Synthesis of Kainoids and C4 Derivatives

Article abstract of DOI:10.1021/acs.joc.8b00179

A unified stereoselective synthesis of 4-substituted kainoids is reported. Four kainic acid analogues were obtained in 8-11 steps with up to 54% overall yields. Starting from trans-4-hydroxy-l-proline, the sequence enables a late-stage modification of C4 substituents with sp² nucleophiles. Stereoselective steps include a cerium-promoted nucleophilic addition and a palladium-catalyzed reduction. A 10-step route to acid 21a was also established to enable ready functionalization of the C4 position.

Full text of DOI:10.1021/acs.joc.8b00179

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Note
Synthesis of Kainoids and C4 Derivatives
Zhenlin Tian, and Frederic Menard
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b00179 • Publication Date (Web): 08 May 2018
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The Journal of Organic Chemistry
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Synthesis of Kainoids and C4 Derivatives
Zhenlin Tian and Frederic Menard*
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Department of Chemistry, University of British Columbia, Kelowna, BC, Canada
Supporting Information Placeholder
O
R
CO2H
CO2H
HO
CO2Me
CO2Me
HO
4
4
3 steps
8–11 steps
COOH
N
H
N
N
H
7–54 % yield
Ns
R = Alkenyl, Aryl
4-hydroxyproline
21a
ABSTRACT: A unified stereoselective synthesis of 4-substituted kainoids is reported. Four kainic acid analogs were obtained in 8
to 11 steps with up to 54% overall yields. Starting from trans-4-hydroxy-L-proline, the sequence enables a late-stage modification
of C4 substituents with of sp2 nucleophiles. Stereoselective steps include a cerium-promoted nucleophilic addition and a palladium-
catalyzed reduction. A 10-step route to acid 21a was also established to enable ready functionalization of the C4 position.
Kainoids are a group of non-proteinogenic amino acids dis-
playing strong neurological activity.1 The title compound of
the family, kainic acid (KA, 1) and its epimer allokainic acid
(2) were isolated from the marine algae Digenea simplex in
1953 (Figure 1).2 The selectivity of 1 for a subset of ionotropic
glutamate receptors even led to naming them as kainate recep-
tors (GluK1-5).3 Kainic acid is now commonly used in neuro-
biology research to study conditions such as epilepsy, Hun-
tington’s, Parkinson’s and Alzheimer’s diseases.4 Biological
testing of KA analogs revealed that the neuroactivity of KA is
highly dependent on its C4 substituent. For instance, synthetic
derivatives such as phenylkainic acid (4),5 acromelic acid (5),5
and thiazole analog 66 all display equal or higher potency than
kainic acid. Herein, we report a synthesis of kainoids that al-
lows access to C4 derivatives via late-stage divergence.
kainic acid have been reported.1,2,7-9 The shortest synthesis of
kainic acid was accomplished by Ohshima in six steps,9 how-
ever, it does not allow for ready variation of the C4 position.
Structure-activity relationship studies have confirmed that
the C4 substituent of kainoids is the only position that can be
exploited to enhance the activity of kainic acid.5,11 Compiling
the biological studies conducted with kainic acid analogs over
the past 30 years reveals that only sp2-hybridized C4 substitu-
ents display a high affinity for GluK receptors.12 As part of our
research program in neuron-glia communication, we required
a GluK chemical probe bearing a reporter tag at C4. We rea-
soned that an ideal approach would enable late-stage introduc-
tion of sp2-hybridized nucleophiles at the C4 position. A route
to phenylkainic acid reported by Baldwin in 1995 provided a
potential framework that could be modified for our purpose.10
Accordingly, we developed a pragmatic route that can deliver
large amounts of C4 kainoid derivatives for cell and animal
assays.13
CO2H
CO2H
CO2H
6
4
3
2
CO2H
CO2H
CO2H
N
H
N
H
N
H
The synthesis begins with a two-step protection of trans-4-
hydroxy-L-proline, followed by a Swern oxidation to obtain
pyrrolidinone 10 (Scheme 1).14 The choice of protecting group
for the amine is crucial for the subsequent C3 alkylation lead-
ing to 11.14b Indeed, Lubell and Rapoport’s phenylfluorenyl
(Pf) protecting group 15,16 was unique in achieving enolate al-
kylation regioselectively at the more hindered alpha position
of ketone 10. Initial trials with Boc or Cbz were unproductive,
leading to poor regioselectivity or stereoselectivity. While Pf’s
large steric bulk was essential to control selectivity, it later
offered its share of challenges.
(–)-kainic acid (1)
(+)-allokainic acid (2)
α-isokainic acid (3)
Ki n/a
Ki 4–53 nM
Ki 470 nM
HO2C
NH2
NH
O
S
N
CO2H
CO2H
CO2H
CO2H
CO2H
CO2H
N
H
N
H
N
H
phenylkainic acid (4)
acromelic acid A (5)
6
Ki 0.22–0.45 KA
Ki 1.7–2.7 KA
Ki 5 nM
Figure 1. Naturally occurring (1–3, 5) and synthetic kainoids
(4 and 6). Potency is indicated as absolute value or relative to
kainic acid's.
Alkylation of the C3 ester side chain to obtain the desired
trans diester 11a is highly sensitive to reactions conditions
(Table 1). We selected a tert-butyl ester because it allowed us
to unambiguously assign the stereochemistry of compounds
Kainoids are a popular synthetic target: close to 70 synthe-
ses of kainic acid, allokainic acid, isokainic acid, and phenyl-
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Scheme 1. Synthesis of pyrrolidin-4-one 11a
Stereoselective Addition of sp2 Nucleophiles. Next, we
1
2
3
4
5
6
7
8
turned to the key installation of side chains at the C4 position
of 11a (Scheme 2). When ketone 11a was treated with alkenyl
or aromatic Grignard reagents, only starting material 11 epi-
merized at C3 was recovered. We explored less basic or-
ganocerium reagents to promote the addition. We found that
using CeCl3×2LiCl (1.3 equiv. for 12a, 3.0 equiv. for 12b)
afforded alcohols 12 as single diastereomers with an almost
quantitative yield.17 The presence of LiCl is essential to full
obtain conversion with 11a, and the reaction worked equally
well with i-Pr- and Ph- organometallic reagents. Baldwin et al.
reported a similar strategy using N-Bz proline ketone with
PhMgBr and CeCl3 that afforded a 52% yield.10 However, the
more sterically demanding N-Pf protected 11a led to poor
conversion with CeCl3. For instance, using i-PrMgBr/CeCl3
gave only 5% yield, and PhMgBr/CeCl3 gave 12b in 16%
yield with C3 epimerized. NOE experiments confirmed the
stereochemistry of the C4 centers (Scheme 2).
1) TMSCl, MeOH
O
2) TMSCl, Et3N, DCM; MeOH;
HO
Pb(NO3)2, PfBr, Et3N;
10% citric acid, MeOH
CO2Me
10
N
CO2H
3) DMSO, (COCl)2, Et3N
77% overall
N
H
Pf
9
i. n-BuLi, THF/HMPA,
–78 o
C
O
O
CO2tBu
CO2tBu
ii. Conditions
3
2
CO2Me
N
CO2Me
N
Ph
9
Pf =
Pf
Pf
11a (86%)
10
11
12
13
14
15
16
17
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11b (8%)
Table 1. Stereoselective alkylation of ketone 10
dra
Conver-
Entry
1b
Halide
Conditions
sion (%)a
100
11a:11b
5:1
BrCH2CO2tBu (3.5 eq) –41 ºC, 3h
2c
3c
4c
ICH2CO2tBu (5.0 eq)
ICH2CO2tBu (6.0 eq)
ICH2CO2tBu (3.5 eq)
–41 ºC, 5.5 h
–78 ºC, 5 h
–78 ºC, 5 h
100
60
9:1d
10:1
14:1
Synthesis of Natural Kainoids. Deoxygenation of tertiary
alcohols 12a and 12b posed a significant challenge of
chemoselectivity. Traditional radical-based deoxygenation
methods failed, so we examined the reactivity of the parent
carbonates (Scheme 2). Allylic carbonate 13a was reduced via
a catalytic transfer hydrogenation using ammonium formate
and palladium acetate18 to afford a 3:1 regioisomeric mixture
of allokainic acid precursor 14a and isokainic acid precursor
14b.19
50
a
b
Determined by 1H NMR analysis of crude products. Reaction condi-
tions: 1:5 HMPA/THF; 0.5 equiv. NaI, halide (Method B in supp. info.). c
1:10 HMPA/THF; halide (Method A in supp. info.). After purification,
d
the ratio of isolated products was 10.8:1.
12–14 (reduced conformational flexibility greatly enhances
NOE). Adapting conditions for a related alkylation using a
catalytic amount of NaI14b,c yielded a 5:1 ratio when the reac-
tion was performed on >5 grams scale using BrCH2CO2tBu
(entry 1). Instead, using tert-butyl iodoacetate directly, the
ratio was improved to 14:1, although the conversion was slug-
gish (entry 4). Increasing equivalents and temperature afford-
ed an acceptable compromise in terms of conversion and se-
lectivity (entries 2 and 3). The stereochemistry of 11a and 11b
The cis-C3,C4 diastereomer of 14 (precursor of kainic acid)
was not observed under these reduction conditions. The steric
bulk of the N-Pf protecting group likely prevents the catalyst
from approaching the top face of the allylic carbonate (viz.
path B, Scheme 2). We surmise that competing catalytic path-
ways take place. For instance, the sterically congested sub-
strate 13 may slow the antiperiplanar addition of Pd(0) to the
allylic system, and a Pd(II) complex may act as Lewis acid
toward the carbonate group. The resulting transient allylic
1
was determined by H NMR analysis: the JH2-3 value for the
trans product 11a is 6.1 Hz, and that of 11b is 8.2 Hz.14
Scheme 2. Synthesis of key intermediates
O
Pd0Ln
R
R
LiHMDS;
ClCO2Me
CeCl3⋅2LiCl;
O
13a, Pd(OAc)2
CO2tBu
CO2tBu
CO2tBu
O
HO
MeO
O
O
RMgBr
N
Ph3P, NH4HCO2
N
O
O
O
O
H
CO2Me
CO2Me
CO2Me
THF, –78 o
C
N
N
N
THF, 60 oC
O
O
PdII
OCO2Me
O
O
Pf
Pf
Pf
MeO
11a
12a: R = 2-propenyl, 98%
12b : R = Ph, 95%
13a: R = 2-propenyl, 96%
13b: R = Ph, 95%
B
A
i. Et3SiH, TFA, DCM
ii. Boc2O, DMAP
H
O
PdII
O
N
N
O
Pd0Ln
O O
O
15a, Pd(OAc)2
O O
O
R
Ph3
P
O
O
O
O
PdII
CO2tBu
H
MeO
O
N
NH4HCO2
N
H
O
O O
O
O O
MeO2CO PdII
CO2Me
C
THF, 60 o
C
N
OCO2Me
O
O
Boc
internal
terminal
internal
15a: R = 2-propenyl, 96%
15b: R = Ph, 95%
via A
internal
terminal
CO2tBu
CO2tBu
CO2tBu
CO2tBu
3,4-cis
not
observed
14b not
observed
NOE
NOE
CO2tBu
CO2tBu
H
CO2Me
CO2Me
OH
CO2Me
CO2Me
OH
N
N
N
N
Ph
Hb
Ha
H
H3C
Hb
Pf
Pf
Boc
Boc
CO2Me
H
CO2Me
N
N
Ha
14a, 51%
14b, 16%
H
16b, 46%
16a, 50%
Pf
Pf
NOE
1. Et3SiH, TFA, DCM
2. LiOH aq, CH3OH
1. LiOH aq, CH3OH
2. TFA, DCM
12a
12b
81%
78%
79%
NOE
H3
C
H2
H
H
NOE
C
CO2tBu
CO2H
CO2H
CO2H
CO2H
CO2H
CO2H
Hb
Ha
H
N
CO2Me
Pf
14a
CO2H
N
H
CO2H
N
H
N
H
N
H
dr = 1 : 1.3
1
2
2
3
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The Journal of Organic Chemistry
carbocation then gets reduced from the same face as the leav- are consistent with reported data (Table S1, SI). The ~1:1 mix-
1
2
3
4
5
6
7
8
ing group. To solve this problem, the amine’s Pf protecting
group was replaced by a less bulky Boc. Extensive optimiza-
tion was needed to selectively cleave the Pf group without
losing the tert-butyl ester or epimerizing C3. We found buff-
ered reductive conditions that are compatible with: tertiary
alcohols, allylic carbonates, tert-butyl carbonates, and esters
(summarized in Table 2). We settled on using Et3SiH with
TFA in dichloromethane, which led to quantitative removal of
N-Pf (entries 2 and 3). Alternatively, Et3SiH with I2 in acetoni-
trile was also efficient, but led to the loss of the tert-butyl
group (entry 4). Using Rapoport’s original conditions (excess
TFA)20 removed the Pf group, however the tert-butyl ester was
quite prone to elimination and high yields were difficult to
reproduce (entry 1). With 13 deprotected, the resulting pyrrol-
idine was directly subjected to standard Boc protection condi-
tions to afford carbamates 15a and 15b in excellent yields.
The catalytic hydrogenolysis of allylic carbonate 15a with
Pd(OAc)2 and formate afforded 16a and 16b as a 1.1:1 mix-
ture of C4 epimers. Improving the ratio for kainic acid precur-
sor 16b remained elusive, despite several attempts.
ture of N-Boc protected diesters 16a and 16b was also depro-
tected with a telescoped sequence: alkaline hydrolysis fol-
lowed by anhydrous TFA treatment led to kainic acid (1, 35%)
and allokainic acid (2, 46%). Separation of 1 and 2 by crystal-
lization and HPLC proved challenging. While it was possible
to obtain analytically pure samples for characterization, the
bulk of the products remained a mixture. Regardless, this 1/2
mixture is useful to us—and we expect to others—as it ena-
bles the gram-scale synthesis of thiazole 6 in four steps, cur-
rently the most potent GluK receptor agonist.6
9
10
11
12
13
14
15
16
17
18
19
20
21
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Synthesis of Non-natural Kainic Acid Derivatives. Inspired
by the lactone hydrogenolysis reaction developed by Baldwin
et al.,10 we attempted to invert the C4 stereocenter with hydro-
genation (Scheme 3). Upon full deprotection, carbonate 15b
spontaneously cyclized to lactone 17 (sparingly soluble in
DMSO). This lactone was then ring-opened with inversion of
stereochemistry under standard hydrogenation conditions us-
ing Pd/C. Phenylkainic acid (4) was obtained as a single dia-
stereomer in 90% yield from 17.
Scheme 3. First attempt of phenylkainic acid synthesis
Table 2. Deprotection of N-phenylfluorenyl (N-Pf)
O
1. LiOH aq,
CH3OH
CO2H
Ph
O
Pd/C, H2
Cmpd
Conditions
Yield
Byproduct
PfOH
PfH
15b
Ph
CO2H
CH3OH/AcOH
90%
2. TFA, DCM
90%
N
H
12a/b
90% a
Quant.
Quant.
TFA (15 eq.), DCM, 1 h
CO2H
• TFA
N
H
4
17
12a/b Et3SiH (3 eq.), TFA (5 eq.), DCM, 1 h
13a/b Et3SiH (3 eq.), TFA (5 eq.), DCM, 1 h
Alternatively, we shortened the above path by directly cy-
clizing alcohol 12b to lactone 18a under excess of TFA. Lac-
tone 18 was hydrogenolyzed to yield phenylkainic ester 19 as
a single diastereomer (Scheme 4). Compared to Baldwin’s
synthesis of phenylkainic acid (9 steps, 11% overall yield),10
the route presented herein compares favorably: 4 is obtained in
8 steps with an overall yield of 54%. Stereochemistry of the
PfH
13a/b Et3SiH (2 eq.), I2 (1 eq.), MeCN, 10 min. Quant. b
PfH
(a) ~10% tert-Butyl ester was converted to the parent acid.
(b) tert-Butyl ester was also deprotected under these conditions.
Kainic acid (1), allokainic acid (2), and isokainic acid (3)
were obtained via global deprotection of 16b, 14a and 14b,
respectively (Scheme 2). A telescoped sequence was used for
14a and 14b: the reductive Pf deprotection was followed by a
mildly alkaline hydrolysis. Natural products 2 and 3 were ob-
tained in 78% and 79% yields, respectively, without epimeri-
zation of the C2 or C4 stereocenters. NMR spectra of 2 and 3
1
phenyl group in 4 was confirmed by H NMR spectroscopic
analyses and is consistent with literature data (Table S1,
SI).6b,21 The cis-C3,C4 stereochemistry was further confirmed
by NOE analysis of the subsequent products 20a and 20b
(Scheme 4).
Scheme 4 Synthesis and Functionalization of Phenylkainic Acid. (p-Ns = p-nitrobenzenesulfonyl)
O
H
Pd
R1
CO2H
Ph
CO2H
CO2H
Ph
19a, LiOH aq,
O
O
N
Pd/C, H2
CH3OH
TFA, DCM
quant.
12b
Ph
R2O
CO2Me
86%
N
CH3OH/AcOH
quant.
N
H
O
CO2Me
N
R
19a: R = H
4
R
O
A
DMAP, Et3N,
18a: R = H
19b: R = Boc
Boc2O, DCM
18b: R = Boc
1. 19a, TMSCl,
CH3OH
O
2. DMAP, Et3N, DCM,
CO2Me
CO2Me
Ph
CO2Me
CO2Me
Ph
CO2Me
CO2Me
p-NsCl for 20a
HO
NaIO4, RuCl3
CH3CN/CCl4/H2O
or Boc2O for 20b
NOE
or
O
N
N
N
H
H
X
R2
R
Boc
Ns
CO2Me
20a: R = Ns, 96%
20b: R = Boc, 92%
21a, 78%
H
21b, 85%
N
CO2Me
R1
Et2N
O
O
O
O
O
O
Et2N
O
20a: R1 = Ns, R2 = Ph
20b: R1 = Boc, R2 = Ph
21a: R1 = Ns, R2 = CO2H
H
N
H
N
COOCH3
COOCH3
NH2
N
H
Et3N,HBTU,DCM
92%
N
22
Ns
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Our need for kainic acid-based chemical probes motivated under reduced pressure to afford the crude tert-butyl bromo-
1
2
3
4
5
6
7
8
us to exploit our most efficient route: that leading to interme-
diate 19a in 7 steps (Scheme 4). Chemoselective oxidation of
the C4-phenyl group to a carboxylic acid would provide a
versatile synthetic handle to append any side chain at a late
stage. Accordingly, diester amine 19a was converted to the N-
Boc diester 20b in two steps. However, when 20b was pre-
sented to RuCl3 and NaIO4, the phenyl group remained un-
touched and only the C5 position of the pyrrolidine was oxi-
acetate as a light-yellow liquid (26.9 g, 96%), which was used
for the following reaction. 1H NMR (400 MHz, CDCl3) δ 3.75
(s, 2H), 1.48 (s, 9H) ppm. 13C NMR (101 MHz, CDCl3) δ
166.2, 82.9, 27.8, 27.7 ppm. tert-butyl iodoacetate was pre-
pared according to a literature procedure from the above tert-
butyl bromoacetate.24 tert-butyl bromoacetate (25.2 g, 128
mmol) was added at r.t. to a suspension of NaI (23.1 g, 154
mmol) in acetone (200 mL) under a nitrogen atmosphere; the
resulting suspension was stirred for 5 hours. The insoluble salt
was removed by vacuum filtration. and the filtrate was con-
centrated under reduced pressure. The residue was redissolved
in diethyl ether (150 mL) and washed with 100 mL basic
Na2S2O3 aqueous solution (5% wt. in aqueous saturated sodi-
um bicarbonate), brine (100 mL). The obtained organic layer
was dried over MgSO4, filtered under vacuum. The resulted
solution was concentrated under reduced pressure to afford
tert-butyl iodoacetate as a pale-yellow liquid (29.4 g, 95%).
The crude product tert-butyl iodoacetate was dried over 4 Å
molecular sieve to remove the trace water residue before use.
1H NMR (400 MHz, CDCl3) δ 3.61 (s, 2H), 1.46 (s, 9H) ppm.
13C NMR (101 MHz, CDCl3) δ 167.8, 82.3, 27.6, -2.6 ppm.
dized to pyrrolidinone 21b. Instead of Boc,
a
p-
9
nitrobenzenesulfonyl (Ns) protecting group was selected for
its ability to deactivate the N-alpha position and its mild
deprotection condition (ArSH, K2CO3, DMF).22 Thus 19a was
converted to the N-Ns amide 20a in two steps and almost
quantitative yield. The phenyl group of 20a was oxidized se-
lectively and afforded the desired acid 21a in 78% yield with-
out epimerization (stereochemistry confirmed by NOE analy-
sis).
10
11
12
13
14
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16
17
18
19
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The C4-acid kainoid 21a provides easy access to a variety
of kainoid derivatives. It enables the synthesis of numbers of
heteroaromatic kainoids.23 In addition, our ongoing work to-
ward kainoid-based GluK imaging probes confirms that 21a
can be readily coupled to fluorescent dyes, further studies will
be reported in due course.
9-Bromo-9-phenylfluorene (PfBr). To a suspension of mag-
nesium turnings (9.2 g, 382 mmol) in anhydrous THF (5 mL)
under argon atmosphere, 0.5 mL bromobenzene was added
neat at r.t. in one portion to initiate the reaction. The reaction
In summary, a unified diastereoselective route to 4-
substituted kainoids was demonstrated from commercially
available trans-4-hydroxy-L-proline. The sequence affords:
kainic acid (1, 11 steps, 19%), allokainic acid (2, 9 steps,
24%), isokainic acid (3, 9 steps, 8%), and phenylkainic acid
(4, 8 steps, 54%). In addition, a mild oxidaton step has been
developed for C4 derivatization from 4. The novel intermedi-
ate 21a (10 steps, 47%) enables the rapid synthesis of a variety
of non-natural kainoids via amide coupling. This report pro-
vides a general access to a range of biologically active 4-
substituted kainoids.
o
vessel was cooled to 0 C on an ice bath, followed by drop-
wise addition of a THF solution of bromobenzene (1.2 M, 270
mL) at 0 oC. The resulted reaction solution was allowed to r.t.
and stirred vigorously for an additional 2 h. The exact concen-
tration of the resulted phenyl magnesium bromide solution
was determined by titration with salicylaldehyde phenylhydra-
zone.25 This phenyl magnesium bromide solution (180 mL,
216 mmol) was transferred into a flame-dried single neck flask
by a syringe. To this solution, CeCl3×2LiCl (15 mL, 0.1 M)
THF solution was added in one portion, 9-fluorenone (30.1 g,
186 mmol) was added by portions. 15 mins later, TLC analy-
sis showed full conversion. The reaction was quenched with
diluted hydrochloric acid (1.0 M, 100 mL) at 0 oC. The result-
ed solution was extracted with Et2O (3´150 mL). The com-
bined organic layer was washed with brine, dried over MgSO4,
filtered and was concentrated under reduced pressure to yield
EXPERIMENTAL SECTION
General Information. NMR spectra were acquired on a 400
MHz Varian NMR AS400 equiped with an ATB-400 probe at
25 °C. Infrared spectra (IR) were obtained using a Nicolet
6700 FT-IR spectrometer (compounds 9-16) as a neat film on
a NaCl plate, or a Perkin-Elmer FT-IR Spectrum Two IR spec-
trometer. High resolution mass spectrometry analyses record-
ed with a HCTultra PTM Discovery System, or with a Waters
Micromass LCT Premier TOF Mass Spectrometer. Melting
points of solid samples were measured with a IA9200 melting
point apparatus (Electrothermal). Column chromatography
was carried on silica gel (230-400, Silicycle, Quebec). The
mixture of kainic acid and allokainic acid was purified using a
Varian ProStar HPLC system with a C18 column (Eclipse
Plus C18, 4.6 ´ 250 mm, 5 µm, Agilent).
o
PfOH as pale-yellow crystals (40.8 g, 95%). Mp 108-109 C.
FTIR (thin film): 3544, 3423, 3059, 1603, 1448, 774, 732,
1
701cm-1. H NMR (400 MHz, CDCl3) δ 7.75 (d, J = 7.5 Hz,
2H), 7.53 – 7.41 (m, 4H), 7.34 (dq, J = 15.1, 7.5 Hz, 7H), 2.79
(s, 1H) ppm. 13C NMR (101 MHz, CDCl3) δ 150.4, 143.2,
139.5, 129.0, 128.4, 128.2, 127.2, 125.4, 124.8, 120.1, 83.6
ppm. PfBr was prepared according to the reported proce-
dure.26 The above PfOH (36.1 g, 140 mmol) was dissolved in
toluene (150 mL) and aqueous HBr (48% w/w, 50 mL) was
added at r.t.. This heterogeneous mixture was stirred vigorous-
ly at r.t. for 48 h, away from light. The mixture was then ex-
tracted with toluene (3 × 100 mL). The combined organic
layer was washed with brine (6 × 150 mL), dried over MgSO4,
filtered, and was concentrated under reduced pressure to af-
ford the crude product as a light-yellow solid. Recrystalliza-
tion with hexane afforded pale-yellow crystals (40.2 g, 90%).
tert-Butyl iodoacetate. To a rigorously stirred solution of
magnesium sulfate (52.0 g, 432 mmol) in dichloromethane
(300 mL) were added concentrated sulfuric acid (3.8 mL, 72
mmol) slowly. After 10 mins, bromoacetic acid (20.0 g, 144
mmol) was added at r.t., followed by addition of tert-butyl
alcohol (41.2 mL, 432 mmol). After 24 h, a second portion of
tert-butyl alcohol (20.5 mL, 216 mmol) was added to take the
reaction to completion in an additional 24 h. The insoluble
matter in resulted reaction solution was removed by vacuum
filtration. The filtrate was transferred to a separatory funnel,
and washed with 200 mL water, 200 mL saturated sodium
bicarbonate. The aqeous layer was extracted with dichloro-
methane (3×50 mL). The combined organic layer was washed
with brine and dried over MgSO4. The solvent was evaporated
o
Mp 100-101 C. FTIR (thin film): 3056, 1603, 837, 738, 694
1
cm-1. H NMR (400 MHz, CDCl3) δ 7.67 (dt, J = 7.6, 0.9 Hz,
2H), 7.60 – 7.49 (m, 4H), 7.36 (td, J = 7.5, 1.2 Hz, 2H), 7.35 –
7.18 (m, 5H) ppm. 13C NMR (101 MHz, CDCl3) δ 149.6,
141.1, 138.1, 129.0, 128.5, 128.3, 128.0, 127.4, 126.1, 120.3,
67.5 ppm.
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The Journal of Organic Chemistry
(2S,4R)-4-hydroxypyrrolidine-2-carboxylic acid methyl
CDCl3) δ 7.70 (dd, J = 7.6, 1.0 Hz, 2H), 7.50 (dt, J = 7.6, 1.6
Hz, 3H), 7.43 (dt, J = 7.5, 0.8 Hz, 1H), 7.43 – 7.33 (m, 2H),
7.32 – 7.21 (m, 5H), 3.78 (d, J = 17.6 Hz, 1H), 3.45 (d, J =
17.6 Hz, 1H), 3.44 (d, J = 6.1 Hz, 1H), 3.10 (s, 3H), 2.82 (dd,
J = 12.4, 6.2 Hz, 1H), 2.45 – 2.38 (m, 2H), 1.31 (s, 9H) ppm.
13C NMR (101 MHz, CDCl3) δ 211.8, 172.9, 169.6, 146.0,
144.4, 141.4, 141.1, 140.4, 128.9, 128.5, 127.9, 127.8, 127.7,
127.3, 127.0, 126.0, 120.2, 120.0, 81.4, 75.8, 63.7, 55.7, 51.5,
49.2, 34.2, 27.9 ppm. HRMS (ESI-TOF) m/z [M+H]+ Calcd
for C31H32NO5 498.2280; Found 498.2278.
1
2
3
4
5
6
7
8
ester hydrochloride (9a). trans-4-hydroxyl-L-proline (10.0 g,
76 mmol) was suspended in methanol (150 mL) under argon.
The suspension was cooled to 0 oC in an ice bath, followed by
dropwise addition of chlorotrimethylsilane (33.9 mL, 267
mmol). After 30 min., it was allowed to warm up to r.t. and
stirred overnight. The solvent was removed under reduced
pressure, the residue was triturated with Et2O (100 mL) and
1
the ester 9a was obtained as white crystals (13.1 g, 95%). H
NMR (400 MHz, D2O) δ: 4.78–4.68 (m, 2H), 3.88 (s, 3H),
3.56 (dd, J = 12.6, 3.8 Hz, 1H), 3.44 (dt, J = 12.5, 1.6 Hz, 1H),
2.52 (ddt, J = 14.3, 7.8, 1.8 Hz, 1H), 2.32 (ddd, J = 14.5, 10.5,
4.3 Hz, 1H) ppm. 13C NMR (101 MHz, D2O) δ 170.2, 69.4,
58.1, 53.8, 53.4, 36.6 ppm. HRMS (ESI-TOF) m/z [M+H]+
Calcd for C6H12NO3 146.0817; Found 146.0817.
9
cis-Ketodiester 11b (minor, 621 mg, 8%):FTIR (thin film):
2974, 1758, 1727, 748, 694 cm-1.1H NMR (400 MHz, CDCl3)
δ 7.76 – 7.71 (m, 1H), 7.68 (d, J = 7.6 Hz, 1H), 7.41 (ddt, J =
9.0, 5.1, 1.6 Hz, 5H), 7.33 (dtd, J = 17.9, 7.4, 1.1 Hz, 3H),
7.24 (dt, J = 7.0, 1.1 Hz, 3H), 4.02 (d, J = 8.2 Hz, 1H), 3.80
(d, J = 17.6 Hz, 1H), 3.58 (dd, J = 17.6, 1.1 Hz, 1H), 3.15 (s,
3H), 3.11 – 3.01 (m, 1H), 2.57 (dd, J = 17.4, 5.2 Hz, 1H), 1.84
(dd, J = 17.4, 8.9 Hz, 1H), 1.34 (s, 9H) ppm. 13 C NMR (101
MHz, CDCl3) δ 212.2, 171.7, 170.3, 146.9, 145.3, 141.7,
141.3, 139.8, 128.9, 128.8, 128.6, 128.1, 127.7, 127.6, 126.8,
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Methyl (2S)-4-oxo-1-(9-phenyl-9H-fluoren-9-yl)pyrrolidine-
2-carboxylate (10). 10 was prepared according to reported
procedures from 9a by two steps with an overall yield of
81%.19a FTIR (thin film): 3060, 2949, 2838, 1758, 1739, 745,
1
701 cm-1. H NMR (400 MHz, CDCl3) δ 7.83 – 7.62 (m, 2H),
7.49 – 7.40 (m, 3H), 7.44 – 7.35 (m, 3H), 7.35 – 7.16 (m, 5H),
3.76 (d, J = 17.8 Hz, 1H), 3.76 (dd, J = 8.6, 2.9 Hz, 1H), 3.48
(dt, J = 17.8, 1.2 Hz, 1H), 3.20 (s, 3H), 2.44 (dd, J = 18.1, 8.6
Hz, 1H), 2.29 (dd, J = 18.1, 2.9 Hz, 1H) ppm. 13C NMR (101
MHz, CDCl3) δ 212.9, 173.1, 146.5, 145.3, 141.8, 140.9,
140.3, 128.9, 128.9, 128.6, 128.0, 127.7, 127.6, 127.0, 126.9,
125.5, 120.3, 120.1, 76.0, 58.2, 55.2, 51.5, 51.5, 41.6 ppm.
HRMS (ESI-TOF) m/z [M+H]+ Calcd for C25H22NO3
384.1600; Found 384.1602.
126.7, 125.4, 120.1, 81.1, 75.4, 61.9, 53.7, 51.1, 48.0, 31.1,
+
28.0 ppm. HRMS (ESI-TOF) m/z (M+H)
Calcd for C
31H32NO5 498.2280; Found 498.2275.
Methyl (2S,3R,4S)-3-(2-(tert-butoxy)-2-oxoethyl)-4-hydroxy-
1-(9-phenyl-9H-fluoren-9-yl)-4-(prop-1-en-2-yl)pyrrolidine-2-
carboxylate (12a). A single-neck flask was charged with mag-
nesium turnings (322 mg, 13.1 mmol) and dry THF (20 mL)
under argon atmosphere, followed by dropwise addition of 2-
bromopropene (1.10 mL, 12.4 mmol) at rt. The suspension
was stirred for 2 h, a gradually turned to a pale-yellow solu-
tion. The resulting isopropenyl magnesium bromide solution
was titrated using salicylaldehyde phenylhydrazone.26
Methyl
phenyl-9H-fluoren-9-yl)pyrrolidine-2-carboxylate (11a) and
Methyl (2S,3S)-3-[2-(tert-butoxy)-2-oxoethyl]-4-oxo-1-(9-
(2S,3R)-3-[2-tert-butoxy]-2-oxoethyl]-4-oxo-1-(9-
Into a single-neck flask, ketone 11a (4.105 g, 8.25 mmol)
was loaded, purged with argon, and a CeCl3×2LiCl solution in
THF (107 mL, 0.1 M) was added via syringe at r.t. The clear
solution was cooled to –78 ºC and stirred for 15 mins. The i-
PrMgBr solution (19.5 mL, 0.55 M) was added dropwise at –
78 ºC. After 2 h, the reaction was quenched at –78 ºC with a
diluted 0.1 M HCl aqueous solution (10 mL). The biphasic
mixture was extracted with Et2O (2 ´ 100 mL), washed with
brine, dried over MgSO4 and filtered. The solvent was evapo-
rated under reduced pressure to afford tertiary alcohol 12a as a
white foam-like solid (4.305 g, 98%), which was used without
further purification. FTIR (thin film): 3510, 3060, 2978, 2949,
phenyl-9H-fluoren-9-yl)pyrrolidine-2carboxylate (11b).
Method A: Ketone 10 (6.0 g, 16 mmol) was dissolved in THF
(41.0 mL) and anhydrous HMPA (4.1 mL) in a single-neck
flask, and under an Ar atmosphere. The solution was cooled to
–78 ºC on a dry ice/acetone bath, followed by a dropwise ad-
dition of n-butyllithium (2.1 M in cyclohexane, 7.8 mL, 16
mmol). After 30 mins, tert-butyl iodoacetate (18.9 g, 79
mmol) was added dropwise to the reaction. The solution was
o
allowed to wamr up to -41 C (dry ice/acetonitrile bath) and
stirred for an additional 5.5 h. The reaction was quenched at -
41 °C by a quick addition of phosphoric acid (10% wt., 15
mL) and allowed warm up to r.t. Water (50 mL) was added
and the mixture was extracted with Et2O (3 × 100 mL). The
combined organic layers were washed with brine, dried over
MgSO4 and filtered. The solvents were evaporated under re-
duced pressure to yield the alkylated products 11a and 11b.
The diastereomeric ratio was determined with 1H NMR of the
crude product; dr = 9:1 with ketones 11a as the major isomer.
The crude products from method A were retaken in MeOH (5
mL) and stored at –20 ºC overnight in a freezer. The white
solid formed was filtered and washed with cold methanol (5
mL) to yield 11a as white solid (6.2 g). The mother liquor's
solvents were removed, and the residue was purified by col-
umn chromatography on silica gel (15-25% EtOAc/hexane as
elution gradient) to yield 11a (525 mg) and 11b (621 mg, 8%)
as white foam-like solid.
1
2885, 1715, 1706, 788, 745, 705 cm-1. H NMR (400 MHz,
CDCl3) δ: 7.74 (d, J = 7.6 Hz, 1H), 7.67–7.50 (m, 4H), 7.44 (t,
J = 7.5 Hz, 1H), 7.39–7.28 (m, 5H), 7.23–7.09 (m, 2H), 5.13
(s, 1H), 4.91 (s, 1H), 3.72 (d, J = 12.1 Hz, 1H), 3.18 (s, 3H),
3.07 (d, J = 3.9 Hz, 1H), 3.06 (d, J = 17.7 Hz, 1H), 2.85 (dt, J
= 9.7, 6.2 Hz, 1H), 2.22 (dd, J = 16.3, 5.4 Hz, 1H), 1.99 (dd, J
= 16.3, 6.7 Hz, 1H), 1.82 (s, 3H), 1.78 (s, 1H), 1.30 (s, 9H)
ppm. 13C NMR (101 MHz, CDCl3) δ: 174.9, 171.5, 146.8,
146.4, 144.6, 142.8, 141.7, 139.7, 129.0, 128.4, 128.3, 127.9,
127.7, 127.4, 127.3, 127.0, 126.2, 120.1, 119.7, 113.0, 81.9,
80.6, 66.3, 61.2, 51.3, 47.0, 32.3, 29.7, 27.9, 19.6 ppm. HRMS
(ESI) m/z [M+H]+ Calcd for C34H38NO5 540.2744; Found
540.2730.
Methyl (2S,3R,4S)-3-(2-(tert-butoxy)-2-oxoethyl)-4-hydroxy-
4-phenyl-1-(9-phenyl-9H-fluoren-9-yl)pyrrolidine-2-
Method B: Adapted from a reported procedure14; BrCH2
COOtBu (3.5 eq) /NaI (0.5 eq). The alkylated ketones 11a and
11b were recovered with a dr = 5:1 (1H NMR analysis of the
crude).
carboxylate (12b). Proceeded as described for 12a with the
following modifications: 11a (2.530 g, 5.08 mmol), phenyl
magnesium bromide (12 mL, 1.1 M), and CeCl3×2LiCl (127
mL, 0.1 M). The crude product was purified by silica-gel
chromatography with ethyl acetate/hexanes as eluent (15-25%
trans-Ketodiester 11a (major: 6.725 g, 86%): FTIR (thin
film): 2974, 1758, 1730, 742, 698 cm-1.1H NMR (400 MHz,
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The Journal of Organic Chemistry
gradient). Alcohol 12b was recovered as a white foam-like (9-phenyl-9H-fluoren-9-yl)-4-(propan-2-ylidene)pyrrolidine-
Page 6 of 10
1
2
3
4
5
6
7
8
solid (2.780 g, 95%). FTIR (thin film): 3506, 3059, 2977,
2946, 2879, 1729, 742, 704 cm-1. 1H NMR (400 MHz, CDCl3)
δ: 7.75 (ddd, J = 7.5, 1.2, 0.6 Hz, 1H), 7.67–7.62 (m, 2H),
7.59–7.53 (m, 4H), 7.45 (dt, J = 7.5, 1.1 Hz, 1H), 7.40–7.36
2-carboxylate (14b). Allylic carbonate 13a (621 mg, 1.36
mmol), palladium acetate (15 mg, 0.066 mmol), triphenyl
phosphine (71 mg, 0.27 mmol), and ammonium formate (427
mg, 6.78 mmol) and THF (10 mL) were charged into a single-
neck flask equipped with a condenser. The flask was purged
with three vaccum/argon cycles. The reaction vessel was heat-
ed at 60 ºC with stirring until full conversion was obsered by
TLC (~5 h). The reaction mixture was filtered on a Celite pad
and washed with Et2O (20 mL). The filtrate was concentrated
under reduced pressure to a brown oil. The crude oil was puri-
fied by silica-gel chromatography with methyl tert-butyl
ether/hexane as eluent (15-25% gradient). The reduced
diesters were recovered as white foam-like solids: 14a (263
mg, 51%), and 14b (82 mg, 16%). Diester 14a: FTIR (thin
(m, 2H), 7.35–7.28 (m, 4H), 7.25–7.24 (m, 1H), 7.24–7.22
(m, 1H), 7.22–7.19 (m, 1H), 7.19–7.16 (m, 1H), 3.86 (d, J =
12.1 Hz, 1H), 3.35 (d, J = 12.1 Hz, 1H), 3.22 (d, J = 12.0 Hz),
3.20 (s, 3H), 3.09–3.01 (m, 1H), 2.20 (dd, J = 16.3, 5.6 Hz,
1H), 2.08 (dd, J = 16.3, 6.7 Hz, 1H), 1.18 (s, 9H) ppm. 13C
NMR (101 MHz, CDCl3) δ: 174.9, 171.4, 146.7, 146.5, 142.8,
141.8, 141.6, 139.8, 129.0, 128.5, 128.4, 128.2, 127.9, 127.8,
127.4, 127.3, 127.1, 127.1, 126.1, 125.6, 120.2, 119.8, 81.1,
80.7, 66.4, 65.3, 51.4, 51.1, 32.5, 27.7 ppm. HRMS (ESI-TOF)
m/z [M+H]+ Calcd for C37H38NO5 576.2744; Found 576.2738.
Methyl (2S,3R,4S)-3-(2-(tert-butoxy)-2-oxoethyl)-4-((meth-
oxycarbonyl)oxy)-1-(9-phenyl-9H-fluoren-9-yl)-4-(prop-1-en-
2-yl)pyrrolidine-2-carboxylate (13a). Allylic alcohol 12a (3.8
g, 7.0 mmol) was dissolved in THF (40 mL) under an argon
atmosphere, cooled to –78 ºC. Lithium bis(trimethylsilyl)-
amide solution (8.5 mL, 1.0 M) was added dropwise, and the
reaction was stirred for 20 min. Methyl chloroformate (1.6 mL,
21.1 mmol) was added slowly and allowed to react for 2 h.
The reaction was quenched at –78 ºC with saturated ammoni-
um chloride aqueous solution (20 mL), extracted with Et2O (2
´ 100 mL). The combined organic layer was washed with
brine, dried over MgSO4 and filtered. The solvent was evapo-
rated under reduced pressure to obtain carbonate 13a as a
white foam-like solid. (4.1 g, 95%). Purity of the crude prod-
uct was excellent, and 13a was used as is for the next step.
FTIR (thin film): 2978, 2950, 1753, 1731, 786, 745, 702 cm-1.
1H NMR (400 MHz, CDCl3) δ: 7.70 (d, J = 7.5 Hz, 1H), 7.62
(d, J = 7.5 Hz, 1H), 7.57–7.49 (m, 2H), 7.44 (d, J = 7.6 Hz,
1H), 7.41–7.34 (m, 2H), 7.33–7.26 (m, 1H), 7.24–7.09 (m,
5H), 4.96 (s, 1H), 4.87 (s, 1H), 3.98 (d, J = 12.9 Hz, 1H), 3.68
(s, 3H), 3.65–3.68 (m, 1H), 3.18 (d, J = 8.5 Hz, 1H), 3.15 (s,
3H), 2.80 (dt, J = 8.1, 4.7 Hz, 1H), 2.42 (dd, J = 16.8, 4.8 Hz,
1H), 2.07 (dd, J = 16.9, 7.8 Hz, 1H), 1.90 (s, 3H), 1.31 (s, 9H)
ppm. 13C NMR (101 MHz, CDCl3) δ: 174.0, 170.8, 153.6,
146.2, 146.1, 142.5, 141.9, 141.1, 140.1, 128.6, 128.4, 128.3,
128.1, 127.4, 127.4, 127.3, 126.9, 126.0, 120.0, 119.7, 90.1,
80.5, 66.0, 55.6, 54.4, 51.3, 48.2, 32.5, 27.9, 20.4 ppm.
HRMS (ESI-TOF) m/z [M+H]+ Calcd for C36H40NO7
598.2805; Found 598.2797.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1
film): 3059, 2977, 2930, 2855, 1728, 741, 702, 638 cm-1. H
NMR (400 MHz, CDCl3) δ: 7.74 (dd, J = 7.6, 1.1 Hz, 1H),
7.60 (d, J = 7.5 Hz, 1H), 7.58–7.48 (m, 3H), 7.44 (dt, J = 7.5,
1.1 Hz, 1H), 7.37–7.27 (m, 2H), 7.25–7.17 (m, 2H), 7.11 (dt, J
= 7.5, 1.1 Hz, 1H), 4.75 (s, 2H), 3.43 (t, J = 11.1 Hz, 1H),
3.38–3.29 (m, 1H), 3.22 (s, 3H), 2.73–2.67 (m, 1H), 2.60
(dddd, J = 14.3, 8.9, 7.0, 5.2 Hz, 1H), 2.27 (dt, J = 11.0, 7.9
Hz, 1H), 2.07 (dd, J = 16.1, 5.2 Hz, 1H), 1.96 (dd, J = 16.1,
7.0 Hz, 1H), 1.69 (s, 3H), 1.29 (s, 9H) ppm. 13C NMR (101
MHz, CDCl3) δ: 175.4, 170.7, 147.6, 146.8, 143.9, 142.7,
142.1, 139.3, 128.7, 128.3, 128.2, 128.0, 127.3, 127.2, 127.2,
127.1, 125.7, 120.0, 119.7, 113.5, 80.3, 67.5, 55.1, 52.2, 51.3,
43.9, 37.3, 27.9, 19.0 ppm. HRMS (ESI) m/z (M+H)+ Calcd.
for C34H38NO4 524.2801; Found 524.2805. Diester 14b: FTIR
(thin film): 3061, 2978, 2934, 2867, 1732, 782, 738, 705 cm-1.
1H NMR (400 MHz, CDCl3) δ: 7.66 (dd, J = 12.5, 7.5 Hz,
2H), 7.53-7.42 (m, 3H), 7.42-7.27 (m, 4H), 7.25–7.16 (m,
5H), 4.01 (d, J = 12.5 Hz, 1H), 3.76 (d, J = 12.5 Hz, 1H),
3.38–3.25 (m, 1H), 3.02 (s, 3H), 2.93–2.81 (m, 1H), 2.44–2.23
(m, 2H), 1.62 (s, 3H), 1.57 (s, 3H), 1.33 (s, 9H) ppm. 13C
NMR (101 MHz, CDCl3) δ: 173.9, 171.2, 147.8, 146.1, 142.7,
140.9, 139.8, 133.2, 128.4, 128.3, 128.2, 127.7, 127.4, 127.3,
127.1, 126.9, 125.6, 123.4, 119.9, 119.7, 80.6, 65.0, 50.7,
49.2, 43.0, 40.1, 27.9, 21.3, 20.4 ppm. HRMS (ESI) m/z
[M+H]+ Calcd for C34H38NO4 524.2801; Found 524.2809.
1-(tert-butyl) 2-methyl (2S,3R,4S)-3-(2-(tert-butoxy)-2-oxo-
ethyl)-4-((methoxycarbonyl)oxy)-4-(prop-1-en-2-yl)pyrrolidi-
ne-1,2-dicarboxylate (15a). N-Pf protected allylic carbonate
13a (3.010 g, 5.04 mmol) was dissolved in dichloromethane
(50 mL) at rt under argon. Triethyl silane (2.4 mL, 15 mmol)
was added, followed by the dropwise addition of trifluoroace-
tic acid (1.9 mL, 25 mmol). The reaction was stirred at rt for 1
h. Solvents were evaporated in vacuo; the TFA ammonium
salt was recovered as a light yellow waxy solid. The residue
was retaken in dichloromethane (50 mL). Di-tert-butyl dicar-
bonate (1.8 g, 8.1 mmol) and 4-dimethylaminopyridine (1.5 g,
10 mmol) were added sequentially, and the reaction was al-
lowed to stir at rt overnight. The reaction mixture was concen-
trated to dryness under reduced pressure. The residue was
purified by column chromatography using ethyl ace-
tate/hexanes as eluent (15-25% gradient). N-Boc carbonate
15a was recovered as a colorless oil (2.226 g, 97%). FTIR
Methyl
(2S,3R,4S)-3-(2-(tert-butoxy)-2-oxoethyl)-4-
((methoxycarbonyl)oxy)-4-phenyl-1-(9-phenyl-9H-fluoren-9-
yl)pyrrolidine-2-carboxylate (13b). Proceeded as described for
13a. Allylic alcohol (450 mg, 0.782 mmol) afforded carbonate
13b (472 mg, 95%) as a foam-like solid. FTIR (thin film):
3060, 2977, 2952, 1749, 1733, 742, 701 cm-1. H NMR (400
1
MHz, CDCl3) δ: 7.72 (ddd, J = 7.6, 1.1, 0.6 Hz, 1H), 7.65–
7.58 (m, 3H), 7.52–7.48 (m, 1H), 7.45–7.30 (m, 7H), 7.30–
7.24 (m, 4H), 7.24–7.15 (m, 2H), 4.34 (d, J = 13.0 Hz, 1H),
3.92 (d, J = 13.0 Hz, 1H), 3.67 (s, 3H), 3.28 (d, J = 8.2 Hz,
1H), 3.12 (s, 3H), 2.87 (dt, J = 8.0, 5.0 Hz, 1H), 2.38 (dd, J =
16.8, 5.0 Hz, 1H), 2.13 (dd, J = 16.8, 7.9 Hz, 1H), 1.22 (s, 9H)
ppm. 13C NMR (101 MHz, CDCl3) δ: 173.8, 170.3, 153.7,
146.2, 146.0, 142.4, 141.3, 140.1, 139.2, 128.7, 128.5, 128.4,
128.4, 128.1, 127.6, 127.5, 127.4, 127.3, 126.9, 126.1, 125.0,
120.1, 119.7, 89.5, 80.4, 66.4, 57.8, 54.5, 52.7, 51.3, 32.5,
27.9 ppm. HRMS (ESI) m/z [M+H]+ Calcd for C39H40NO7
634.2799; Found 634.2802.
1
(thin film): 2979, 1758, 1730, 1707, 850, 791, 770 cm-1. H
NMR (400 MHz, CDCl3, two rotamers) δ: 5.03 (s, 1H), 4.88
(s, 1H), 4.44 (d, J = 13.2 Hz, 1H), 4.00 (dd, J = 26.5, 9.6 Hz,
1H), 3.87–3.79 (m, 2H), 3.71 (s, 3H), 3.68 (s, 3H), 2.84–2.73
(m, 1H), 2.62 (dd, J = 17.5, 4.3 Hz, 1H), 2.39 (ddd, J = 17.6,
14.9, 7.6 Hz, 1H), 1.81 (s, 3H), 1.46–1.29 (m, 19H) ppm. 13C
NMR (101 MHz, CDCl3, two rotamers) δ: 172.5, 172.4, 170.7,
170.6, 154.2, 153.5, 153.4, 153.3, 139.5, 139.4, 114.1, 90.5,
Methyl (2S,3S,4R)-3-(2-(tert-butoxy)-2-oxoethyl)-1-(9-phen-
yl-9H-fluoren-9-yl)-4-(prop-1-en-2-yl)pyrrolidine-2-carboxyl-
ate (14a) and methyl (2S,3S)-3-(2-(tert-butoxy)-2-oxoethyl)-1-
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The Journal of Organic Chemistry
89.7, 81.0, 80.7, 63.5, 63.0, 54.8, 54.7, 52.5, 52.3, 52.2, 52.1,
11.8, 10.6 Hz, 1H), 2.86 (dt, J = 10.5, 8.5 Hz, 1H), 2.72–2.59
(m, 2H), 2.41–2.32 (m, 1H), 1.73 (bs, 3H) ppm. 13C NMR
(101 MHz, D2O) δ: 179.3, 173.7, 140.6, 114.6, 64.9, 51.5,
48.2, 42.6, 39.7, 17.7 ppm. HRMS (ESI-TOF) m/z [M+H]+
Calcd for C10H16NO4 214.1074; Found 214.1074.
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2
3
4
5
6
7
8
47.2, 46.6, 31.9, 31.6, 28.3, 28.1, 20.5, 20.4 ppm. HRMS
(ESI-TOF) m/z [M+Na]+ Calcd for C22H35NO9Na 480.2204;
Found 480.2221.
1-(tert-Butyl) 2-methyl (2S,3R,4S)-3-(2-(tert-butoxy)-2-
oxoethyl)-4-((methoxycarbonyl)oxy)-4-phenylpyrrolidine-1,2-
dicarboxylate (15b). Proceeded as described for 15a. Allylic
carbonate 13b (451 mg, 0.712 mmol) afforded N-Boc car-
bonate 15b as a colorless liquid (331 mg, 94%). FTIR (thin
Isokainic acid (3). Proceeded as described for 2. Diester 14b
(50 mg, 0.95 mmol) was fully deprotected to afford isokainic
3 acid as a white crystalline solid (16 mg, 79%). MP 241–244
oC. FTIR (thin film): 3421, 3182, 2920, 1732，1554, 706 cm-
1
film): 2974, 1755, 1727, 1705, 843,764, 698 cm-1. H NMR
1
1. H NMR (400 MHz, D2O) δ: 4.16 (d, J = 1.6 Hz, 1H), 4.07
9
(400 MHz, CDCl3) δ: 7.41–7.35 (m, 2H), 7.34–7.28 (m, 3H),
4.66 (d, J = 13.3 Hz, 1H), 4.27 (t, J = 13.0 Hz, 1H), 4.16 (dd, J
= 23.4, 9.5 Hz, 1H), 3.75 (s, 3H), 3.70 (s, 3H), 2.89 (ddd, J =
9.6, 7.8, 4.3 Hz, 1H), 2.61–2.43 (m, 2H), 1.45 (s, 9H), 1.31 (s,
9H) ppm.13C NMR (101 MHz, CDCl3) δ: 172.3, 172.2, 170.1,
170.0, 154.1, 153.5, 153.4, 153.3, 136.2, 128.8, 128.8, 128.2,
128.2, 124.7, 124.7, 90.0, 89.3, 80.9, 80.9, 80.7, 80.6, 64.0,
63.5, 54.8, 54.7, 53.9, 53.6, 52.4, 52.2, 52.0, 51.4, 31.7, 31.4,
30.9, 28.3, 28.2, 28.0, 27.9 ppm. HRMS (ESI-TOF) m/z
[M+H]+ Calcd. for C25H36NO9 494.2385; Found 494.2408.
1-(tert-Butyl) 2-methyl (2S,3S,4R)-3-(2-(tert-butoxy)-2-
oxoethyl)-4-(prop-1-en-2-yl)pyrrolidine-1,2-dicarboxylate
(16a) and 1-(tert-Butyl) 2-methyl (2S,3S,4S)-3-(2-(tert-but-
oxy)-2-oxoethyl)-4-(prop-1-en-2-yl)pyrrolidine-1,2-dicarboxy-
late (16b). Proceeded as described for 14a and 14b. Allylic
carbonate 15a (2.010 g, 4.84 mmol) afforded a crude oil which
was purified by silica-gel chromatography with ethyl ace-
tate/hexane as eluent (15-25% gradient). Diesters 16a and 16b
were recovered as an inseparable mixture: cololess oil (1.592
g, 96%). The combined diastereomers were used as is for the
next step. HRMS (ESI) m/z [M+Na]+ Calcd for C20H33NO6Na
406.2200; Found 406.2232.
(d, J = 14.6, 1H), 3.96 (dd, J = 14.6, 1.9 Hz, 1H), 3.55 (t, J =
6.9 Hz, 1H), 2.51 (dd, J = 14.7, 6.9 Hz, 1H), 2.44 (dd, J =
14.6, 6.9 Hz, 1H), 1.74 (bs, 3H), 1.65 (bs, 3H) ppm. 13C
NMR (101 MHz, D2O) δ: 179.7, 173.7, 129.8, 125.7, 66.4,
46.7, 42.1, 41.2, 20.4, 20.3 ppm. HRMS (ESI-TOF) m/z
[M+H]+ Calcd for C10H16NO4 214.1074; Found 214.1075.
Kainic acid (1) and allo-Kainic acid (2). The diastereomeric
mixture of 16a and 16b (605 mg, 1.46 mmol) was dissolved in
methanol (20 mL), and a LiOH aqueous solution (2.5 N, 9.4
mL) was added and stirred at rt for 5 h. The reaction was neu-
tralized at 0 ºC with diluted 1.0 N HCl aqueous solution, ex-
tracted with ethyl acetate (3 ´ 50 mL). The combined organic
layer was dried over MgSO4, and filtered. The solvents were
evaporated under reduced pressure to obtain a colorless oil.
The crude oil was retaken in dichloromethane (5.0 mL) under
Ar atmosphere, and trifluoroacetic acid (3.0 mL) was added at
rt. The solution was stirred for 3 h, then was concentrated to
dryness under reduced pressure to afford a light-yellow fluid
oil. This crude product was purified and recrystallized as de-
scribed above for 2. The product was recovered as a white
powder (273 mg, 81%), and consisted of kainic acid (1, 43%)
and allo-kainic acid (2, 57%), as determined by 1H NMR. The
mixture of diastereomers proved inseparable with HPLC.
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17
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60
Allokainic acid (2). N-Pf protected diester 14a (103 mg,
0.197 mmol) was dissolved in dichloromethane (1 mL) at rt
under argon. Triethyl silane (0.30 mL, 2.0 mmol) was added,
followed by trifluoroacetic acid (0.19 mL, 3.0 mmol). The
reaction was stirred at rt for 3 h, at which point a second por-
tion of triethyl silane (0.63 mL, 3.9 mmol) and trifluoroacetic
acid (0.39 mL, 5.9 mmol) was added. TLC showed full depro-
tection of the N-Pf group after an additional 4 h. The reaction
was evaporated to dryness under reduced pressure. The resi-
due was retaken in methanol (3 mL), LiOH aqueous solution
(1 mL, 2.5 M) was added, and the reaction was stirred at rt
overnight. The reaction was then neutralized at 0 ºC with 1.0
M HCl aqueous solution, and the mixture was diluted with
water (20 mL). The Pf-H byproduct was removed by extrac-
tion with petroleum ether (2 ´ 50 mL). The aqueous layer was
concentrated to dryness under high vacuum. The residue was
retaken in water (5 mL) and purified by ion-exchange chroma-
tography: ion-exchange resin Dowex 50WX4 100-200, eluting
with 0.5 N aqueous ammonia. The eluting fractions were col-
lected, and analyzed by TLC for presence of the desired prod-
uct (TLC plates were dried gently with a heatgun before being
stained with ninhydrin; further heating revealed the presence
of the amino acid 2 as yellow spots). The fractions containing
the product were combined, flash frozen, and the solvents
were removed by lyophilization to yield a pale yellow solid.
This product was recrystallized with aqueous ethanol (The
product was dissolved with minimal water, followed by drop-
wise addition of ethanol until the crystal formed. It was then
1
Kainic acid (1): H NMR (400 MHz, D2O) δ: 5.08 (bs, 1H),
4.78 (bs, 1H), 4.18 (d, J = 3.5 Hz, 1H), 3.67 (dd, J = 11.9, 7.3
Hz, 1H), 3.46 (t, J = 11.4 Hz, 1H), 3.18–3.00 (m, 2H), 2.53
(dd, J = 16.9, 6.3 Hz, 1H), 2.44 (dd, J = 16.8, 8.2 Hz, 1H),
1.78 (s, 3H) ppm. HRMS (ESI-TOF) m/z [M+H]+ Calcd for
C10H16NO4 214.1074; Found: 214.1076. MS analysis was
conducted with the mixture of 1 and 2.
(3R,4S,6S)-4-carboxy-2-oxo-6a-phenyl-5-(2,2,2-trifluoro-
acetyl)hexahydro-2H-furo[2,3-c]pyrrol-5-ium (17). Benzylic
carbonate 15b (205 mg, 0.356 mmol) was dissolved in metha-
nol (5 mL), a LiOH aqueous solution (1 mL, 2.5 N) was added
at rt and was stirred for 6 h. The reaction was neutralized at 0
ºC with diluted 1.0 N HCl aqueous solution, and extracted
with ethyl acetate (3 ´ 10 mL). The combined organic layer
was dried over MgSO4, and filtered. The solvents were evapo-
rated under reduced pressure. This residue was retaken in di-
chloromethane (5 mL) under argon atmosphere, trifluoroacetic
acid (1 mL) was added, and the solution was stirred at rt for 3
h. The reaction mixture was concentrated to dryness under
reduced pressure to obtain a light yellow solid. This crude
product was triturated with acetone (5 mL) and afforded lac-
tone 17 as a white powder (90 mg, 90%). The solubility of
lactone 17 is extremely poor in either H2O, acetone, chloro-
form, or DMSO. FTIR (thin film): 3521, 3017, 2983, 1783,
1
1605, 754, 705 cm-1. H NMR (400 MHz, D2O) δ: 7.59–7.46
(m, 5H), 4.36 (d, J = 4.8 Hz, 1H), 4.14 (d, J = 13.7 Hz, 1H),
3.92 (d, J = 13.7 Hz, 1H), 3.76–3.64 (m, 1H), 3.21 (dd, J =
19.1, 8.7 Hz, 1H), 2.98 (d, J = 19.2 Hz, 1H) ppm. 13C NMR
(101 MHz, DMSO-d6) δ: 175.5, 171.6, 139.5, 128.7, 128.2,
124.7, 95.0, 67.6, 58.1, 49.5, 34.7 ppm. 19F NMR (376 MHz,
o
left standing at 0 C, and allokainic acid was recovered after
tirtuation as a white crystalline solid (33 mg, 78%). MP 239–
o
1
242 C. FTIR (thin film): 3451, 2971, 1578 cm-1. H NMR
(400 MHz, D2O) δ: 4.95 (bs, 1H), 4.94 (m, 1H), 3.92 (d, J =
8.1 Hz, 1H), 3.51 (dd, J = 11.8, 7.8 Hz, 1H), 3.33 (dd, J =
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The Journal of Organic Chemistry
Page 8 of 10
DMSO-d6) δ: -73.45 ppm. HRMS (ESI-TOF) m/z [M+H]+
Calcd for C13H14NO4 248.0917; Found 248.0912.
drogen atmosphere (balloon) for 24 h. The resulted reaction
1
2
3
4
5
6
7
8
solution was filtered over a Celite pad. The filtrate was con-
centrated to dryness to under reduced pressure and afford 19b
as a colorless oil (174 mg, 99%). FTIR (thin film): 2977,
(3R,4S,6S)-4-(Methoxycarbonyl)-2-oxo-6a-phenyl-5-(2,2,2-
trifluoroacetyl)hexahydro-2H-furo[2,3-c]pyrrol-5-ium (18a).
To a stirred solution of allylic alcohol 12b (2.268 g, 3.94
mmol) in dichloromethane (25 mL) was added trifluoroacetic
acid (9.2 mL, 0.12 mol). After 3 h, a TLC showed the full
conversion. The reaction mixture was concentrated to dryness
under reduced pressure. The residue was triturated in methanol
(20 mL), and the insoluble byproduct (PfOH) was removed by
filtration. The filtrate was concentrated to afford the trifluoro-
acetate salt of 18a as a pale-yellow oil (1.465 g, 99%) which
was used as is for the next step. FTIR (thin film): 2955, 2914,
1
2917, 1742, 1698 cm-1. H NMR (400 MHz, CDCl3, two rota-
mers) δ: 7.33–7.27 (m, 3H), 7.09–7.05 (m, 2H), 4.14 (d, J =
6.6 Hz, 0.4H), 4.02 (d, J = 6.7 Hz, 0.6H), 3.94 (ddd, J = 15.7,
10.9, 6.9 Hz, 1H), 3.84 (dd, J = 11.0, 4.7 Hz, 0.6H), 3.77 (d, J
= 3.1 Hz, 3H), 3.74–3.73 (m, 0.4H), 3.71–3.65 (m, 1H), 2.99
(dt, J = 10.7, 6.9 Hz, 1H), 2.30 (ddd, J = 17.4, 14.7, 7.1 Hz,
1H), 2.14–2.02 (m, 1H), 1.45 and 1.49 (2s, 9H). 13C NMR
(101 MHz, CDCl3) δ: 176.8, 172.8, 172.5, 154.3, 153.5, 138.5,
138.4, 128.9, 127.7, 127.7, 127.4, 80.7, 80.6, 77.3, 77.0, 76.7,
63.3, 63.0, 52.5, 52.3, 50.8, 50.4, 45.2, 44.3, 44.1, 43.3, 33.1,
28.4, 28.3. HRMS (ESI-TOF) m/z [M+Na]+ Calcd. for
C19H25NO6Na 386.1580; Found 386.1556.
Phenyl kainic acid (4). Synthetic route from 17: To a solu-
tion of lactone 17 (50 mg, 0.21 mmol), acetic acid (1 mL) and
methanol (5 mL) was added Pd/C (5 mg). This reaction mix-
ture was stirred under a hydrogen atmosphere (balloon) for 24
h. The resulted reaction mixture was filtered over a Celite pad,
and the filtrate was concentrated to dryness under reduced
pressure. The residual acetic acid was removed under high
vacuum. The crude product was purified and recrystallized as
described above 2. Phenylkainic acid (4) was recovered as a
white crystalline solid (45 mg, 90%).
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16
17
18
19
20
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23
24
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36
37
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46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1
2847, 1777，1733 cm-1. H NMR (400 MHz, CD3OD) δ:
7.27–7.21 (m, 4H), 7.21–7.15 (m, 1H), 4.50 (d, J = 6.6 Hz,
1H), 3.86 (d, J = 13.5 Hz, 1H), 3.69 (d, J = 13.5 Hz, 1H), 3.69
(s, 3H), 3.48 (ddd, J = 8.3, 7.1, 1.9 Hz 1H), 2.74 (dd, J = 18.7,
7.5 Hz, 1H), 2.67 (dd, J = 18.7, 2.0 Hz, 1H). 13C NMR (101
MHz, CD3OD) δ: 175.0, 169.0, 137.3, 130.5, 130.3, 126.0,
94.4, 65.5, 57.6, 54.4, 50.4, 49.6, 49.4, 49.2, 49.0, 49.0, 48.6,
48.4, 34.2. HRMS (ESI-TOF) m/z [M+H]+ Calcd. for
C14H16NO4 262.1079; Found 262.1070.
5-(tert-Butyl) 4-methyl (3R,4S,6S)-2-oxo-6a-phenylhexa-
hydro-5H-furo[2,3-c]pyrrole-4,5-dicarboxylate (18b). To a
solution of 18a (TFA salt, 215 mg, 0.573 mmol) in dichloro-
methane (2 mL) was added DMAP (7 mg, 0.06 mmol), tri-
ethylamine (145 µL, 1.43 mmol) and Boc2O (188 mg, 0.859
mmol). After 6 h, the reaction mixture was concentrated to
dryness under reduced pressure. The crude residue was puri-
fied by column chromatography using dichloro-
methane/methanol as eluent (10-20% gradient). 18b was re-
covered as a colorless oil (192 mg, 93%). FTIR (thin film):
Synthetic route from 18a: To a solution of methyl ester 18
(386 mg, 1.21 mmol) in methanol (15 mL) was added a LiOH
aqueous solution (8 mL, 2.5 N). Full conversion was observed
after 4 h. The reaction was neutralized at 0 ºC with 1.0 N
aqueous HCl. The solvents were evaporated under high vac-
cum. The resulted residue was purified and recrystallized as
described above 2. Phenylkainic acid (4) was recovered as a
white crystalline solid (256 mg, 86%). MP 255–257 oC. FTIR
1
2974, 2917, 1793, 1749, 1698 cm-1. H NMR (400 MHz,
CDCl3, two rotamers) δ: 7.41 (m, 7.42–7.40, 4H), 7.39–7.33
(m, 4H), 4.41 (d, J = 4.6 Hz, 0.5H), 4.33–4.25 (m, 4H), 4.25–
4.21 (m, 0.5H), 3.89 (dd, J = 13.0, 9.0 Hz, 1H), 3.83 (s, 3H),
3.23–3.17 (m, 1H), 2.90–2.62 (m, 2H), 1.44 and 1.46 (2s, 9H).
13C NMR (101 MHz, CDCl3, two rotamers) δ: 174.2, 172.1,
172.0, 154.0, 153.0, 137.5, 137.2, 129.0, 128.9, 128.8, 124.8,
94.4, 93.4, 81.3, 81.2, 77.4, 77.2, 77.0, 76.7, 65.9, 65.8, 59.2,
58.6, 52.8, 52.6, 51.4, 50.5, 35.1, 34.9, 28.3, 28.2. HRMS
(ESI-TOF) m/z [M+Na]+ Calcd for C19H23NO6Na 384.1423;
Found 384.1455.
(2S,3S,4S)-1-Acetyl-3-(carboxymethyl)-2-(methoxycarbonyl)-
4-phenylpyrrolidin-1-ium (19a). To a solution of 18a (TFA
salt, 1.126 g, 2.97 mmol) in methanol (30 mL) was added
acetic acid (0.71 mL) and Pd/C (56 mg). The reaction mixture
was stirred under a hydrogen atmosphere (balloon) for 24 h.
The resulted reaction mixture was filtered over a Celite pad.
The filtrate was evaporated to dryness under reduced pressure
and then high vaccum to afford the acetate salt of 19a as a
pale-yellow powder (956 mg, 99%). FTIR (thin film): 3390,
2521, 1739, 1673, 1635 cm-1. 1H NMR (400 MHz, CD3OD) δ:
7.42–7.31 (m, 4H), 7.29–7.22 (m, 2H), 4.59 (d, J = 6.2 Hz,
1H), 4.01–3.93 (m, 1H), 3.91 (s, 3H), 3.88–3.82 (m, 1H),
3.84–3.75 (m, 1H), 3.30–3.19 (m, 1H), 2.33 (ddd, J = 17.0,
7.8, 1.2 Hz, 1H), 2.22 (dd, J = 16.9, 6.1 Hz, 1H), 1.98 (s, 3H).
13C NMR (101 MHz, CD3OD) δ: 173.8, 172.9, 168.9, 134.4,
128.6, 128.2, 127.7, 68.2, 58.1, 53.4, 53.1, 48.2, 48.0, 47.8,
47.6, 47.4, 47.2, 46.9, 43.9, 42.5, 33.0. HRMS (ESI-TOF) m/z
[M+H]+ Calcd for C14H18NO4 264.1236; Found 264.1257.
2-((2S,3S,4S)-1-(tert-Butoxycarbonyl)-2-(methoxycarbonyl)-
4-phenylpyrrolidin-3-yl)acetic acid (19b). To a solution of
18b (175 mg, 0.484 mmol) in methanol (20 mL) was added
Pd/C (25 mg). The reaction mixture was stirred under a hy-
1
(thin film): 3429, 3185, 3042, 2920, 1736, 765, 699 cm-1. H
NMR (400 MHz, D2O) δ: 7.49–7.34 (m, 3H), 7.27–7.18 (m,
2H), 4.05 (d, J = 7.3 Hz, 1H), 3.94 (dd, J = 11.4, 7.8 Hz, 1H),
3.87 (q, J = 7.8 Hz，1H), 3.72 (dd, J = 11.4, 8.1 Hz, 1H), 3.15
(q, J = 7.2 Hz, 1H), 2.38 (dd, J = 16.3, 6.5 Hz, 1H), 2.02 (dd, J
= 16.3, 8.7 Hz, 1H) ppm. 13C NMR (101 MHz, D2O) δ: 178.5,
173.3, 136.2, 128.8, 128.2, 127.6, 65.1, 48.0, 44.8, 44.0, 35.7
ppm. HRMS (ESI-TOF) m/z [M+H]+ Calcd. for C13H16NO4
250.1074; Found 250.1086.
1-(tert-Butyl) 2-methyl (2S,3S,4S)-3-(2-methoxy-2-oxoethyl)-
4-phenylpyrrolidine-1,2-dicarboxylate (20b). To a solution of
19a (acetate salt, 136 mg, 0.421 mmol) in methanol (2.5 mL)
was added TMSCl (0.1 mL, 1 mmol) at r.t. After 6 h, the reac-
tion was concentrated to dryness to afford the methyl ester a
white solid residue. This residue was retaken in dichloro-
methane (3 mL), and trimethylamine (150 µL, 1.08 mmol)
was added, followed by DMAP (5 mg, 0.04 mmol) and Boc2O
(138 mg, 0.631 mmol). The white suspension was stirred
overnight at rt. The solvent was evaporated under reduced
pressure. The resulted residue was purified by column chro-
matography using ethyl acetate/hexane as eluent (10-20%
gradient) to afford Boc-protected diester 20b as a colorless oil
(146 mg, 92%). FTIR (thin film): 2917, 2854, 1742, 1691 cm-1.
1H NMR (400 MHz, CDCl3, two rotamers) δ: 7.37–7.20 (m,
3H), 7.06 (dd, J = 7.9, 3.6 Hz, 2H), 4.07 (dd, J = 39.4, 6.4 Hz,
1H), 3.99–3.89 (m, 1H), 3.87–3.80 (m, 1H), 3.80–3.75 (m,
3H), 3.69 (q, J = 5.8 Hz, 1H), 3.65–3.58 (m, 3H), 3.01 (h, J =
7.2 Hz, 1H), 2.27 (dt, J = 17.7, 7.2 Hz, 1H), 2.04 (ddd, J =
17.2, 7.8, 4.9 Hz, 1H), 1.43 and 1.45 (2s, 9H). 13C NMR (101
MHz, CDCl3, two rotamers) δ: 172.8, 172.5, 172.1, 172.1,
154.2, 153.5, 138.6, 138.4, 128.8, 127.7, 127.7, 127.3, 80.5,
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The Journal of Organic Chemistry
77.3, 77.0, 76.7, 63.4, 63.1, 52.4, 52.2, 51.8, 51.7, 50.7, 50.2, layer was then acidified with 0.5 N aqueous HCl solution at 0
oC until the pH ~ 2. It was then reextracted with ethyl acetate
(3 ´ 30 mL) and washed with brine. The organic layer was
dried over MgSO4 and concentrated to dryness under reduced
pressure. The residue was purified by column chromatography
using methanol/dichloromethane as eluent (5-10% gradient) to
afford 21a (652 mg, 78%) as a white foamy solid. FTIR (thin
1
2
3
4
5
6
7
8
45.2, 44.3, 44.3, 43.5, 33.3, 33.2, 28.4, 28.3. HRMS (ESI-TOF)
m/z [M+Na]+ Calcd. for C20H27NO6Na 400.1736; Found
400.1717.
Methyl
(2S,3S,4S)-3-(2-methoxy-2-oxoethyl)-1-((4-nitro-
phenyl)sulfonyl)-4-phenylpyrrolidine-2-carboxylate (20a). To
a solution of 19a (acetate salt, 696 mg, 2.15 mmol) in metha-
nol (5.0 mL) was added TMSCl (690 µL, 5.39 mmol). The
solution was stirred at rt for 6 h. The reaction mixture was
concentrated to dryness to afford the methyl ester as a white
solid. This residue was retaken in dichloromethane (10 mL),
and trimethylamine (746 µL, 5.39 mmol) was added, followed
by DMAP (26 mg, 0.22 mmol) and p-NsCl (716 mg, 3.23
mmol). The reaction was stirred at rt overnight. The reaction
concentrated to dryness. The resulted residue was purified by
column chromatography using ethyl acetate/hexane as eluent
(10-30% gradient). N-Ns-protected diester 20a was recovered
as a white foam-like solid (953 mg, 96%). FTIR (thin film):
1
film): 2914, 2851, 1723, 1591, 1524 cm-1. H NMR (400
MHz, CDCl3) δ 8.37 (d, J = 9.1 Hz, 1H), 8.05 (d, J = 9.1 Hz,
1H), 4.14 (d, J = 7.0 Hz, 1H), 3.81 (s, 3H), 3.79-3.74 (m, 1H),
3.69 (s, 3H), 3.67 -3.64 (m, 1H), 3.37-3.32 (m, 1H), 3.06 –
2.98 (m, 1H), 2.47 (dd, J = 17.0, 7.4 Hz, 1H), 2.36 (dd, J =
17.0, 7.5 Hz, 1H). 13C NMR (101 MHz, CDCl3) δ 175.8,
170.9, 170.8, 150.3, 143.3, 128.8, 124.2, 64.6, 53.1, 52.2,
49.2, 44.9, 42.0, 32.3. HRMS (ESI-TOF) m/z [M+H]+ Calcd.
for C16H19N2O10S 431.0676; Found 431.0648.
9
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12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
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60
Methyl
(2S,3S,4R)-4-((6-(7-(diethylamino)-2-oxo-2H-
1
3100, 2952, 1736, 1527 cm-1. H NMR (400 MHz, CDCl3) δ:
chromene-3-carboxamido)hexyl)carbamoyl)-3-(2-methoxy-2-
8.40 (d, J = 9.0 Hz, 1H), 8.13 (d, J = 9.1 Hz, 1H), 7.35–7.27
(m, 3H), 7.09–6.99 (m, 2H), 4.43 (d, J = 4.5 Hz, 1H), 3.81–
3.78 (m, 3H), 3.78 (s, 3H), 3.60 (s, 3H), 3.10–2.99 (m, 1H),
2.02 (d, J = 1.8 Hz, 1H), 2.00 (d, J = 3.3 Hz, 1H). 13C NMR
(101 MHz, CDCl3) δ: 171.6, 171.4, 150.2, 144.5, 136.4, 129.0,
128.8, 127.7, 127.7, 124.2, 77.4, 77.0, 76.7, 65.2, 52.9, 51.9,
50.7, 45.2, 44.4, 32.5. HRMS (ESI) m/z [M+H]+ Calcd for
C21H23N2O8S 463.1175; Found 463.1174.
oxoethyl)-1-((4-nitrophenyl)sulfonyl)pyrrolidine-2-
carboxylate (22). To a stirred solution of 21a (26 mg, 0.060
mmol) in dichloromethane (1 mL) was added triethylamine
(13 µL, 0.090 mmol), HBTU (25 mg, 0.071 mmol) and the
coumarin amine (33 mg, 0.091 mmol). After 1 h, 21a was
fully converted to 22. The reaction mixture was poured into
water and extracted with diethyl ether (3´15 mL). The com-
bined ether layer was then washed with ammonium chloride
aqueous solution, sodium bicarbonate aqueous solution and
brine. The organic layer was then dried over MgSO4, filtered
and concentrated to dryness under reduced pressure. The re-
sulted residue was purified by column chromatography on
silica gel (10-35% EtOAc/hexane as elution gradient) to yield
22 (43 mg, 92%) as a yellow solid. FTIR (thin film): 3452,
3338, 2923, 2851, 1672, 1616, 1581, 1533, 1515, 755 cm-1. 1H
NMR (400 MHz, CDCl3) δ 8.83 (t, J = 6.0 Hz, 1H), 8.68 (s,
1H), 8.37 (d, J = 8.8 Hz, 2H), 8.06 (d, J = 8.8 Hz, 2H), 7.48 (d,
J = 8.9 Hz, 1H), 6.66 (dd, J = 9.0, 2.5 Hz, 1H), 6.51 (d, J = 2.4
Hz, 1H), 6.24 (t, J = 5.6 Hz, 1H), 4.22 (d, J = 6.4 Hz, 1H),
3.74-3.78 (m, 4H), 3.68 (t, J = 4.9 Hz, 1H), 3.65 (s, 3H), 3.50
– 3.39 (m, 6H), 3.28 (td, J = 7.1, 5.2 Hz, 1H), 3.08 (dt, J =
15.0, 6.7 Hz, 2H), 3.03 – 2.95 (m, 1H), 2.42 (dd, J = 17.2, 8.2
Hz, 1H), 2.26 (dd, J = 17.2, 6.8 Hz, 1H), 1.60 (t, J = 6.7 Hz,
2H), 1.40 (d, J = 6.3 Hz, 2H), 1.37 – 1.30 (m, 4H), 1.25 (t, J =
7.1 Hz, 6H). 13C NMR (101 MHz, CDCl3) δ 172.0, 171.3,
169.3, 163.3, 162.9, 152.6, 150.2, 148.1, 143.4, 131.2, 129.0,
124.2, 110.0, 108.4, 96.5, 65.3, 52.9, 52.0, 50.0, 45.8, 45.1,
42.8, 38.7, 38.7, 32.3, 29.3, 28.8, 25.7, 25.6, 12.4 ppm. HRMS
(ESI-TOF) m/z [M+H]+ Calcd. for C36H46N5O12S 772.2864;
Found 772.2876.
1-(tert-Butyl) 2-methyl (2S,3S,4S)-3-(2-(tert-butoxy)-2-oxo-
ethyl)-5-oxo-4-phenylpyrrolidine-1,2-dicarboxylate (21b).
N-Boc protected diester 20b (52 mg, 0.14 mmol) was dis-
solved in a solvent mixture composed of: acetonitrile (0.5
mL), carbon tetratchloride (0.5 mL), and water (1 mL). Sodi-
um metaperiodate (471 mg, 2.20 mmol) was added, followed
by ruthenium trichloride monohydrate (2 mg, 0.009 mmol).
After 8 h, TLC analysis showed only partial conversion. A
second portion of metaperiodate (118 mg, 0.552 mmol) and
ruthenium trichloride (1 mg, 0.04 mmol) was added. After an
additoinal 24 h of reaction, the mixture was diluted with ethyl
acetate (30 mL). The organic layer was washed with 0.5 N
HCl aqueous solution (5 mL), brine (20 mL). It was then dried
over MgSO4 and filtered. The solvent was removed under re-
duced pressure. The resulted residue was purified by column
chromatography using ethyl acetate/hexane as eluent (10-30%
gradient). N-Boc pyrrolidin-5-one 21b was recovered as a
colorless oil (45 mg, 86%). FTIR (thin film): 2950, 2912,
1
2851, 1796, 1743 cm-1. H NMR (400 MHz, CDCl3) δ: 7.36–
7.29 (m, 3H), 7.17–7.10 (m, 2H), 4.43 (d, J = 3.3 Hz, 1H),
4.22 (d, J = 8.7 Hz, 1H), 3.86 (s, 3H), 3.57 (s, 3H), 3.08 (m,
1H), 2.30 (dd, J = 16.9, 8.8 Hz, 1H), 2.09 (dd, J = 16.9, 6.4
Hz, 1H), 1.53 (s, 9H). 13C NMR (101 MHz, CDCl3) δ: 171.9,
171.3, 170.9, 149.5, 133.2, 129.5, 128.8, 127.9, 84.2, 62.0,
52.9, 51.9, 51.4, 37.0, 34.4, 27.9. HRMS (ESI) m/z [M+Na]+
Calcd for C20H25NO7Na 414.1529; Found 414.1492.
ASSOCIATED CONTENT
Supporting Information
Spectroscopic data for all compounds, spectra of 1H/13C/2D NMR
analyses. The Supporting Information is available free of charge
on the ACS Publications website.
(3R,4S,5S)-4-(2-Methoxy-2-oxoethyl)-5-(methoxycarbonyl)-
1-((4-nitrophenyl)sulfonyl)pyrrolidine-3-carboxylic
acid
(21a). Proceeded according to the procedure described for 21b,
and started with 20a (901 mg, 1.95 mmol). The resulted reac-
tion mixture was acidified with 0.5 N aqueous HCl solution at
0 oC until the pH ~ 2. The resulted mixture was extracted with
ethyl acetate (2 ´ 20 mL), followed by washing with brine (50
mL). The combined organic layers were then extracted with
saturated aqueous sodium bicarbonate solution (3 ´ 20 mL),
the desired product was thus extracted to the aqueous layers.
The aqueous layers were combined and extracted with diethyl
ether (3 ´ 20 mL) to remove impurities. The resulted aqueous
AUTHOR INFORMATION
Corresponding Author
* E-mail: frederic.menard@ubc.ca
Notes
The authors declare no competing financial interest.
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(21) Determined by the chemical shift difference of between the C3
ACKNOWLEDGMENT
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4
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protons: δHa–δHb (3,4-cis) > δHa–δHb (3,4-trans). Literature δHa-b
(3,4-cis) = 0.37 ppm and δHa-b (3,4-cis) = 0.18 ppm (300 MHz, D2O);
(ref.6b). This work: δHa-b = 0.36 ppm (400 MHz, D2O).
(22) Matoba M.; Kajimoto T.; Node M. Synth. Commun. 2008, 38,
1194.
(23) Related synthetic procedures can be found in Layton M.E. et al.
ACS Chem. Neurosci. 2011, 2, 352.
(24) Meguellati, K.; Koripelly, G.; Ladame, S. Angew. Chem. Int. Ed.
Engl. 2010, 49, 2738.
This work was supported by the Natural Sciences and Engineer-
ing Research Council of Canada (NSERC DG), the Canadian
Fund for Innovation (CFI-JELF), and the University of British
Columbia. ZT gratefully acknowledges UBC for University
Graduate Fellowships. We thank Dr. Paul Shipley for NMR sup-
port and guidance. We are grateful to Dr. Vojtech Kapras for
insightful discussions. Thanks to Dr. Zandberg and Matt
Noestedem for HPLC and mass spectrometry analyses.
8
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(25) Love, B. E.; Jones, E. G., J. Org. Chem. 1999, 64, 3755.
(26) Jamison, T.; Lubell, W.; Dener, J.; Krisché, M.; Rapoport, H.,
Org. Synth. 1993, 71, 226.
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REFERENCES
(1) Stathakis, C. I.; Yioti, E. G.; Gallos, J. K. Eur. J. Org. Chem.
2012, 25, 4661.
(2) Parsons, A. F. Tetrahedron 1996, 52, 4149.
(3) There are five isoforms of the kainate receptors: ionotropic gluta-
mate receptors iGluK1-5. Werner, P.; Voigt, M. Nature 1991, 351,
742.
(4) (a) Wang, Q.; Yu, S.; Simonyi, A.; Sun, G. Y.; Sun, A. Y. Mol.
Neurobiol. 2005, 31, 3. (b) Greenamyre, J. T.; Young, A. B. Neurobi-
ol. Aging 1989, 10, 593. (c) Stein, B. A.; Sapolsky, R. M. Brain Res.
1988, 473, 175. (d) Köhler, C.; Schwarcz, R. Neuroscience 1983, 8,
819. (e) Victor Nadler, J. Life Sci. 1981, 29, 2031.
(5) Ishida, M.; Shinozaki, H. Br. J. Pharmacol. 1991, 104, 873.
(6) (a) Baldwin, J. E.; Fryer, A. M.; Pritchard, G. J. J. Org. Chem.
2001, 66, 2588. (b) Ibid. J. Org. Chem. 2001, 66, 2597.
(7) For reviews, see Ref. 1, 2, and: (a) Moloney, M. G. Nat. Prod.
Rep. 1998, 15, 205. (b) Moloney, M. G. Nat. Prod. Rep. 2002, 19,
597. (c) Fujii, M.; Yokoshima, S.; Fukuyama, T. Eur. J. Org. Chem.
2014, 22, 4823. (d) Bhat, C.; Kumar, A. Asian J. Org. Chem. 2015, 4,
102. (d) Clayden, J.; Read, B.; Hedbitch, K. Tetrahedron 2005, 61,
5713.
(8) Most recent kainoids syntheses relying on: (a) Lei H.; Xin S.; Qiu
Y.; Zhang X. Chem. Comm. 2018, 54, 727. (b) Suzuki J.; Miyano N.;
Yashiro S.; Umezawa T.; Matsyda F. Org. Biomol. Chem. 2017, 15,
6557. (c) Shi H.; Li J.; Liu Y.; Du Z.; Huang Z.; Zhao N.; Li N.;
Yang J. Tetrahedron 2016, 72, 5502. (d) Shimamoto, K.; Hamashima,
Y.; Kan, T. Org. Lett. 2014, 16, 564.
(9) Zhang, M.; Watanabe, K.; Tsukamoto, M.; Shibuya, R.; Morimo-
to, H.; Ohshima, T. Chem. Eur. J. 2015, 21, 3937.
(10) Baldwin, J. E.; Rudolph, M. Tetrahedron Lett. 1994, 35, 6163.
(11) Chevliakov, M. V.; Montgomery, J. Angew. Chem. Int. Ed. Engl.
1998, 37, 3144.
(12) Tian, Z.; Clark, B. L.; Menard, F. Manuscript in preparation.
(13) Fukuyama has reported a practical synthesis that delivered 14 g
of kainic acid in 13 steps from carvone. However, it does not allow
variation of C4. Takita, S.; Yokoshima, S.; Fukuyama, T. Org. Lett.
2011, 13, 2068.
(14) The N-protection and oxidation sequence was assembled from
reference: (a) Blanco, M.-J.; Sardina, F. J. J. Org. Chem. 1996, 61,
4748. (b) Poisson, J.-F.; Orellana, A.; Greene, A. E. J. Org. Chem.
2005, 70, 10860. (c) Zanato, C.; Watson, S.; Bewick, G. S.; Harrison,
W. T.; Zanda, M. Org. Biomol. Chem. 2014, 12, 9638.
(15) (a) Lubell, W. D.; Rapoport, H. J. Am. Chem. Soc. 1987, 109,
236. (b) Lubell, W. D; Rapoport, H. J. Am. Chem. Soc. 1988, 110,
7447. (c) Lubell, W.D.; Rapoport, H. J. Org. Chem. 1989, 54, 3824.
(d) Lubell, W. D.; Jamison, T. F.; Rapoport, H. J. Org. Chem. 1990,
55, 3511. (e) Gill, P.; Lubell, W. D. J. Org. Chem. 1995, 60, 2658. (f)
Sharma, R.; Lubell, W. D. J. Org. Chem. 1996, 61, 202.
(16) Preparation of Pf-Br on 40 grams scale is straight-forward and
efficient (two steps, 86% yield).
(17) Krasovskiy, A.; Kopp, F.; Knochel, P. Angew. Chem. Int. Ed.
Engl. 2006, 45, 497. See supp. info. for the preparation of the
CeCl3•2LiCl reagent.
(18) Tsuji, J.; Yamakawa, T. Tetrahedron Lett. 1979, 20, 613.
(19) Using N-Pf protected 13a was advantageous over Ma’s report
using N-Bz proline carbonate, as it minimized the numbers of iso-
mers: only two products were observed instead out of four possible.
Ma D.; Wu W.; Deng P. Tetrahedron Lett. 2001, 42, 6929.
(20) Koskinen, A.M.P.; Rapoport, H. J. Org. Chem. 1989, 54, 1859.
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