3.2 g,
1-O-Acetyl-3,4-O-isopropylidene-5-O-p-methoxybenzyl-D-
lyxitol (6)
To a solution of 5 (1.83 g, 5.86 mmol) in dry CH2Cl2
(60 mL) cooled to –78°C was added freshly distilled 2,4,6-
collidine (1.55 mL, 2 equiv.). After stirring for 5 min, acetyl
chloride (0.55 mL, 1.2 equiv.) was added dropwise and the
solution was stirred for 2 h at –78°C. Dichloromethane
(200 mL) was added and the resulting organic phase was
washed with 0.5 N HCl (1 × 50 mL), saturated aqueous
NaHCO3 (1 × 50 mL) and water (2 × 50 mL). The organic
layer was dried (MgSO4) and evaporated. Flash chromatog-
raphy on silica gel (EtOAc:hexane, 3:7) gave 6 (1.87 g,
25
90%) as a colorless oil. [α]D –1.0 (CHCl3, c 3.38); IR
(neat) νmax: 3700–3000, 3000–2800, 1380, 1370, 1300–
1
1100 cm–1; H NMR (CDCl3) δ: 4.05 (1H, dd, J1 = 8.4 Hz,
J2 = 6.0 Hz), 3.92 (3H, m), 4.27 (1H, dd, J1 = 11.7 Hz, J2 =
4.3 Hz), 3.62 (2H, m), 2.75 (1H, s), 1.33 (3H, s), 1.31
(3H, s), 1.29 (3H, s), 1.25 (3H, s); 13C NMR (CDCl3) δ:
109.57, 109.20, 80.75, 78.65, 76.95, 67.62, 62.51, 26.77,
26.70, 26.42, 24.97; HRMS (CI, NH3) m/z: calcd. for
C11H21O5: 233.1389 (MH+), found 233.1393 ± 0.0007.
25
90%) as a colorless oil. [α]D –1.0 (CHCl3, c 2.45); IR
(neat) νmax: 3600–3150, 3010–2980, 2980–2800, 1740, 1380,
1370, 1300–1000 cm–1; 1H NMR (CDCl3) δ: 7.24 (2H, d, J =
8.6 Hz), 6.87 (2H, d, J = 8.6 Hz), 4.52 (2H, s), 4.37 (1H, dd,
J = 2.3 Hz), 4.06 (2H, m), 3.79 (4H, m), 3.70 (2H, m), 3.53
(1H, dd, J1 = 9.3 Hz, J2 = 6.7 Hz), 3.43 (1H, br s), 2.09
(3H, s), 1.36 (6H, s); 13C NMR (CDCl3) δ: 171.05, 159.31,
129.41, 129.11, 113.75, 109.37, 78.97, 78.51, 73.27, 70.98,
70.11, 66.08, 55.06, 26.67, 20.70; HRMS (CI, NH3) m/z:
calcd. for C18H26O7: 354.1678 (M+), found 354.1672 ± 0.0010.
1,2:3,4-Di-O-isopropylidene-5-O-p-methoxybenzyl-D-
lyxitol (4)
To a suspension of NaH (359.6 mg, 14.98 mmol) in dry
DMF (10 mL) at 0°C and under dry nitrogen atmosphere was
added dropwise a solution of 3 (2.23 g, 9.99 mmol) in DMF
(50 mL). The mixture was stirred for 30 min at 0°C and the
p-methoxybenzyl chloride (1.625 mL, 11.98 mmol) was
added. The mixture was stirred for 1 h at 0°C and then over-
night at room temperature. The mixture was then filtered
through Celite and the filtrate was diluted with ether
(300 mL), washed with water (3 × 60 mL). The organic phase
was dried (MgSO4) and evaporated. Flash chromatography on
(3R,4R)-1-Acetyl-3,4-isopropylidene-5-p-methoxybenzyl-
1,3,4,5-tetrahydroxy-2-pentanone (7)
A solution of DMSO (872 µL, 11.33 mmol) in dichloro-
methane (6 mL) was added to a solution of oxalyl chloride
(546 µL, 6.2 mmol) in dichloromethane (6 mL) at –78°C.
The mixture was stirred for 20 min under dry nitrogen atmo-
sphere. A solution of alcohol 6 (1.70 g, 4.783 mmol) in di-
chloromethane (50 mL) was added dropwise and the
solution was stirred for 2 h at –78°C. Triethylamine
(2.96 mL, 23.92 mmol) was added and the solution was al-
lowed to reach room temperature. The reaction was diluted
with dichloromethane (250 mL) and washed with water
(40 mL), saturated aqueous NaHCO3 (40 mL) and brine (3 ×
40 mL). The organic phase was dried (MgSO4) and evapo-
rated. The crude product was purified by flash chromatogra-
phy on silica gel (AcOEt:hexane, 3:7) to give ketone 7
silica gel (ether:petroleum ether, 1:4) gave 4 (3.21 g, 91%) as
25
a colorless oil. [α]D +8.0 (CHCl3, c 3.0); IR (neat) νmax
:
3090–3000, 2900–2800, 1380, 1370, 1300–1100 cm–1; 1H
NMR (CDCl3) δ: 7.24 (2H, d, J = 8.6 Hz), 6.84 (2H, d, J =
8.6 Hz), 4.50 (2H, s), 4.42–3.99 (3H, m), 3.86 (1H, m), 3.75
(3H, s), 3.70 (2H, m), 3.52 (1H, dd, J1 = 10 Hz, J2 = 6 Hz),
1.38 (3H, s), 1.36 (3H, s), 1.31 (3H, s), 1.29 (3H, s); 13C
NMR (CDCl3) δ: 159.04, 130.95, 129.16, 113.55, 109.56,
109.39, 79.58, 77.57, 77.43, 72.93, 70.20, 67.39, 55.05,
26.92, 26.47, 25.10; HRMS (CI, NH3) m/z: calcd. for
C19H28O6: 352.1886 (M+), found 352.1893 ± 0.0010.
25
(1.466 g, 87%) as a colorless oil. [α]D –12.65 (CHCl3, c
2.6); IR (film) νmax: 3100–3000, 3000–2800, 1760, 1730,
1380, 1370, 1300–1100 cm–1; 1H NMR (CDCl3) δ: 7.17
(2H, d, J = 8.6 Hz), 6.79 (2H, d, J = 8.6 Hz), 4.99 (1H, d, J
= 17.8 Hz), 4.79 (1H, d, J = 17.8 Hz), 4.45 (2H, s), 4.26
(1H, d, J = 7.9 Hz), 4.20 (1H, m), 3.71 (3H, s), 3.64 (2H,
dd, J1 = 11.0 Hz, J2 = 2.9 Hz), 3.51 (2H, dd, J1 = 11.0 Hz,
3,4-O-Isopropylidene-5-O-p-methoxybenzyl-D-lyxitol (5)
To a solution of diacetonide 4 (4.0 g, 11.34 mmol) in an-
hydrous methanol (40 mL) maintained at - 10°C was added
p-toluenesulfonic acid (80 mg). The reaction was followed
by thin layer chromatography (EtOAc; 4, rf = 0.9; 5, rf =
0.2, corresponding tetraol, rf = 0) and stopped after - 10 h.
© 2000 NRC Canada

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Can. J. Chem. Vol. 78, 2000
J2 = 5.1 Hz), 2.08 (3H, s), 1.39 (3H, s), 1.37 (3H, s); 13C
NMR (CDCl3) δ: 202.46, 170.01, 159.13, 129.76, 129.18,
113.66, 111.34, 80.14, 77.38, 73.10, 69.18, 66.10, 55.08,
26.53, 25.97, 20.21. HRMS (CI, NH3) m/z: calcd. for
C18H24O7: 352.1522 (M+), found 352.1529 ± 0.0010.
mg, 1.5 equiv.) was added and the mixture was stirred at
room temperature. The reaction was followed by TLC and
quenched after 4 h. The reaction mixture was diluted with
CH2Cl2 (60 mL), filtered and washed with saturated aqueous
NaHCO3 (10 mL) and brine (3 × 10 mL). The organic layer
was dried (Na2SO4) and evaporated. The crude product was
purified by flash chromatography on silica gel (ethyl ace-
(3S,4R)-3,4-Isopropylidene-5-p-methoxybenzyl-1,3,4,5-
tetrahydroxy-2-pentanone (8)
tate:hexane, gradient 1:4 to 2:3) to give 12 (215 mg, 83%) as
25
a colorless oil. [α]D –60.0 (CHCl3, c 1.6); IR (film) νmax
:
Candida antarctica lipase (800 mg, total weigh enzyme
on carrier, 500 U/mg dry carrier) and ethanol (1.0 mL,
17.0 mmol) were added to a solution of compound 7 (1.2 g,
3.4 mmol) in diisopropyl ether (30 mL) and the reaction
mixture was stirred at room temperature. The reaction was
followed by thin layer chromatography (ether:hexane, 1:1)
and stopped when all the starting material was gone (-15 h).
The reaction mixture was filtered and evaporated. Flash
chromatography on silica gel (ether:hexane, 1:1) gave 8 (897
mg, 85%) as a colorless oil. [α]D25 –7.7° (CHCl3, c 1.45); IR
(film) νmax: 3600–3100, 3100–3000, 2990–2800, 1730, 1380,
3600–3100, 2990–2800, 1790, 1710, 1620, 1370, 1360,
1
1300–1100 cm–1; H NMR (CDCl3) δ: 5.39 (1H, d, J = 8.0
Hz), 5.10 (1H, d, J = 19.8 Hz), 4.85 (1H, d, J = 19.8 Hz),
3.96–3.80 (3H, m), 3.09 (1H, m), 2.92 (1H, m), 1.58
(2H, m), 1.45 (6H, s), 1.27 (8H, m) 0.97 (3H, t, J = 7.0 Hz);
13C NMR (CDCl3) δ: 198.75, 173.33, 169.74, 126.75,
110.54, 81.44, 73.82, 69.02, 61.42, 41.92, 31.49, 28.91,
28.84, 26.65, 23.04, 22.46, 13.93; HRMS (CI, NH3) m/z:
calcd. for C18H29O6: 341.1964 (MH+), found 341.1956 ±
0.0010.
1
1370, 1300–1100 cm–1; H NMR (CDCl3) δ: 7.18 (2H, d, J
Syringolide 2 (13)
= 8.6 Hz), 6.80 (2H, d, J = 8.6 Hz), 4.47 (2H, s), 4.44
(2H, m), 4.31 (1H, d, J = 8.0 Hz), 4.11 (1H, m), 3.72
(3H, s), 3.67 (1H, dd, J1 = 3.1 Hz, J2 = 10.7 Hz), 3.55 (1H,
dd, J1 = 5.2 Hz, J2 = 10.7 Hz), 2.88 (1H, t, J = 5.0 Hz), 1.39
(3H, s), 1.34 (3H, s); 13C NMR (CDCl3) δ: 209.10, 159.21,
129.67, 129.22, 113.72, 111.36, 79.98, 76.90, 73.21, 69.23,
66.12, 55.13, 26.65, 26.01; HRMS (CI, NH3) m/z: calcd. for
C16H22O6: 310.1416 (M+), found 310.1421 ± 0.0009.
A solution of 12 (140 mg, 0.411 mmol) and p-toluene-
sulfonic acid (782 mg, 10 equiv.) in acetone:water (1:1.2,
10 mL) was stirred for 56 h at room temperature. The solu-
tion was neutralized with saturated aqueous NaHCO3 (2 mL)
and extracted with ethyl acetate (3 × 20 mL). The organic
layer was washed with brine (3 × 5 mL), dried over Na2SO4
and evaporated. The residue was redissolved in EtOAc
(3 mL) and the solution was filtered through silica gel and
evaporated. The product was washed with hexane to give
syringolide 2 (95 mg, 77%) as a pale yellow solid. A frac-
tion was recrystallized in pentane:ether 2:1 to give pure
syringolide 2 (66.5 mg, 54%) as a colorless solid. Mp 122–
(1NR,2NR)-3-[1N,2N-(Isopropylidenedioxy)-3N-(p-methoxy-
benzyloxy)propyl]-2-octanoyl-2-buten-4-olide (11)
The octanoyl Meldrum’s derivative 9 was prepared by
stirring equimolar amounts of Meldrum’s acid (2,2-
dimethyl-1,3-dioxane-4,6-dione) and n-octanoyl chloride in
dichloromethane at 0°C for 8 h.
25
123°C; [α]D –75.1 (CHCl3, c 0.06); lit. (1) mp 123–124°C
24
[α]D –75.91 (CHCl3, c 0.22); IR (KBr) νmax: 3600–3100,
2990–2800, 1770, 1300–1100 cm–1; 1H NMR (acetone-d6) δ:
5.38 (1H, d, J = 1.9 Hz), 4.68 (1H, d, J = 10.2 Hz), 4.49
(1H, br s), 4.33 (1H, d, J = 10.2 Hz), 4.30 (1H, d, J = 4.2
Hz), 4.13 (1H, m), 3.94 (1H, dd, J1 = 10.1 Hz, J2 = 1.0 Hz),
3.83 (1H, dd, J1 = 10.1 Hz, J2 = 2.8 Hz), 3.10 (1H, s), 1.89
(2H, m), 1.70–1.40 (2H, m), 1.30 (8H, m), 0.87 (3H, t, J =
7.0 Hz); 13C NMR (acetone-d6) δ: 172.70, 108.72, 99.00,
92.16, 75.50, 75.32, 74.81, 59.63, 39.36, 32.40, 30.45,
29.40, 24.27, 23.15, 14.18. HRMS (CI, NH3) m/z: calcd. for
C15H25O6: 301.1651 (MH+), found 301.1657 ± 0.0009.
A solution of alcohol 8 (380 mg, 1.224 mmol) and Meldrum’s
derivative 9 (397 mg, 1.469 mmol) in THF was brought to
reflux during 3 h. The solvent was evaporated and the residue
was dissolved in hexane:ethyl acetate 9:1 (15 mL) in the
presence of silica gel (1 g). The mixture was stirred at room
temperature for 18 h. Silica gel was filtered and the solvents
were evaporated. Flash chromatography on silica gel (ethyl
acetate:hexane, 3:7) gave 11 (402 mg, 71%) as a colorless oil.
25
[α]D –23.6 (CHCl3, c 1.2); IR (film) νmax: 3080–2990,
2990–2800, 1780, 1700, 1640, 1370, 1360, 1300–1100 cm–1;
1H NMR (CDCl3) δ: 7.23 (2H, d, J = 8.6 Hz), 6.84 (2H, d, J
= 8.6 Hz), 5.43 (1H, d, J = 8.2 Hz), 5.03 (1H, d, J = 19.8 Hz),
4.83 (1H, d, J = 19.8 Hz), 4.55 (1H, d, J = 11.6 Hz), 4.45
(1H, d, J = 11.6 Hz), 3.97 (1H, m), 3.78 (3H, s), 3.72 (2H, d,
J = 4.8 Hz), 2.91 (2H, t, J = 7.0 Hz), 1.57 (2H, m), 1.43
(6H, s), 1.27 (8H, m), 0.86 (3H, t, J = 5.7 Hz); 13C NMR
(CDCl3) δ: 196.56, 172.47, 170.08, 159.16, 129.69, 129.31,
126.71, 113.61, 110.96, 80.69, 73.76, 73.15, 69.78, 68.88,
55.10, 41.84, 31.53, 28.95, 28.87, 26.64, 23.03, 22.45, 13.92;
HRMS (CI, isobutane) m/z: calcd. for C26H36O7: 460.2461
(M+), found 460.2464 ± 0.0014.
Acknowledgements
The authors would like to thank the Natural Sciences and
Engineering Research Council of Canada (NSERC) for fi-
nancial support.
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© 2000 NRC Canada

Chênevert and Dasser
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© 2000 NRC Canada