Formal total synthesis of (-)-physostigmine

Article abstract of DOI:10.3987/COM-08-S(N)29

The formal total synthesis of (-)-physostigmine via the chiral malonic acid mono-ester ((R)-2-(2-chlorophenyl)-2-methoxycarbonylpropanoic acid, 99% ee) newly prepared by the pig liver esterase (PLE) mediated asymmetric hydrolysis of the corresponding di-e

Full text of DOI:10.3987/COM-08-S(N)29

HETEROCYCLES, Vol. 76, No. 1, 2008
183
HETEROCYCLES, Vol. 76, No. 1, 2008, pp. 183 - 190. © The Japan Institute of Heterocyclic Chemistry
Received, 6th March, 2008, Accepted, 4th April, 2008, Published online, 8th April, 2008. COM-08-S(N)29
FORMAL TOTAL SYNTHESIS OF (–)-PHYSOSTIGMINE
Kaori Asakawa, Naoyoshi Noguchi, and Masahisa Nakada*
Department of Chemistry and Biochemistry, Faculty of Science and Engineering,
Waseda University, 3-4-1 Ohkubo, Shinjuku-ku, Tokyo 169-8555, Japan
Abstract – The formal total synthesis of (–)-physostigmine via the chiral malonic
acid mono-ester ((R)-2-(2-chlorophenyl)-2-methoxycarbonylpropanoic acid, 99%
ee) newly prepared by the pig liver esterase (PLE) mediated asymmetric
hydrolysis of the corresponding di-ester is described. The CuI-mediated
intramolecular aryl amidation under modified Buchwald’s conditions is a key
reaction to constructing the oxindoline core in the target.
(-)-physostigmine
(-)-phenseline
(-)-esermethole
(-)-eserethole
: R=OCONHMe, X=NMe
: R=OCONHPh, X=NMe
: R=OMe, X=NMe
: R=OEt, X=NMe
R
X
(-)-eseroline
: R=OH, X= NMe
: R=H, X=NMe
N H
(-)-deoxyeseroline
(-)-N8-norphysostigmine : R=OCONHMe, X=NH
(-)-physovenine
: R=OCONHMe, X=O
Figure 1. Structure of (–)-physostigmine and derivatives
(–)-Physostigmine (Figure 1)1 was initially isolated from the seeds of Physostigma venenosum in 1864,2
and its structural determination3 disclosed its characteristic hexahydropyrrolo[2,3-b]indole core with a
quaternary stereogenic center at its benzylic position. (–)-Physostigmine has been clinically used for the
treatment of glaucoma, myasthenia gravis, atropine and organophosphate intoxication, and for the relief
of intoxication induced by overdoses of tricyclic antidepressants, antihistamines, antipsychotics, and
benzodiazepines.1,4-6 Moreover, (–)-physostigmine has been evaluated in clinical trials for the
symptomatic treatment of Alzheimer’s disease.1,4-6 (–)-Physostigmine is a potent inhibitor of
acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE), thereby showing wide biological
activities. (–)-Physostigmine acts as a pseudosubstrate, transferring a carbamate residue to the enzyme’s
active site to bring about a stereoselective inhibition, but spontaneous hydrolysis regenerates the native

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HETEROCYCLES, Vol. 76, No. 1, 2008
enzyme and function. Therefore, various derivatives of (–)-physostigmine with improved
pharmacological profile against Alzheimer’s disease have been prepared,1,7 stimulating further synthetic
studies. Indeed, although the first total synthesis of physostigmine was reported in 1935, a number of total
syntheses as well as preparations of its derivatives have been reported to date.8-10 Interestingly, the
inhibition of AChE is found to be enantioselective through the studies using acetylcholinesterase obtained
from human tissues;11 that is, (–)-physostigmine is some 1000 times more potent than its
(+)-enantiomer,11 implying the importance of the enantioselective total synthesis of (–)-physostigmine
and its derivatives.
The unique bioactivity and structural features of (–)-physostigmine described above make this compound
a still-attractive target. We report herein a formal total synthesis of (–)-physostigmine via the chiral
malonic acid mono-ester ((R)-2-(2-chlorophenyl)-2-methoxycarbonylpropanoic acid, 99% ee) newly
prepared by the pig liver esterase (PLE) mediated asymmetric hydrolysis of the corresponding di-ester.
OMOM
COR1
OH
O
MeHNOCO
N
X
N H
(-)-physostigmine
N
intramolecular
aryl amidation
2
1
CO2R3
CO2R3
CO2H
CO2R3
R2
R2
asymmetric
hydrolysis
with PLE
3
4
Scheme 1. Retrosynthetic analysis of (–)-physostigmine
The retrosynthetic analysis of (–)-physostigmine is shown in Scheme 1. The total synthesis of
(–)-physostigmine presents some problems; 1) construction of the hexahydropyrrolo[2,3-b]indole core; 2)
construction of a quaternary stereogenic center at the benzylic position; and 3) obtaining the chiral
intermediate. Since lactam 1 has been converted to (–)-physostigmine,9,10 we planned its formal total
synthesis, namely, the enantioselective total synthesis of lactam 1.
Although lactam 1 incorporates the oxindoline core with a quaternary stereogenic center at its benzylic
position, the oxindoline core was expected to be constructed by the intramolecular lactam formation of
compound 2, which would be derived from mono-ester 3. Chiral mono-ester 3 was thought to be obtained
by asymmetric hydrolysis of the corresponding malonic acid di-ester 4 with pig liver esterase (PLE).12
Mono-ester 3 would be a useful intermediate for the synthesis of congeners of (–)-physostigmine (Figure

HETEROCYCLES, Vol. 76, No. 1, 2008
185
1) and other compounds with a quaternary stereogenic center at the benzylic position. Consequently,
preparation of di-ester 4 and its asymmetric hydrolysis with PLE was examined.
CO2Me
CO2Me
NO2
CO2Me
CO2Me
F
dimethyl malonate
MeI, NaH, THF, rt
100%
NaH, THF, reflux, 46%
NO2
NO2
5
6
PLE
potassium phosphate
bufer (pH=8), 30 oC
H2, Pd/C
THF, rt
CO2Me
O
CO2Me
N
H
NO2
8
7
Scheme 2. Preparation of di-ester 6 and its attempted conversion
If substituent R2 of di-ester 4 is an amine or its derivative, synthesis of lactam 1 via mono-ester 3 would
be easy. Therefore, we first prepared di-ester 6 because a nitro group is easily reduced to an amine. Since
o-fluoro nitrobenzene has been reported to undergo a nucleophilic substitution, dimethyl malonate was
reacted with o-fluoro nitrobenzene using NaH as a base to afford diester 5,13 which was methylated to
provide di-ester 6 (Scheme 2). Although asymmetric hydrolysis of di-ester 6 with PLE proceeded
smoothly, the product obtained was not the corresponding mono-ester but, rather, decarboxylated methyl
ester 7. This result would be well explained by the fact that the anion at the benzylic position of the
mono-ester, which was generated from di-ester 6, could be stabilized by an electron withdrawing nitro
group through the conjugation system. In addition, catalytic hydrogenation of a nitro group of di-ester 6
smoothly provided undesired lactam 8 even in the presence of acid or a trapping reagent such as acetic
anhydride or Boc2O, directing our attention to preparing another type of di-ester.
1) (MeO)2CO, NaH
THF, 50 oC, 94%
CO2Me
CO2Me
CO2Me
2) MeI, NaH, THF
rt, 80%
I
I
10
9
Scheme 3. Preparation of di-ester 10

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HETEROCYCLES, Vol. 76, No. 1, 2008
Next we examined the preparation of di-ester 4 with a halogen substituent as R2, because the
corresponding mono-ester would not undergo decarboxylation. Furthermore, the intramolecular aryl
amidation of amide 2, which would be easily prepared from mono-ester 3, was expected to provide the
oxindol core.
Iodide 10 was successfully prepared from known methyl ester 914 by introduction of methyl ester15 and
subsequent methylation (Scheme 3).
CO2Me
CO2Me
CO2Me
CO2Me
1) H2, Pd/C, EtOAc, rt
CH3CH(CO2Me)2
Cl
Cl
NaH, THF, 70 oC
12 h, 63%
2) NaNO2, 50% H3PO2 aq
0 oC, 93% (2 steps)
O2N
Cl
12
O2N
Cl
11
Scheme 4. Preparation of di-ester 12
Chloride 12 was prepared starting from 3,4-dichloronitrobenzene, which was easily obtained from
o-dichlorobenzene (Scheme 4). As 3,4-dichloronitrobenzene was reported to react with dimethyl
malonate with high regioselectivity,16 we examined the reaction of 3,4-dichloronitrobenzene with an
anion of dimethyl methylmalonate and found that the reaction in DMF proceeded at 70 °C, providing
di-ester 11 in 63% after 12 h. The catalytic hydrogenation of di-ester 11 in methanol caused
over-reduction, affording a product lacking no chlorine atom, but the reaction in ethyl acetate was
chemoselective, cleanly providing the arylamine corresponding to compound 11. The arylamine was
treated with sodium nitrite in aqueous hypophosphorous acid solution, successfully giving di-ester 12.
Table 1. PLE mediated hydrolysis of 10, 10’, and 12
CO2R
CO2R
CO2H
CO2R
PLE
potassium phosphate
bufer (pH=8), 30 oC
X
X
10 (R=Me, X=I)
10' (R=Et, X=I)
12 (R=Me, X=Cl)
13 (R=Me, X=I)
13' (R=Et, X=I)
14 (R=Me, X=Cl)
Entry
1
Substrate
Time (d)
2
Yield (%)a
78
Ee (%)b
99
10
2
3
10’
12
1
3
63
92
44
99
aIsolated yield. bEe determined by HPLC using the corresponding anilide.

HETEROCYCLES, Vol. 76, No. 1, 2008
187
PLE-mediated asymmetric hydrolysis of di-esters 10, 10’,17 and 12 was performed under the general
conditions12 (Table 1). The hydrolysis of dimethyl ester 10 proceeded with high enantioselectivity,
affording the corresponding mono-ester 13 in 78% yield with 99% ee. Interestingly, the hydrolysis of
diethyl ester 10’ gave the product 13’ with low ee (44% ee). On the other hand, the hydrolysis of dimethyl
ester 12, which was a chloride, provided mono-ester 14 with 92% yield and 99% ee. Comparing the
preparation method of iodide 10 with that of chloride 12, chloride 12 was easier to prepare and suitable
for a large-scale synthesis. Consequently, we decided to employ mono-ester 14 for further
transformations. Although the absolute configuration of 14 was undetermined at this point, the hidden
symmetry in the structure of chiral mono-ester 14 implied that mono-ester 14 would be converted to both
enantiomers of alcohol 1. Hence, we continued further synthetic studies from 14 with assuming its
absolute structure as shown in Scheme 5.
CO2H
COCl
NaBH4, MeOH, THF
-30 oC, 86% (2 steps)
(COCl)2, DMF, CH2Cl2
0 oC to rt
CO2Me
CO2Me
Cl
Cl
14
15
OMOM
OH
CO2Me
MOMCl, DIPEA, NaI
CH2Cl2, reflux, 91%
1N-NaOH, MeOH
reflux, ovn, 89%
CO2Me
Cl
Cl
17
16
1) (COCl)2, DMF, CH2Cl2
0 oC
OMOM
CONH2
OMOM
CO2H
2) NH3, THF, 0 oC
77% (2 steps)
Cl
Cl
18
19
Scheme 5. Preparation of amide 19 from mono-ester 14
Mono-ester 14 was converted to acid chloride 15 (Scheme 5), which was reduced with sodium
borohydride to provide alcohol 16 in 86% yield (2 steps).18 Alcohol 16 was converted to MOM ether 17,
which resisted conversion to amide 19 under any conditions. This can probably be attributed to the steric
hindrance derived from the quaternary carbon adjacent to the ester group. Therefore, MOM ether 17 was
subjected to hydrolysis, and the resulting carboxylic acid 18 was converted to the reactive acid chloride,
followed by the reaction with ammonia to provide amide 19.
In general, reaction of a functional group adjacent to a quaternary carbon is slow due to the steric
hinderance. In addition, the copper(I)-mediated amidation of aryl chloride is slow. However, the reaction
points of amide 19 are set closed; hence, its intramolecular aryl amidation was expected to provide lactam

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HETEROCYCLES, Vol. 76, No. 1, 2008
20. First, we examined the copper(I)-mediated intramolecular reaction of amide 19 under Buchwald’s
conditions, using N, N’-dimethyl ethylenediamine (Table 2).19 The reaction carried out with potassium
carbonate in toluene provided lactam 20 in 66% (entry 1), but a certain amount of amide 19 remained
unreacted. Use of one equivalent of CuI did not improve the yield, and the reactions by use of potassium
phosphate (entry 2) or cesium carbonate (entry 3) reduced the yield. The reaction in dioxane at reflux
temperature (entry 4) lowered the yield. However, although the reaction in DMF at 100 °C (entry 5) was
unsatisfactory, the reaction in DMF at reflux temperature (entry 6) gave lactam 20 in 76% yield.
Table 2. Intramolecular aryl amidation of amide 19
NHMe
(1.0 equiv)
NHMe
OMOM
O
OMOM
CONH2
CuI (0.5 equiv),
base (1.5 equiv)
conditions, 1 d
N
Cl
H
19
20
Entry
Solvent
toluene
toluene
toluene
dioxane
DMF
Base
K2CO3
K3PO4
Cs2CO3
K2CO3
K2CO3
K2CO3
Temp
reflux
reflux
reflux
reflux
100 °C
reflux
Yield (%)a
1
66
33
57
24
53
76
2
3
4
5
6
DMF
aIsolated yield.
OMOM
O
OH
1) NaH, MeI, THF, 0 oC, 97%
26
[α]D -8.6 (c 0.42, CHCl3)
O
( lit. [α]D29 -8.5 (c 0.42 CHCl3)10)
N
H
2) c HCl, MeOH, 50 oC, 81%
N
20
1
Scheme 6. Formal total synthesis of (–)-physostigmine
Lactam 20 thus obtained was N-methylated, and subsequent acid treatment successfully deprotected the
MOM group, providing alcohol 1. Synthesized alcohol 1 was identical with the known compound 1 in all
respects (1H-NMR, IR, MS, [α]D, and 13C-NMR)10, indicating that the formal total synthesis of
(–)-physostigmine was achieved. This enantioselective total synthesis proved the absolute structure of

HETEROCYCLES, Vol. 76, No. 1, 2008
189
mono-ester 14 is as shown in Scheme 5, too.
In summary, we have developed the synthetic method of di-ester 12, and the PLE-mediated asymmetric
hydrolysis of di-ester 12 afforded chiral mono-ester 14 with 92% yield and 99% ee. The
copper(I)-mediated intramolecular aryl amidation of amide 19 under modified Buchwald’s conditions
successfully provided lactam 20, which was converted to known alcohol 1 to complete the formal total
synthesis of (–)-physostigmine. Chiral mono-ester 14 possessing a stereogenic quaternary carbon center at
the benzylic position could be used for enantioslective synthesis of other natural and unnatural products.
All reactions in this synthesis can be carried out on a large scale, allowing supply of a considerable
amount of alcohol 1, which would be a key intermediate for the library of (–)-physostigmine.
ACKNOWLEDGEMENTS
This work was financially supported in part by a Waseda University Grant for Special Research Projects
and a Grant-in-Aid for Scientific Research on Priority Areas (Creation of Biologically Functional
Molecules (No. 17035082)) from MEXT, Japan. We are also indebted to 21COE "Practical
Nano-Chemistry."
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