Tandem approaches for the synthesis of functionalized pyrrolidones: Efficient routes toward allokainic acid and kainic acid

Article abstract of DOI:10.1016/j.tetlet.2012.11.007

Tandem approaches for the synthesis of pyrrolidone precursor of allokainic acid and kainic acid are described. The synthesis of pyrrolidone intermediate of allokainic acid is achieved by tandem Wittig-Michael reaction and tandem amidation-Michael reaction

Full text of DOI:10.1016/j.tetlet.2012.11.007

Tetrahedron Letters 54 (2013) 245–248
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Tetrahedron Letters
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Tandem approaches for the synthesis of functionalized pyrrolidones:
efficient routes toward allokainic acid and kainic acid
⇑
Chinmay Bhat, Santosh G. Tilve
Department of Chemistry, Goa University, Taleigao-Plateau, Goa 403 206, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Tandem approaches for the synthesis of pyrrolidone precursor of allokainic acid and kainic acid are
described. The synthesis of pyrrolidone intermediate of allokainic acid is achieved by tandem Wittig–
Michael reaction and tandem amidation-Michael reaction while one pot amidation—Ene-esterification
is employed for the synthesis of Ganem intermediate (2:1) of kainic acid.
Received 29 August 2012
Revised 31 October 2012
Accepted 2 November 2012
Available online 9 November 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Pyrrolidin-2-one
Kainoids
Allokainic acid
Tandem
Substituted five member heterocycles 2-oxopyrrolidines or
pyrrolidin-2-ones, popularly known as -lactams are prevalent
due to their powerful neuroexcitatory activity18 in the central ner-
vous system (CNS). A highly functionalized trisubstituted pyrroli-
dine ring with three contiguous stereogenic centers18 and global
deficiency19 are the other factors which makes the synthesis of
these molecules challenging. Numerous reports are available for
the synthesis of allokainic acid.20 Some of the recent examples in-
clude palladium-catalyzed carbocyclization by Cook and Sun,20v
diastereoselective Rh(II)-carbenoid C–H insertion by Zhang and
Wee.20y Jung et al. achieved the synthesis of allokainic acid by
Rh(II)-catalyzed intramolecular C–H insertion through control of
stereochemistry over 3, 4 positions.20x It is stereoselectively syn-
thesized by trimethylstannyl-mediated radical carbocyclization
and oxidative destannylation by Hanessian and Ninkovic20o and
tandem aza-Cope/Mannich reaction by Couty and co-workers.20m
Allokainic acid was also synthesized by tandem Michael reaction
methodology by Barco et al.20k Momose and co-workers achieved
the formal synthesis of allokainic acid by synthesizing a useful
functionalized pyrrolidone through intramolecular Michael
c
structural motifs present in various natural products.1 They consti-
tute an important role in pharmacological2 studies due to their
application in the treatment of cancers, diabetes, and hepatitis.3
The SAR studies confirmed the usefulness of these moieties for
binding with the receptor proteins.4 They have also been used as
a source of monomers for the synthesis of various functional poly-
mers.5 Several syntheses have shown that pyrrolidin-2-ones serve
as an efficient precursor for the synthesis of pyrrolidines6 and
c-
aminoacids.7 Due to its unique structure and intriguing biological
activities, it is a target of synthetic studies.8 Most of the strategies
involve mainly radical cyclization,9 ring expansion,10 metal medi-
ated cyclization reactions,11 cycloaddition,12 or intramolecular
cyclization of esters.13 Many tandem approaches, which nowadays
an important synthetic tool,14 have also been investigated.15
Majority of them include metal mediated, radical cyclization or
cycloaddition.
Earlier we had approached the synthesis of kainic acid16
through tandem Wittig–Ene reaction.16b We envisioned that a sim-
ilar tandem Wittig–Michael reaction can lead us to a pyrrolidone
moiety for the synthesis of allokainic acid. Herein we report two
domino and one ‘one pot’ processes for the synthesis of pyrroli-
done precursor of allokainic acid. Allokainic acid 1 was isolated
along with its C-4 epimer kainic acid 2 from the Japanese marine
algae Digenea simplex AG17 in 1953. Over the last few decades sev-
eral members of kainoids (Fig. 1) have attracted synthetic chemists
reaction20h
.
Our syntheses use readily available starting materials. The first
synthesis involved tandem Wittig–Michael reaction as a key step
(Scheme 1). The p-methoxybenzylamine (PMB) was alkylated with
methyl vinyl ketone (MVK) to form amine 5 which was immedi-
ately converted into bromoamide 6 by reacting with bromoacetyl
chloride in a biphasic medium of water/chloroform (4:1) in 80%
yield. In organic solvents the acylation was unsatisfactory. The bro-
moamide 6 was then converted into phosphonium salt 7. The at-
tempted conversion of it to the corresponding phosphorane in
aqueous basic medium always resulted in its decomposition. So
it was decided to prepare the phosphorane in situ. Thus the salt
⇑
Corresponding author.
E-mail address: santoshtilve@unigoa.ac.in (S.G. Tilve).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.11.007

246
C. Bhat, S. G. Tilve / Tetrahedron Letters 54 (2013) 245–248
HOOC
HOOC
HOOC
HOOC
HOOC
N
H
HOOC
N
H
N
H
COOH
Allokainic acid
Kainic acid 2
1
Domoic acid
3
R
HOOC
HOOC
N
H
Kainoids4
Figure 1. Members of kainoids.
NH2
O
O
ClCOCH2Br/
Et3N/
O
O
Br-Ph3+P
O
Br
O
PPh3
THF, rt,
toluene, rt, 90%
HN
PMB
5
N
H2O/CHCl3 (4:1),
80%
N
O
PMB
PMB
7
6
O
O
O
Ph3P
60%
HOOC
EtOOC
HOOC
CHO-COOH
Et3N, toluene, 800 C,
O
N
O
N
O
N
PMB
PMB
PMB
8
O
Br-PPh3+CH3 / n-BuLi
SOCl2
/
CAN/EtOH
rt, 75%
EtOOC
EtOOC
THF, 0 0C - rt, 60%
EtOH, rt, 90%
O
N
O
O
N
N
H
PMB
9
PMB
10
11
HOOC
HOOC
N
H
1
Scheme 1. Tandem Wittig–Michael approach.
7 was treated with excess triethylamine in the presence of ethyl
glyoxalate. The reaction took place displaying three close spots
on TLC, of the product, the decomposed phosphorane product
and triphenylphosphine oxide making it difficult for separation.
Hence to make the separation process easy, the Wittig reaction
was carried out with glyoxalic acid with excess triethylamine in
toluene at 80 °C. The expected Wittig product 8 was then separated
by chemical separation without involving column chromatogra-
phy. The 1H NMR of the crude acid confirmed the formation of
domino Wittig–Michael reaction; however at this stage we could
not assign the configuration at C3–C4. The crude acid 8 as such
was esterified using SOCl2 in ethanol to give ester 9 in 54% overall
yield. The product obtained was purified by column chromatogra-
phy and characterized by spectroscopic methods. The compound 9
was then alkenated to give 10. The PMB group was easily deprotec-
ted using ceric ammonium nitrate (CAN) in EtOH to get the com-
pound 11.
= 2.76 Hz
Ha
= 2.20 Hz
Hb
= 2.41 Hz
Hb
= 2.66HHaz
EtOOC
CH3
= 1.58Hz
CH3
= 1.69Hz
EtOOC
O
N
H
O
N
H
14
11
(Ha-Hb) = 0.56Hz
δ
δ (Ha-Hb) = 0.25Hz
Figure 2. The differentiation of cis and trans isomer.
of the kainoids was well explained by Baldwin et al.21 based on d
values of the methylene protons attached to carboxyl part of the
side chain and they demonstrated that (i) one of the protons of
CH2 (either Ha or Hb) shows significantly lower chemical shift in
the case of cis isomer compared to the corresponding proton in
the trans isomer due to shielding effect (Ha or Hb of cis < Ha or
Hb of trans) and (ii) The difference in the chemical shift values
At this stage we compared the 1H NMR of 11 with that reported
in the literature.16b,c The differentiation of the cis and trans isomers

C. Bhat, S. G. Tilve / Tetrahedron Letters 54 (2013) 245–248
247
O
O
O
O
HOOC
EtOOC
SOCl2
/
92%
HOOC
Maleic anhydride,
toluene, reflux
O
N
O
N
EtOH, rt, 95%
O
HN
PMB
N
PMB
crude 8
PMB
PMB
9
Scheme 2. Tandem amidation-Michael approach.
once the primary amine was totally consumed and further stirred
till the disappearance of the secondary amine. SOCl2 was added in
the same pot and stirred further at room temperature to afford the
key intermediate 9 in 95% yields.
Further we visualized that similar one pot strategy can be
adapted for the synthesis of all important Ganem intermediate
1416c employed in the synthesis of kainic acid. Thus a mixture of
prenylated PMB 12 and maleic anhydride was refluxed in toluene
and then treated with EtOH and SOCl2 in the same pot. The three
reactions namely, amidation, Ene, and esterification took place in
one pot to give a mixture of diastereomers 13 and 10 in a 3:1 ratio.
The PMB group was then deprotected using CAN to give 14
(Scheme 4).
In summary we have developed metal-free, ligand-free intra-
molecular tandem, and one pot approaches for the synthesis of a
functionalized pyrrolidin-2-one which are effective precursors for
allokainic acid and kainic acid. The challenging asymmetric fusion
of C3 and C4 which serves as a route for (À)-allokainic acid and
(À)-kainic acid is under way.
NH2
O
(i) MVK, EtOH, rt
(ii) Maleic anhydride,
(iii) SOCl2, rt
EtOOC
O
N
O
PMB
9
"one pot"
95%
Scheme 3. One-pot approach for allokainic acid precursor.
H
Maleic anhydride,
toluene, reflux
HO
HN
PMB
O
O
N
PMB
12
13
Acknowledgments
EtOOC
SOCl2/EtOH
We are thankful to DST, New Delhi, for providing funds for the
research and acknowledge IISc Bangalore for HRMS facility. One of
the authors (C.B.) is thankful to CSIR, New Delhi, for awarding Se-
nior Research Fellowship.
HOOC
O
N
rt, 72%
one pot
O
N
PMB
13
PMB
cis:trans = 3:1
Supplementary data
EtOOC
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.
11.007.
CAN/EtOH,
rt, 70%
O
N
H
14
cis:trans = 2:1
Scheme 4. One-pot approach for kainic acid precursor.
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