Enantioselective total synthesis of (-)-kainic acid and (+)-acromelic acid C: Via Rh(i)-catalyzed asymmetric enyne cycloisomerization

Article abstract of DOI:10.1039/c7cc07967b

A diversity-oriented synthetic strategy was developed for the total synthesis of kainoid amino acids, which led to the enantioselective synthesis of (-)-kainic acid and the first total synthesis of (+)-acromelic acid C. Rh(i)-catalyzed asymmetric enyne cycloisomerization served as the key reaction in this strategy for the rapid construction of highly functionalized lactam, and the resulting vinyl acetate moiety was further utilized as a versatile building block for the installation of both isopropylidene and 2-pyridone units existing in natural kainoids.

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Enantioselective Total Synthesis of (-)-Kainic Acid and (+)-
Acromelic Acid C via Rh(I)-Catalyzed Asymmetric Enyne
Cycloisomerization
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Honghui Lei, Shan Xin, Yifan Qiu, and Xumu Zhang*
www.rsc.org/
A diversity-oriented synthetic strategy was developed for the total
synthesis of kainoid amino acids, which led to the enantioselective
synthesis of (-)-kainic acid and the first total synthesis of (+)-
R
4
3
2
CO H
HO
2
C
HO
2
C
HO
2
C
2
N
N
N
H
HO
2
C
HO
2
C
HO2C
H
H
acromelic
acid
C.
Rh(I)-catalyzed
asymmetric
enyne
general structure (1)
kainic acid (2)
domoic acid
cycloisomerization served as the key reaction in this strategy for
the rapid construction of highly functionalized lactam, and the
resulting vinyl acetate moity was further utilized as a versatile
building block for the installation of both isopropylidene and 2-
pyridone units existed in natural kainoids.
O
NH
2
CO H
2
HO C
HO C
2
N
HO
2
C
N
H
2
HO C
H
The kainoid family is a class of pyrrolidine dicarboxylic acids
isodomoic acid C
acromelic acid C (3)
with a general structure
core which differs solely in the nature of the substituent at the
C4-position (Figure 1). Kainic acid ( ), the parent member of
1, a trans-C2, C3/cis-C3, C4 pyrrolidine
Figure 1 Natural Kainoid Amino Acid
2
Most routes face the difficulty to readily modify the C4
position of the pyrrolidine ring. Additionally, it has been
confirmed by structural-activity investigations that the nature
of the C4 substituent and its conformation take full
this family, was originally isolated in 1953 from the seaweed
1
Digenea simplex. After that, a series of structurally related
natural products have been discovered, including domoic
2
3
4
acid , isodomoic acids and acromelic acids (Figure 1). These
kainoid amino acids have received considerable attention over
decades due to their potent neuroexcitatory activity, which
makes them extremely valuable as pharmacological probes to
stimulate nerve cells and mimic disease states, such as
5
g,10
responsibility for its biological activities.
Thus, a unified
strategy for the synthesis of different types of kainoid amino
acids is highly desirable, which would offer an easier access to
various kainoid-type compounds and provide opportunities for
achieving more biologically active analogues. Herein, we report
5
epilepsy, Alzheimer’s disease, and Huntington’s chorea. So
6
,7
a
diversity-oriented synthetic strategy towards the
far, more than 60 synthesis of kainic acid,
and several
9
8
enantioselective total synthesis of (-)-kainic acid and the first
total synthesis of (+)-acromelic acid C.
synthesis of isodomoic acids and acromelic acids, have been
reported in the literature using diverse synthetic strategies.
However, many of those synthetic approaches for isodomoic
acids or acromelic acids require fundamentally different
strategies compared with those for the synthesis of kainic acid
itself. To the best of our knowledge, there are only limited
unified routes by which one can prepare different types of
Transition metal-catalyzed cycloisomerization of 1,6-
enynes has emerged as a powerful tool for the synthesis of
1
1
various five-membered rings. Previously, we have developed
a Rh(I)-catalyzed enantioselective cycloisomerization reaction
1
2
of 1,6-enynes. High enantioselectivity and efficiency can be
achieved for a wide range of substrate scope including
nitrogen-, oxygen-, ester- and amide tethered 1,6-enynes. We
believe that this methodology can offer us a new entry into
kainoid amino acids.
7
c,7e
kainoid amino acids, including kainic acid,
8
domoic acid,
a-c,8e
isodomoic acids,
acromelic acids and their unnatural
a
n
a
l
o
g
u
e
s
.
As depicted in Scheme 1, our general strategy relied on the
preparation of compound
6 as a late-stage common
intermediate for the synthesis of kainic acid, acromelic acid C
and other kainoid congeners varied at C4 position (
anticipated that the enal functionality of compound
7
). We
6
could
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conditions. Fortunately, after carefully optimization, we found
that the combination of BOP-Cl with NaHCO gave the
expected enyne in 93% yield.
With compound in hand, we began our investigation on
3
9
9
Rh(I)-catalyzed asymmetric cycloisomerization reaction. As
shown in Table 1, the initial trials were conducted for the
evaluation of several metal precursors, such as [Rh(COD)Cl]2,
[Rh(NBD)Cl] and [Rh(CH CH ) Cl] , in combination with (R)-
2 2 2 2 2
BINAP (L1) as the ligand and AgBF as the additive (entries 1-3).
4
Among them, [Rh(COD)Cl] provided the relatively better result
2
giving cyclization product
8 in 98% yield and 81% ee. We then
focused on the identification of an optimal chiral ligand to
further improve the stereoselectivity. In addition to (R)-BINAP
(
L1), various types of ligands were tested. Interestingly,
theresult indicated that the reactivity was quite sensitive to
the steric environment of ligands. For example, when (R)-
Segphos (L2) and (R)-BIPHEP (L5) were tested, the cyclization
Scheme 1 Retrosynthetic Analysis
product
8 could be obtained in excellent yield, albeit with
enable an easy access to numerous required substituents
present in kainoid amino acids. For instance, direct
Table 1 Optimization of the Rh(I)-Catalyzed Asymmetric Cycloisomerization of
a
Enyne
9
deoxygenation of the enal group of
6 would produce the
desired C4 isopropylidene unit in kainic acid. Also, a one-pot 2-
pyridone synthesis from the same enal, which utilized the
1
3
method reported by Fukuyama and co-workers, could afford
the required 2-pyridone substituent in acromelic acid C.
Further conversion of the lactam functionality of
4 or 5 to the
desired C2 carboxylic acid could be readily achieved via a
b
yield (%)
c
ee (%)
entry
catalyst/ligand
additive
7
d
similar procedure developed by Ganem and co-workers. On
the other hand, common intermediate was supposed to be
easily derived from lactam , which would be prepared via
Rh(I)-catalyzed asymmetric cycloisomerization of the enyne
substrate
The synthesis of common intermediate
1
2
3
4
5
6
7
8
9
[Rh(COD)Cl]
2
/
/
L1
L1
AgBF
AgBF
AgBF
AgBF
AgBF
4
4
4
4
4
98
96
98
97
92
<5
99
21
24
96
N.R.
98
98
96
81
78
79
69
53
--
6
[Rh(NBD)Cl]
2
8
2 2 2 2
[Rh(CH CH ) Cl] /L1
[Rh(COD)Cl]
[Rh(COD)Cl]
2
/
/
L2
L3
9
.
2
6
commenced with
As shown in Scheme 2,
substitution of bromide 10 with PMBNH provided the
corresponding allylic amine 11. Subsequent coupling with acid
proved to be difficult under classical peptide coupling
[Rh(COD)Cl]2/L4
AgBF4
1
4
the known allylic bromide 10
.
[Rh(COD)Cl]
[Rh(COD)Cl]
[Rh(COD)Cl]
[Rh(COD)Cl]
[Rh(COD)Cl]
2
/
2
/
2
/
2
/
2
/
L5
L6
AgBF
AgBF
AgBF
AgBF
AgBF
AgBF
4
4
4
4
4
4
77
71
73
61
--
2
L7
1
1
1
1
1
0
1
2
3
4
L8
12
L9
[Rh(COD)Cl]
[Rh(COD)Cl]
[Rh(COD)Cl]
[Rh(COD)Cl]
2
2
2
2
/
/
/
/
L10
L10
L10
L10
91
92
90
92
AgOTf
AgSbF
6
d
e
1
5
AgOTf
98 (95)
O
O
O
PAr2
PAr2
PAr2
PAr2
MeO
MeO
PR2
PR2
O
Ar = C6H5
L1
Ar = C6H5
L2
L3
R
= C6H5
L5
t
3,5-di-Me-C6H3
3,5-di- Bu-4-OMe-C6H2 L6
t
t
3
,5-di- Bu-4-OMe-C6H2 L4
3,5-di- Bu-C6H3
L7
3
,5-di-Me-C6H3
iPr
,4,5-tri-OMe-C6H2
L8
L9
3
L10
a
All reactions were carried out on a 0.06 mmol scale in 1 mL of DCE in a
b
closed 10 mL screw Schlenk tube; Yield was determined by H NMR with
1
c
maleic acid as internal standard; determined by chiral stationary phase
d
HPLC using a chiral AD-H column; The reaction was carried out with 2.5
e
mol % of [Rh(COD)Cl] , 6 mol % of L10, and 10 mol % of AgOTf in 2 hours;
2
Scheme 2 Synthesis of the Common Intermediate
6
Isolated yield.
2
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O
S
O
moderate stereoselectivity (entries 4, 7). However, when the
more sterically hindered ligands were used (like L4 L6 L7 L9),
O
Ph
NH2
,
,
,
1
8
NH
O
the yields dramatically dropped (entries 5-11). In addition to
the steric effect, the result also implied that ligands with more
electro-donating groups would give better enantioselectivity.
To our delight, when electron-rich ligand L10 was involved,
significantly increased enantiocontrol was achieved (entry 12).
Further evaluation of the counter-ion effect provided us the
EtO2C
1
8, DBU, LiCl, CH3CN
Tf2O, DIPEA
DCM, 88%
N
then, HOAc, CH3CN, reflux
EtO2C
O
5
8%
PMB
N
O
common intermediate (6)
PMB
1
9
OTf
N
OTf
N
2
optimal condition with [Rh(COD)Cl] as the catalyst, L10 as the
CAN, THF/H2O
1%
a) Cp ZrHCl, THF
ligand and AgOTf as the additive. Moreover, the catalyst
loading could be reduced to 2.5 mol % without sacrificing the
enantioselectivity and yield, despite longer reaction time was
required for full conversion (entry 15).
EtO C
2
2
EtO2C
7
b) TMSCN, DCM
N
O
N
O
PMB
H
2
0
21
OTf
N
O
With compound
intermediate was straightforward. The α,β-unsaturated
double bond of was diastereoselectively reduced according
to Buchwald’s protocol, which delivered the required product
with a cis-C3, C4 configuration. The vinyl acetate group of
compound 13 was then converted to the desired enal, after
alcoholysis under K CO /EtOH condition and subsequent
treatment with Eschenmoser’s reagent and Et N in DCM.
The final stage for the synthesis of kainic acid started with
Luche reduction of compound and acetylation of the
8 in hand, the rest synthesis of common
6
NH
8
HCl/MeOH
then, 2 N KOH/H2O
1
5
EtO2C
HO2C
5
2%
13
N
N
NC
HO2C
H
H
(
+)-acromelic acid C (3)
2
2
2
3
Scheme 4 Synthesis of (+)-Acromelic Acid C
3
According to our strategy shown in Scheme 1, common
intermediate could be also utilized for the total synthesis of
acromelic acid C. The enal group of compound was a very
good start point for the construction of the desired 2-pyridone
6
6
corresponding alcohol, which provided allylic acetate 15 in 93%
yield. The expected isopropylidene at C4 was then successfully
installed by a palladium catalyzed hydrogenolysis of allyllic
6
1
3
1
6
at C4 position. Inspired by Fukuyama’s work, treatment of
intermediate with 2-(phenylsulfinyl)acetamide 18 under
basic condition promoted a 1,4-addition of the acetamide 18
to the enal . The corresponding adduct was then refluxed in
CH CN in the presence of HOAc, which led to the formation of
-pyridone functionality of 19 through cyclization and
acetate.
The removal of the PMB protecting group
O, which offered
It was further converted to kainic
6
underwent smoothly with CAN in THF/H
2
7
d
Ganem’s intermediate 4.
6
acid through several transformations without isolation of any
intermediates including amide-to-imine reduction, Strecker
3
2
reaction, alcoholysis to
a
diester, epimerization and
elimination. Unfortunately, further deprotection of the PMB
group was problematic, which gave rise to either
decomposition or no reaction under oxidative conditions or
acidic conditions, respectively. We speculated that the
relatively electron-rich diene moiety of 19 might not tolerate
the oxidative conditions. Thus, the 2-pyridone unit was
saponification. Purification of
2
was then carried out using an
+
ion-exchange resin (DOWEX 20WX8 100-200 mesh, H form, 2%
2
0
aqueous ammonia) yielded (-)-kainic acid (
20
2
), [α]
D
= -14.2 (c
7
s
O) [lit. [α]
0.2, H
2
D
= -14.6 (c 0.49, H O)], in 60% yield.
2
protected as its triflate ester
(22), which should be
spontaneously removed during the final saponification.
Gratifyingly, PMB group was removed smoothly under
standard oxidative condition in a decent yield. Finally, similar
procedures, as we mentioned before, were applied
successfully to finish the first total synthesis of (+)-acromelic
2
0
4b
acid C (
O)], in 52% yield.
In conclusion, we have successfully developed a diversity-
D 2 D
3), [α] = +30.3 (c 0.15, H O) [lit. [α] = +31.9 (c 0.23,
H
2
oriented synthetic strategy for the total synthesis of both (-)-
kainic acid and (+)-acromelic acid C. Our strategy highlights the
employment of the common intermediate
6, which can be
accessed via a Rh(I)-catalyzed asymmetric enyne
cycloisomerization, for the divergent synthesis of kainiod
amino acids. The application of this strategy for the synthesis
of other kainoids is in progress, and will be reported in due
course.
Scheme 3 Synthesis of (-)-Kainic Acid
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Conflicts of interest
There are no conflicts to declare
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