Chemoenzymatic enantioselective synthesis of (-)-enterolactone

Article abstract of DOI:10.1139/v98-231

Enterolactone, a lignan isolated from biological fluids of animals and humans, was synthesized via enzymatic desymmetrization of 2-(3-methoxybenzyl)-1,3-propanediol.

Full text of DOI:10.1139/v98-231

223
Chemoenzymatic enantioselective synthesis of
(–)-enterolactone
Robert Chênevert, Ghodsi Mohammadi-Ziarani, Dave Caron,
and Mohammed Dasser
Abstract: Enterolactone, a lignan isolated from biological fluids of animals and humans, was synthesized via
enzymatic desymmetrization of 2-(3-methoxybenzyl)-1,3-propanediol.
Key words: enterolactone, synthesis, lipase, desymmetrization, lignan.
Résumé : L’entérolactone, une lignane isolée des fluides biologiques des animaux et des humains, a été préparée via
une désymétrisation enzymatique du 2-(3-méthoxybenzyl)-1,3-propanediol.
Mots clés : entérolactone, synthèse, lipase, désymétrisation, lignane.
Chênevert et al. 226
composition of 3 was measured by 19F NMR (282 MHz, in
benzene-d6) analysis of the corresponding (+)-α-methoxy-α-
trifluoromethyl-α-phenyl acetate (MTPA, Mosher’s ester).
Lignans, naturally occurring secondary plant metabolites
consisting of two phenylpropanoid units, are widely distrib-
The enantiomeric excess was determined to be 94% (10).
uted in the plant kingdom (1). Enterolactone and enterodiol
(The accuracy of this method is ±3%.) The absolute configu-
were the first lignans to be found in humans and animals (2,
25
ration of monoester 3 (R; [α]D +26.8 (c 2.26, CHCl3)) was
3). Enterolactone, like other lignans, exhibits a wide range
determined by correlation with enterolactone 8 of known ab-
of biological activity (inhibition of cardiac Na+, K+-ATPase,
solute configuration (the absolute configuration of entero-
antiestrogenic properties, platelet activating factor antago-
lactone was established by X-ray crystal diffraction (11)).
nist, inhibitor of human estrogen aromatase) and has at-
tracted great interest on account of its cancer-protective
properties (4). Epidemiological studies reveal a lowered risk
of hormone-dependent cancers (breast, prostate) among veg-
etarians consuming large amount of lignans. Enterolactone
and enterodiol are formed in the intestinal tract by the action
of bacteria on precursors present in the diet (5).
Many syntheses of racemic enterolactone have been re-
The use of Pseudomonas fluorescens lipase gave similar re-
sults (quantitative yield, ee = 92%, reaction time = 6.5 h).
Monoester (R)-(+)-3 was treated with mesyl chloride in
the presence of triethylamine to give mesylate (S)-(+)-4
(Scheme 2). Displacement of the mesylate with cyanide in
DMSO afforded nitrile (R)-(–)-5. Hydrolysis of the nitrile
and the acetate groups in acidic medium was followed by
ring closure in a one-pot procedure to give lactone (R)-(+)-6,
ported (6) but only a few enantioselective syntheses have
25
24
[α]D +6.15 (c 1.86, CHCl3) (lit. (7e) [α]D 6.06 (c 7.92,
been achieved (7). We report here a chemoenzymatic enantio-
selective synthesis of enterolactone via enzymatic desym-
metrization of 2-(3-methoxybenzyl)-1,3-propanediol.
CHCl3).
The anion of lactone 6 was generated by treatment with
lithium diisopropylamide and stereoselectively alkylated
with m-methoxybenzyl bromide (7c) to give lactone (R,R)-
25
23
(–)-7, [α]D –39.0 (c 2.5, CHCl3) (lit. (7a) [α]D –39.2 (c
0.78, CHCl3)). Demethylation of lactone 7 with boron tribro-
mide provided enterolactone (R,R)-(–)-8. Comparison of the
The known diester 1 (8) was reduced to the diol 2 by lith-
ium aluminum hydride in ether (Scheme 1). Diol 2 was sub-
jected to enzyme-catalyzed esterification (for related
desymmetrization of propane-1,3-diol derivatives, see ref. 9)
by treatment with Pseudomonas cepacia lipase using vinyl
acetate as acyl donor and solvent to give the optically active
monoester (R)-(+)-3 in quantitative yield. The enantiomeric
25
optical rotation values ([α]D –38.4 (c 0.37, CHCl3)) for the
sample obtained with those reported in the literature (7a)
23
([α]D –38.4 (c 0.25, CHCl3)) confirmed the absolute con-
figuration of (–)-8 to be (R,R), which requires that the enzy-
matic desymmetrization of 2 leads to diol (+)-3 with R
configuration at the newly created stereogenic center.
In conclusion, we have completed the enantioselective
synthesis (ee = 94%) of (R,R)-(–)-enterolactone in seven
steps and 35% yield from diester 1. The key step is the en-
zymatic desymmetrization of diester 2.
Received August 4, 1998.
R. Chênevert,1 G. Mohammadi-Ziarani, D. Caron, 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
Melting points were recorded on a Thomas–Hoover appa-
ratus, and are uncorrected. Infrared spectra were recorded
Can. J. Chem. 77: 223–226 (1999)
© 1999 NRC Canada

224
Can. J. Chem. Vol. 77, 1999
Scheme 1.
M
O
e
OMe
OMe
Enzyme
AcO
LiAlH4
EtOEt
EtO
OEt
O
O
OH OAc
(R)-(+)-3
OH OH
1
2
Scheme 2.
MeO
OMe
OMe
MeSO2Cl
NEt3
NaCN
DMSO
CN OAc
OH OAc
OMs OAc
(S)-(+)-4
(R)-(-)-5
(R)-(+)-3
H2O
H2SO4
AcOH
110 °C
OMe
OMe
MeO
1) LDA, -78 °C
OMe
O
O
2)
(R,R)-(-)-7
O
O
Br
BBr3, CH2Cl2
OH
(R)-(+)-6
OH
O
O
(R,R)-(-)-8
using a Perkin Elmer 781 spectrometer. NMR spectra were
recorded in CDCl3 solutions at 300 MHz (1H), 282 MHz
(19F), 75 MHz (13C) on a Bruker AC-300 instrument. Opti-
cal rotation values were obtained from a JASCO DIP-300
polarimeter. Column purifications were conducted by flash
chromatography on silica gel 60 (230–400 mesh). Pseudo-
monas cepacia lipase and Pseudomonas fluorescens lipase
were purchased from Amano and Aldrich, respectively.
were washed with saturated aqueous NaHCO3 (2 × 50 mL)
and brine (2 × 50 mL), dried (MgSO4), and evaporated. The
crude product was purified by flash chromatography (ethyl
acetate:hexane 2:3, to pure ethyl acetate) to yield diol 2 as a
white solid (1.76 g, 75%); mp 82°C (recrystallized from
ether) (lit. (6l) mp 81–82°C). IR (KBr): 3350, 3020–2800,
1605, 1588, 1491, 1264, 1036, 873, 778, 695 cm–1; 1H NMR
(MeOD): 1.90 (1H, m), 2.61 (2H, d, J = 7.0 Hz), 3.54 (4H,
d, J = 5.6 Hz), 3.74 (3H, s), 6.76 (3H, m), 7.15 (1H, m); 13
C
NMR (CDCl3): 34.17, 43.75, 55.03, 64.49, 111.17, 114.77,
121.55, 129.28, 141.49, 159.49.
2-(3-Methoxybenzyl)-propane-1,3-diol (2)
Lithium aluminum hydride (1.825 g, 48.0 mmol) was sus-
pended in anhydrous ether (100 mL) at 0°C under a dry N2
atmosphere. Diester 1 (3.37 g, 12.0 mmol) was added
dropwise and the reaction mixture was stirred at 25°C for
4 h. The mixture was cooled and acidified with 3 N HCl
(65 mL), filtered through a bed of Celite, and then extracted
with ethyl acetate (2 × 100 mL). The combined extracts
Lipase-catalyzed acylation of diol 2. 3-Hydroxy-2-(3-
methoxybenzyl)propyl acetate ((R)-3)
To a solution of diol 2 (40 mg, 0.167 mmol) in vinyl ace-
tate (3 mL) was added Pseudomonas cepacia lipase (35 mg),
and the mixture was stirred at room temperature. The prog-
© 1999 NRC Canada

Chênevert et al.
225
ress of the reaction was monitored by TLC and the reaction
was stopped after disappearance of the starting material
(26 min). The enzyme was filtered and washed with diethyl
ether. The solvent was evaporated and the crude product was
purified by flash chromatography (ethyl acetate:hexane,
10:90 to 50:50) to give monoester (R)-(+)-3 as a colorless
oil in quantitative yield (ee = 94%). The use of Pseudomo-
nas fluorescens lipase gave similar results (quantitative
HRMS (EI) calcd. for C14H17O3N (M+): 247.1208; found:
247.1200.
4-[(3-Methoxyphenyl)methyl]dihydro-2(3H)-furanone
((R)-(+)-6)
Nitrile 5 (114 mg, 0.461 mmol) was dissolved in water
(5 mL), acetic acid (10 mL), and concentrated sulfuric acid
(1 mL). The solution was stirred at 110°C for 2 h. Ethyl ace-
tate (300 mL) and 2 M NaOH (60 mL) were added to the so-
lution and the layers were decanted. The organic layer was
washed with water (3 × 50 mL), dried (MgSO4), and evapo-
rated. The crude product was purified by flash chromatogra-
phy (silica gel, ethyl acetate:hexane, 1:4) to afford lactone
25
yield, ee = 92%, reaction time 6.5 h). [α]D +26.8 (c 2.26,
CHCl3); IR (neat): 3425, 1738, 1605, 1588, 1491, 1043 cm-1;
1H NMR (CDCl3): 2.07 (3H, s), 2.10 (2H, m), 2.62 (2H, m),
3.50 (1H, dd, J = 11.2 Hz, J = 6.2 Hz), 3.60 (1H, dd, J =
11.2 Hz, J = 4.5 Hz), 3.79 (3H, s), 4.07 (1H, dd, J =
11.1 Hz, J = 6.5 Hz), 4.18 (1H, dd, J = 11.1 Hz, J = 4.6 Hz),
6.76 (3H, m), 7.21 (1H, t, J = 7.5 Hz); 13C NMR (CDCl3):
20.71, 34.25, 42.22, 55.01, 61.95, 63.92, 111.39, 114.76,
121.31, 129.32, 140.86, 159.63, 171.45; HRMS (EI) calcd.
for C13H18O4 (M+): 238.1205; found: 238.1199.
25
(R)-(+)-6 (89.4 mg, 94%) as a colorless oil. [α]D +6.15 (c
24
1.86, CHCl3) (lit. (7e) [α]D 6.06 (c 7.92, CHCl3) (lit. (7d)
20
[α]D +6.41 (c 2.08, CHCl3)). IR (film): 3100–3000, 3000–
1
2800, 1775, 1601, 1584, 1492, 1300–1000 cm–1; H NMR
(CDCl3): 2.24 (1H, dd, J = 17.5 Hz, J = 7.2 Hz), 2.57 (1H,
dd, J = 17.2 Hz, J = 7.6 Hz), 2.71 (2H, dd, J = 9.3 Hz, J =
3.2 Hz), 2.81 (1H, m), 3.76 (3H, s), 4.00 (1H, dd, J =
9.1 Hz, J = 6.3 Hz), 4.31 (1H, d, J = 8.7 Hz, J = 6.9 Hz),
6.74 (3H, m), 7.22 (1H, m); 13C NMR (CDCl3): 34.10,
36.92, 38.78, 55.06, 72.53, 111.73, 114.47, 120.85, 129.68,
139.76, 159.75, 176.77.
1-Acetoxy-2-(3-methoxybenzyl)-3-(mesyloxy)propane
((S)-4)
A solution of 3 (296.4 mg, 1.244 mmol) in anhydrous
THF (10 mL) was prepared under nitrogen. The solution
was cooled to 0°C and triethylamine (0.184 mL, 1.32 mmol)
was added first, and then methanesulfonyl chloride
(0.152 mL, 1.96 mmol) was added dropwise. The solution
was stirred for 2 h while the temperature was increased to
25°C. The solvent was evaporated and the residue was taken
up in ether. The organic phase was washed with a 1 N HCl
solution, with a saturated NaHCO3 solution (three times),
dried, and concentrated. The crude product was purified by
flash chromatography (silica gel, elution with hexanes:ethyl
acetate, 3:1) to yield (S)-(+)-4 (380 mg, 97%) as a colorless
3,4-Bis[(3-methoxyphenyl)methyl]-dihydro-2(3H)-
furanone (3R,4R)-(–)-7
To lactone 6 (40.2 mg, 0.195 mmol) in anhydrous THF
(1.5 mL) at –78°C was added LDA (2 M solution in CH2Cl2,
0.176 mL, 0.351 mmol) and HMPA (0.105 mL,
0.585 mmol). The solution was stirred for 0.5 h at –78°C
and 3-methoxybenzyl bromide (70.6 mg, 0.351 mmol) in an-
hydrous THF (0.5 mL) was added in one portion. The mix-
ture was stirred overnight (15 h) at –78°C and then warmed
slowly to 0°C. The excess of base was neutralized at 0°C
with a saturated aqueous NH4Cl solution (3 mL) and the
mixture was extracted with ether (5 mL) and ethyl acetate (2
× 10 mL). The combined organic layer was washed with wa-
ter (3 × 20 mL) and brine (10 mL), dried (MgSO4), and
evaporated under reduced pressure. The crude product was
purified by flash chromatography (pure hexane, to ether:hex-
ane, 15:85) to give lactone 7 (38 mg, 60%) as a colorless oil.
25
oil. [α]D 5.40 (c 2.34, CHCl3); IR (neat): 3100–3000,
1
3000–2800, 1738, 1602, 1587, 1490, 1350, 1155 cm–1; H
NMR (CDCl3): 1.99 (3H, s), 2.32 (1H, m), 2.63 (2H, d, J =
7.4 Hz), 2.91 (3H, s), 3.71 (3H, s), 3.93–4.16 (4H, m), 6.69
(3H, m), 7.14 (1H, t, J = 7.8 Hz); 13C NMR (CDCl3): 20.58,
33.78, 36.85, 39.32, 54.96, 62.78, 68.57, 111.83, 114.63,
121.16, 129.52, 139.47, 159.68, 170.54; HRMS (EI) calcd.
for C14H20O6 S (M+): 316.0980; found: 316.0990.
25
23
[α]D –39.0 (c 2.5, CHCl3) (lit.(7a) [α]D –39.2 (c 0.78,
CHCl3)). IR(neat): 3100–3000, 3000–2800, 1770, 1600,
1585, 1490, 1300–1100, 870, 782, 696 cm-1; 1H NMR
(CDCl3): 2.43–2.64 (4H, m), 2.91 (1H, dd, J = 13.9 Hz, J =
7.0 Hz), 3.06 (1H, dd, J = 13.9 Hz, J = 5.0 Hz), 3.76 (3H, s),
3.78 (3H, s), 3.78 (1H, dd, J = 15.0 Hz, J = 6.2 Hz), 4.10
(1H, dd. J = 9.0 Hz, J = 6.8 Hz), 6.76 (4H, m), 6.56
(2H, m), 7.20 (2H, m). 13C NMR (CDCl3): 35.01, 38.43,
41.14, 46.22, 55.00, 55.03, 71.05, 111.74, 112.24, 114.39,
114.72, 120.77, 121.47, 129.53, 129.59, 139.42, 139.17,
159.70, 178.32.
4-Acetoxy-3-(3-methoxybenzyl)butyronitrile ((R)-(–)-5)
To a solution of mesylate 4 (152.8 mg, 0.483 mmol) in
anhydrous DMSO (5 mL) was added finely pulverized so-
dium cyanide (31.9 mg, 0.651 mmol). The solution was
stirred at 45°C for 12 h. Ethyl acetate (200 mL) was added
and the organic layer was washed with water (8 × 50 mL)
dried (MgSO4), and evaporated. The crude product was puri-
fied by flash chromatography (silica gel, hexane:ethyl ace-
tate, 3:1) to give nitrile (R)-(–)-5 (118.4 mg, 99%) as a
colorless oil. [α]D25–10.77 (c 1.96, CHCl3); IR (neat): 3100–
3000, 3000–2800, 2250, 1742, 1604, 1589, 1493, 1250–
1040 cm–1; 1H NMR (CDCl3): 2.07 (3H, s), 2.13–2.42
(2H, m), 2.38 (1H, m), 2.66 (1H, dd, J = 13.7 Hz, J =
7.4 Hz), 2.76 (1H, dd, J = 13.7 Hz, J = 6.3 Hz), 3.77 (3H, s),
3.98 (1H, dd, J = 11.5 Hz, J = 6.7 Hz), 4.17 (1H, dd, J =
11.5 Hz, J = 4.4 Hz), 6.72 (3H, m), 7.22 (1H, m); 13C NMR
(CDCl3): 18.94, 20.62, 36.44, 36.61, 55.05, 64.99, 112.06,
114.62, 117.73, 121.13, 129.71, 138.94, 159.80, 170.51;
3,4-Bis[(3-hydroxyphenyl)methyl]dihydro-2(3H)-
furanone (enterolactone, 8)
A solution of BBr3
in CH2Cl2 (1 M, 0.421 mL,
0.421 mmol) was added dropwise to a solution of lactone 7
(34.3 mg, 0.105 mmol) in anhydrous CH2Cl2 at 0°C. The so-
lution was stirred at 0°C for 1 h and then at –18°C for 12 h.
Water (1 mL) was added to the solution and the aqueous
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Can. J. Chem. Vol. 77, 1999
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layer was extracted with CH2Cl2 (3 × 5 mL). The combined
organic layers were washed with brine, dried (MgSO4), and
evaporated. The crude product was purified by flash chro-
matography (ethyl acetate:hexane, 3:7) to yield
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25
142°C (lit. (6l) mp 141–143°C). [α]D –38.4 (c 0.37,
23
CHCl3) (lit. (7a) [α]D –38.4 (c 0.25, CHCl3)). IR (KBr):
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1350, 1230, 875, 785, 698 cm-1; 1H NMR (acetone-d6):
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