A new synthesis of Entacapone and report on related studies

Article abstract of DOI:10.1007/s12039-015-0961-4

A new synthesis of the catechol-O-methyltransferase (COMT) inhibitor, entacapone (E-isomer) has been achieved under mild conditions by amine-mediated demethylation of the precursor 2-Cyano-3-(3- hydroxy-4-methoxy-5-nitrophenyl) prop-2-eneamide, wherein the methoxyl group adjacent to a nitro group gets demethylated under nucleophilic attack. Similar demethylation was achieved on ethyl 2-cyano-3-(3, 4-dimethoxy-5-nitrophenyl) prop-2-enoate, 2-cyano-3-(3,4-dimethoxy-5-nitrophenyl)-N,N-diethylprop-2-enamide, ethyl 2-cyano-3-(3-hydroxy-4-methoxy-5-nitrophenyl) prop-2-enoate and ethyl 2-cyano-3-(4-methoxy-3-nitrophenyl) prop-2-enoate. The scope of demethylation has been studied. Analogues of ethyl 2-cyano-3-(3, 4-dimethoxy-5-nitrophenyl) prop-2-enoate wherein a methoxyl group is not adjacent to a NO ₂ group are unaffected and phenolic derivatives yield the amine salts. Entacapone has been converted to salts with organic bases. The crystal structure of the isomer of entacapone (Z-isomer), a significant human metabolite of E-isomer has been established. NMR methods for deriving E and Z geometry and other similar molecules have been successfully established, mainly by studying the proton coupled ¹³C spectra. Preliminary studies reveal in vitro activity for some compounds against tuberculosis (TB) and dengue. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s12039-015-0961-4

c
J. Chem. Sci. Vol. 127, No. 11, November 2015, pp. 1977–1991. ꢀ Indian Academy of Sciences.
DOI 10.1007/s12039-015-0961-4
A new synthesis of Entacapone and report on related studies
ATTIMOGAE SHIVAMURTHY HARISHAa,b, SURESH PARAMESHWAR NAYAKb,
PAVAN M Sc, SHRIDHARA Ka, SUNDARRAJA RAO Ka, RAJENDRA Kd, KOTEPPA PARId,
SIVARAMKRISHNAN Hd, GURU ROW T Nc,∗ and KUPPUSWAMY NAGARAJANa,∗
aAlkem Laboratories Ltd., R & D Centre, Industrial Estate, 4th Phase, Bangalore 560 058, India
bDepartment of Chemistry, Mangalore University, Mangalagangothri, Mangalore 574 199, India
cSolid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India
dPiramal Life Sciences, Piramal Health Care, Mumbai, Maharashtra 400 030, India
e-mail: ssctng@sscu.iisc.ernet.in; dknb69@yahoo.com
MS received 30 April 2015; revised 1 June 2015; accepted 28 August 2015
Abstract. A new synthesis of the catechol-O-methyltransferase (COMT) inhibitor, entacapone (E-isomer)
has been achieved under mild conditions by amine-mediated demethylation of the precursor 2-Cyano-3-(3-
hydroxy-4-methoxy-5-nitrophenyl) prop-2-eneamide, wherein the methoxyl group adjacent to a nitro group
gets demethylated under nucleophilic attack. Similar demethylation was achieved on ethyl 2-cyano-3-(3,
4-dimethoxy-5-nitrophenyl) prop-2-enoate, 2-cyano-3-(3,4-dimethoxy-5-nitrophenyl)-N,N-diethylprop-2-ena-
mide, ethyl 2-cyano-3-(3-hydroxy-4-methoxy-5-nitrophenyl) prop-2-enoate and ethyl 2-cyano-3-(4-methoxy-
3-nitrophenyl) prop-2-enoate. The scope of demethylation has been studied. Analogues of ethyl 2-cyano-3-(3,
4-dimethoxy-5-nitrophenyl) prop-2-enoate wherein a methoxyl group is not adjacent to a NO2 group are unaf-
fected and phenolic derivatives yield the amine salts. Entacapone has been converted to salts with organic bases.
The crystal structure of the isomer of entacapone (Z-isomer), a significant human metabolite of E-isomer has
been established. NMR methods for deriving E and Z geometry and other similar molecules have been suc-
cessfully established, mainly by studying the proton coupled 13C spectra. Preliminary studies reveal in vitro
activity for some compounds against tuberculosis (TB) and dengue.
Keywords. Mild demethylation; crystal structure; entacapone; Z-isomer; carbon, proton NMR;
E, Z geometry.
the bioavailability of L-DOPA.8 Among them, the nitro
catechol derivative entacapone 1 has found high accep-
tance as an adjunct for treatment by L-DOPA. Enta-
capone, having the E geometry shown in formula 1,
has been the object of some synthetic, X-ray crystallo-
graphic and NMR studies. The crystal structure of 2 (Z
isomer), which has been identified as one of the major
metabolites in humans9 has not been reported so far.
We have achieved a new approach to the synthesis of 1
and some analogues under mild conditions and studied
its scope. The results have been reported in a prelim-
inary communication.23 We present full experimental
details of the demethylation reaction and extension to
some similar substrates where demethylation does not
occur Amine salts are formed or starting materials are
recovered.23 A few amine salts of water insoluble 1 have
been prepared.23 Also recorded are derivation of the sin-
gle crystal X-ray structure of 2 and extensive applica-
tion of NMR techniques for deriving the geometry of
such molecules.
1. Introduction
Parkinson’s disease (PD)1 described by James Parkinson
in 1817, is one of the most common, intriguing and
debilitating neurological diseases affecting almost five
million people today. PD is a progressive degenerative
disease that usually causes severe disability after 10 to
15 years, causing a heavy social and financial burden,
especially on the elderly, estimated to cost about 10 to
11 billion USD in terms of drug treatment.2,3 Among
the currently available symptomatic interventions for
the pharmacological treatment of PD, therapy with L-
DOPA in combination with a DOPA decarboxylase is
widely prescribed. During such treatment, the enzyme
COMT4,5 metabolizes L-DOPA by another mechanism
and reduces its availability.6,7 COMT inhibitors have
been developed to inhibit this enzyme and increase
∗For correspondence
1977

1978
Attimogae Shivamurthy Harisha et al.
The aqueous layer was evaporated to isolate methyl-
2. Experimental
triethylammonium iodide salt 7, MR: 327-328◦C; 1H
NMR (400MHz, D2O): δ 1.30 (m, 9H), 2.93 (s, 3H),
3.32(q, J = 7.0Hz, 6H), identical with a sample pre-
pared as per literature procedure,22 MR: 329-330◦C
(mixed MR: 327-328◦C).
2.1 General information
All reagents were purchased from commercial sources
and used without further treatment, unless otherwise
indicated. The products were purified by column chro-
Compound 6 obtained above was identical with an
authentic sample got by heating ethyl cyanoacetate and
3-methoxy-4-hydroxy-5-nitrobenzaldehyde in ethanol
using the conditions employed for 4. MR: 165-166◦C
(mixed MR: 164-165◦C).
1
matography over silica gel 230-400 mesh. H NMR
spectra were recorded on 300, 400 or 500 MHz and 13C
spectra on 75 or 125 MHz. CDCl3 was the solvent used
generally and DMSO-d6 occasionally with TMS as an
internal standard. Melting points were determined with
a Buchi-545 instrument. HPLC chromatograms were
recorded with an Agilent 1200 instrument.
2.1c 2-Cyano-N,N-diethyl-3-(4-hydroxy-3-methoxy-5-
nitrophenyl) prop-2-enamide (10): Compound 9 (3.3
g, 0.01 mol) was subjected to reaction with triethy-
lamine in ethanol as under 6. Ethanol was distilled
off and the residue was dissolved in water, acidified
under cooling with dilute hydroiodic acid; the product
was extracted into ethyl acetate, dried organic layer
was evaporated to get the crude demethylated product.
This was purified by silica gel column chromatogra-
phy using (hexane/ethyl acetate as mobile phase) to
afford 10. 56% isolated yield as yellow solid. MR:
109-110◦C; 1H NMR (400 MHz, CDCl3): δ 1.27 (br s,
6H), 3.50 (br s, 4H), 4.02 (s, 3H), 7.60 (s, 1H), 7.98 (d,
J = 1.9Hz, Ar-H), 8.02(d, J = 1.9Hz, Ar-H), 11.06
(br s, -OH); ms: m/z 318.1 (negative mode). This was
identical with an authentic sample,11 MR: 106-108◦C
(mix MR: 105-108◦C).
2.1a Ethyl 2-cyano-3-(3, 4-dimethoxy-5-nitrophenyl)
prop-2-enoate (4): Ethyl cyanoacetate (11.3 g, 0.1
mol) was taken in ethanol (100 mL) to which 2-3 drops
piperidine were added and the mixture stirred for 10-15
min. 3,4-Dimethoxy-5-nitrobenzaldehyde (21.1 g, 0.1
mol) was added slowly to the mixture which was then
heated to reflux until a TLC test showed the reaction
was complete. The mixture was cooled to 25-30◦C, the
precipitate was filtered off, washed with ethanol and
then recrystallized from the same solvent to afford 4.
81% was the isolated yield as off white solid. MR:118-
119◦C; H-NMR (400MHz, CDCl3): δ 1.40 (t, J =
1
7.1Hz, 3H), 4.00 (s, 3H), 4.08 (s, 3H), 4.40 (q, J =
7.1Hz, 2H), 7.70 (d, J = 1.7Hz, Ar-H), 8.04 (d, J =
1.7Hz, Ar-H), 8.14 (s, 1H), ms: m/z 305.9(negative
mode).
2.1d Ethyl 2-cyano-3-(3-hydroxy-4-methoxy-5-nitrophenyl)
prop-2-enoate (11): This was prepared from isoni-
trovanillin and ethyl cyanoacetate according to the con-
ditions used for 4. It was recrystallized from ethanol to
afford pure 11. 86% isolated yield as off white solid.
MR: 189-190◦C; 1H-NMR (400 MHz, DMSO): δ 1.29
(t, J = 7.1Hz, 3H), 3.95 (3H, s), 4.29(q, J = 7.0Hz,
2H), 7.91(d, J = 2.1Hz, Ar-H), 7.95 (d, J = 2.1Hz,
Ar-H), 8.33 (s, 1H), 11.12(br s,-OH); ms: m/z 291.1
(negative mode); Analysis: Calcd (%) for C13H12N2O6
(292.24): C 53.42, H 4.14, N 9.58. Found (%): C 53.43,
H 3.65, N 9.52.
2.1b Ethyl 2-cyano-5-(4-hydroxy-3-methoxy-5-nitrophenyl)
prop-2-enoate (6)- demethylation of (4): A solution
of 4 (3.0 g, 0.01 mol) and triethylamine (3.03 g, 0.03
mol) in ethanol was heated to 70-75◦C for 12 h. Ethanol
was distilled off, the residue was dissolved in water, the
solution acidified with dilute hydrochloric acid and the
product extracted into ethyl acetate. The organic layer
was dried with sodium sulphate and evaporated to get
the crude demethylated product. This was purified using
silica gel column chromatography (hexane/ethylacetate
as mobile phase) to afford 6, 60% isolated yield as light
yellow solid. MR: 165-166◦C; 1H-NMR (400MHz,
DMSO): δ 1.28 (t, J = 7.0Hz, 3H), 3.90(s, 3H), 4.30(q, 2.1e Ethyl 2-cyano-3-(3,4-dihydroxy-5-nitrophenyl)prop-
J = 7.0Hz, 2H), 7.96(unresolved doublet, Ar-H), 8.30 2-enoate (12): Demethylation of 11 (2.92 g, 0.01 mol)
(unresolved doublet, Ar-H), 8.35(1H, s), 11.01(br s, with triethylamine in ethanol was carried out as per
–OH), ms: m/z 291.8.
6. The crude product was recrystallized with ethyl
In another experiment, the crude reaction mass acetate to afford 12. 48% isolated yield as light yellow
1
was dissolved in water, the solution acidified with powder. MR: 205-206◦C ( lit.22:205-206◦C); H-NMR
hydroiodic acid and the product 6 extracted into ethy- (400 MHz, DMSO): δ 1.28 (t, J = 7.0Hz, 3H), 4.29
lacetate.
(q, J = 7.1Hz, 2H), 7.82 (d, J = 2.0Hz, Ar-H), 8.07 (d,

Synthesis of Entacapone and related studies
1979
J = 2.0Hz, Ar-H), 8.22 (s,1H), 10.71 (br s,2-OH); ms: 4H), 3.64 (s,3H), 4.23 (q, J J = 7.1Hz, 2H), 7.47(s,1Ar-
m/z 277.0(negative mode); HRMS(EI):m/z(%): calcd H), 7.89 (s, -H), 7.90(s, 1Ar-H), 8.10 (s,1Ar-H),8.25
for C12H10N2O6Na: 301.0437; found: 301.0455.
(br s, -OH).
2.1i Ethyl 2-cyano-3-(4-methoxy-3-nitrophenyl) prop-
2.1f 2-Cyano-3-(3-hydroxy-4-methoxy-5-nitrophenyl) 2-enoate (16): This was prepared from reaction of
prop-2-eneamide (13): To a solution of N, N- ethyl cyanoacetate and 4-methoxy-3-nitrobenzaldehyde
diethylcyanoacetamide (14.1 g, 0.1 mol) in toluene was (18.1 g, 0.1 mol) in ethanol as usual. The product was
added 0.2 g IRA 96 resin and isonitrovanillin (19.7 recrystallized with ethanol to afford 16. 65% isolated
g, 0.1 mol). The mixture was heated to reflux with yield as off white solid. MR: 187-188◦C. 1H NMR (400
azeotropic distillation of water. The reaction was con- MHz; CDCl3) δ 1.39 (t, J = 7.2Hz, 3H), 4.06 (s, 3H),
tinued until a TLC test showed completion of reaction. 4.39 (q, J = 7.2Hz, 2H), 7.23 (d, J = 5.4Hz, Ar-H),
The mass was filtered hot. The filtrate was concentrated 8.16 ( s, 1H), 8.36 (d, J = 2.8Hz, Ar-H), 8.38 (d, J =
under vacuum to give the crude product, which was 5.4, 2.8Hz, Ar-H); ms: m/z 277.2 (positive mode).
recrystallized from absolute alcohol to afford 13. 61%
isolated yield as light yellow solid. MR: 175-176◦C;
2.1j Ethyl 2-cyano-3-(4-hydroxy-3-nitrophenyl) prop-
2-enoate (17): Demethylation was carried out on 16
(2.7 g, 0.01 mol) under standard conditions. The crude
1H-NMR (400 MHz, DMSO): δ 1.13 (br s, 6H), 3.39
(br s, 4H), 3.92 (s, 3H), 7.67 (s, 1H), 7.76 (d, J = 2.1Hz,
Ar-H), 7.78 (d, J = 2.1Hz, Ar-H), 10.98 (br s,-OH); ms:
product was crystallized from ethanol to afford crude
m/z 318.1 (negative mode).
17. 57% isolated yield as red solid. MR: 169-170◦C. 1H-
NMR (400 MHz, CDCl3) δ :1.39 (t, J = 7.2Hz, 3H),
4.04 (q, J = 7.2Hz, 2H), 7.31 (d, J = 8.7Hz, Ar-H),
2.1g (2E)-2-Cyano-3-(3, 4-dihydroxy-5-nitrophenyl)- 8.18 (s, 1H), 8.40 (dd, J = 8.8, 2.2Hz, Ar-H), 8.60
N,N-diethylprop-2-enamide, entacapone (1): Acry- (d, J = 2.1Hz, Ar-H),10.9 (br s, -OH); ms: m/z 261.0
lamide 13 (3.19, 0.01 mol) was demethylated with tri- (negative mode).
ethylamine in ethanol at 70-75◦C for 12 h. After com-
pletion of the reaction, the intermediate salt (2.9 g)
was isolated by distilling off the solvent. The salt was
2.1k Ethyl 2-cyano-3-(3-methoxy-2-nitrophenyl) prop-
2-enoate (19): This was prepared from 3-methoxy-2-
dissolved in water, acidified with hydrochloric acid,
nitrobenzaldehyde (18.1 g) under standard conditions.
and the product extracted into ethyl acetate. The dried
85% isolated yield as colorless solid. MR: 139-140◦C;
organic layer was evaporated to get the crude demethy-
1H-NMR (400 MHz, CDCl3): δ 1.41 (t, J = 7.2Hz,
lated product. This was purified through silica gel
column chromatography using hexane/ethyl acetate as
3H), 3.98 (s, 3H), 4.39 (q , J = 7.2Hz, 2H), 7.27 (d,
J = 8.4Hz, Ar-H), 7.61 (t, J = 8.1Hz, Ar-H), 7.80 (d,
mobile phase to afford pure 1. 45% isolated yield as
J = 8.1Hz, Ar-H), 8.15 (s, 1H); ms: m/z 295.6 water
1
yellow solid. MR: 162-163◦C (lit.10:162-163◦C). H-
adduct (positive mode).
NMR (400 MHz, CDCl3): δ 1.20 (br s, 6H), 3.43 (br
s,4H), 7.44 (s,1H), 7.84 (unresolved doublet, Ar-H),
7.94 (unresolved doublet, 1ArH), 9.67(br s, -OH), 10.79
2.1l Diethylamine salt of 2-cyano-N, N-diethyl-3-(4-
(br s, -OH); 13C-NMR( CDCl3,75MHz) : δ 162.90,
148.96, 148.36, 146.88, 133.81,124.03 120.69, 118.41,
116.62, 107.08, 43.42, 40.72, 13.50, 12.50; ms: m/z
=304.1 (negative mode); HPLC purity >99%. The
yield of 1 was increased to 55% when the reaction time
was extended to 14 h.
hydroxy-3-methoxyphenyl)prop-2-enamide 22 (R=N
Et2): Attempted demethylation of 20 (R=N Et2)
(2.74 g 0.01 mol) made from vanillin and N, N-
diethylcyanoacetamide under standard conditions with
diethylamine gave only the salt. 76% isolated yield as
1
yellow solid. MR: 140-141◦C; H NMR (400 MHz,
CDCl3): δ 1.2 (t, 6H), 1.27 ( br t, 6H), 2.82(q,
J =7.1Hz, 4H), 3.52 (q, J = 7.1Hz, 4H), 3.90 (s, 3H),
6.81 (unresolved dd,1ArH), 7.26 (d, J = 6.9Hz, 1ArH),
7.62 (s, H), 7.72 (d, J = 2.1Hz, 1ArH).
2.1h Diethylamine salt of ethyl 2-cyano-3-(4-hydroxy-
3-methoxy-5-nitrophenyl) prop-2-enoate (15): Attempted
demethylation of 6 with diethylamine in ethanol gave
only the salt 15. 88% isolated yield as yellow solid. MR: 2.1m Diethylamine salt (23) of (21): Attempted
128-129◦C; 1H-NMR(400 MHz, CDCl3): δ 1.25 (t, J = demethylation of 21 (2.47 g, 0.01 mol) made from
7.2Hz, 6H), 1.32(t, J = 7.2Hz, 3H), 2.94 (q, J = 7.1Hz, vanillin and ethyl cyanoacetate with diethylamine

1980
Attimogae Shivamurthy Harisha et al.
(2.2g, 0.03mol) in ethanol at 50-55◦C for 11 h gave 23. When salt formation was complete, the precipitated salt
85% isolated yield as light red solid. MR: 134-135◦C; solid was filtered and dried to afford the product.
1H-NMR (400 MHz, DMSO): δ 1.09 (t, J = 7.2Hz,
6H),1.26 (t, J = 7.0Hz, 3H), 2.80 (q, J = 7.2Hz, 4H),
2.1s Diethyl amine salt (30): 92% isolated yield as
light yellow solid. MR: 205-206◦C.
3.68 (s, 3H), 4.18 (q, J= 7.0Hz, 2H), 6.35 (d,
J = 8.6Hz, 1ArH), 7.34(q, J = 6.7,1.8Hz, 1ArH), 7.66
(d, J = 1.8Hz, 1ArH), 7.80 (s, 1H).
2.1t Triethylamine salt (31): 78% isolated yield as
red colored solid. MR: 203-204◦C.
2.1n Ethyl 2-cyano-3-(4-hydroxy-3, 5-dimethoxyphenyl)
prop-2-enoate (24): Condensation of 4-hydroxy-3, 5-
2.1u Piperidine salt (32): 77% isolated yield as yel-
dimethoxybenzaldehyde (18.2 g, 0.1 mol) with ethyl
low solid. MR: 156-159◦C.
cyanoacetate (11.3 g, 0.1 mol) gave 24. 80% isolated
1
yield as yellow solid. MR: 177-178◦C; H-NMR (300
2.1v Di-isopropyl ethylamine salt (33): 80% iso-
MHz, DMSO): δ 1.3 (t, J = 6.0Hz, 3H), 3.82 (s, 6H),
lated yield as yellow solid. MR: 102-103◦C.
4.31 (q, J =6.0Hz, 2H), 7.53 (s, 2Ar-H), 8.26 (s, 1H),
9.98 (s,-OH); ms: m/z 276.0 (negative mode).
2.1w Morpholine salt (34): 76% isolated yield as
red colored solid. MR: 156-157◦C.
2.1o Diethylamine salt of ethyl 2-cyano-3-(4-
hydroxy-3, 5-dimethoxyphenyl)prop-2-enoate (25).
(B=Et2 N): Attempted demethylation of 24 with
3. Results and Discussion
diethylamine in ethanol gave the salt 25. 88% iso-
lated yield as dark yellow solid. MR: 190-191◦C.
3.1 Demethylation by nucleophilic attack
1H-NMR(300MHz, DMSO) : δ 1.14 (t, J = 6.0Hz, 6H),
1.25 (t, J = 6.0 Hz, 3H), 2.85 (q, J = 6.0Hz, 4H), 3.77
Currently available methods for the synthesis of enta-
capone 1 are generally covered by scheme 1.10–15
In the above processes, the nitro catechol moiety
is formed at an early or late stage by demethyla-
(s, 6H), 4.25 (q, J = 6.0Hz, 2H), 7.28 (s, 2Ar-H), 7.34
(s, 1H), 7.97 (s,-OH); ms: m/z274.9 (positive mode).
2.1p Piperidine salt of ethyl-2-cyano-3-(4-hydroxy- tion of a nitrovertraldehyde or nitrovanillin derivative
3, 5-dimethoxyphenyl) prop-2-enoate (26): The pipe- carrying the entacapone side chain using harsh and
ridine salt 26 of 24 resulted when demethylation was corrosive reagents such as aluminum chloride, hydro-
attempted with piperidine. 80% isolated yield as yellow bromic acid, boron trichloride, thiophenol etc. In gen-
solid. MR: 146-147◦C.1H-NMR (300 MHz, DMSO) δ: eral methyl aryl ethers can be cleaved under similar
1.23 (t, J = 7.06Hz, 3H), 1.55 (6H, m), 2.95 (4H, m), harsh conditions using pyridine hydrochloride alone
3.80 (s, 6H), 4.18 (q, J = 7.06Hz, 2H), 7.27 (br s, 2H), under microwave irradiation16 or in a high boiling
7.73(s, 1H); ms: m/z 277.0 positive mode).
solvent. Buckle et al.,17 reported dealkylation of (2-
methoxyphenyl) ethynes using lithium iodide in reflux-
ing 2, 4, 6-trimethylpyridine.18 Nishioka et al.,19 found
that ortho anisic acid can be demethylated with aliphatic
amines in high boiling aprotic solvents like DMSO,
DMF, and dimethyl acetamide at temperatures around
150◦C. It occurred to us that it may be possible to syn-
thesize 1 under mild basic conditions from nitrocate-
chol precursors like 3.
2.1q 2-Cyano-N, N-diethyl-3-(3-hydrox-4-methoxyphenyl)
prop-2-enamide (28): Condensation of 3-hydroxy-4-
methoxybenzaldehyde under conditions used for 13
gave 28, which was recrystallized with methanol. 74%
isolated yield as light yellow solid. MR: 131-132◦C;
1H-NMR (400 MHz, CDCl3) δ :1.24 (br s, 6H), 3.3 (br
s, 4H), 3.96 (s, 3H), 6.92 (d, J = 8.4Hz, Ar-H), 7.45
(dd, J = 6.2, 2.1Hz, Ar-H), 7.5 (d, J = 2.1Hz, Ar-H),
7.57(s, 1H); ms m/z 275.2 (positive mode). Reaction of
28 with triethylamine under standard conditions led to
its recovery.
2.1r Amine salts of Entacapone: These were pre-
pared by treating a solution of entacapone (0.01 mol)
in ethanol with the appropriate amine (0.01 mol). The wherein the methoxyl group on C-4 having strong elec-
reaction mass was stirred for 25-30 min at 25-30◦C. tron withdrawing groups (EWG) at the ortho (NO2)

Synthesis of Entacapone and related studies
1981
Scheme 1. Currently available method for the synthesis of 1.
and para (EWG) positions may be susceptible to nucle- acidified with hydrochloric acid and extracted into ethyl
ophilic attack by bases under mild conditions. There acetate. Evaporation of ethyl acetate layer gave crude
are some reports in the literature which encouraged us demethylated product 6 which was purified by silica gel
in this pursuit. Cahn et al.,20 found a method for the column chromatography to afford pure 6 in a gratify-
dealkylation of phenolic ethers ortho to a nitro group ing yield of 58%. This unknown compound had correct
by pyridine or piperidine. It is also reported by Con- NMR and mass spectral properties and was identical
reaux et al.,21 that 3-alkynyl-4-methoxy-2-pyridinones to the product obtained by condensing 5-nitrovanillin
heated in a reaction mixture containing triethylamine 8 with ethyl cyanoacetate. A preliminary communi-
for a prolonged time underwent dealkylation by SN2 cation has reported these results.23 In another experi-
mechanism.
ment acidification was done by aqueous hydroiodic acid
and the acidic layer from the experiment was evapo-
rated, washed with diethyl ether to give methyltrie-
thylammonium iodide 7, MR 327-328◦C, which was
3.2 Successful demethylation- synthesis of
entacapone
1
characterized by H-NMR spectrum in D2O. The qua-
ternary salt was identical with the product (MR and mix
MR 327-328◦C) obtained by reaction of triethylamine
with methyl iodide in ether22 at 20-25◦C. This estab-
lished that reaction had occurred, as expected by nucle-
ophilic attack of the methyl group on the C-4 methoxyl
by triethylamine to give the demethylated product as
an ammonium salt 5 with the phenoxide anion
We carried out initial model experiments with the sub-
strate ethyl 2-cyano-3-(3, 4-dimethoxy-5-nitrophenyl)
prop-2-enoate 4 which was obtained readily by conden-
sation of 5-nitroveratraldehyde with ethyl cyanoacetate
(scheme 2). Compound 4 was treated with triethylamine
in ethanol at 70-75◦C for 12 h and the solvent evap-
orated to a sticky solid. This was dissolved in water,
Scheme 2. Model demethylation.

1982
Attimogae Shivamurthy Harisha et al.
stabilized by extensive conjugation with the ortho
placed NO2 group and acrylic derivative at the para
position (scheme 3). It is obvious that an analogue
of 4 with no methoxyl in the ortho position but only
in the meta position which will be also meta located
with respect to the acrylic side chain will not undergo
demethylation since the methoxyl oxygen atom is
not conjugating with the two electron withdrawing
groups.
The optimum temperature for demethylation with tri-
ethylamine in ethanol was 70-75◦C; at a lower tempera-
ture 40-45◦C there was no observable demethylation in
ethanol during 12 h. Demethylation was also observed
with diethyl amine and piperidene in ethanol. Surpris-
ingly, nucleophilic addition of these secondary amines
to the electron-deficient double bond was not observed
to any detectable level.
We then extended the demethylation to com-
pound 2-cyano-3-(3,4-dimethoxy-5-nitrophenyl)-N,N-
diethylprop-2-enamide 9 from 5-nitrovertraldehyde and
N, N-diethyl cyanoacetamide.11 Reaction of 9 with tri-
ethylamine in ethanol at 70-75◦C for 12 h and the
product worked up as earlier, followed by crystal-
lization from ethyl acetate gave the expected known
demethylated compound 10 in 56% yield (scheme 4).
This was identical to the product made by condensing
5-nitrovanillin with N, N-diethyl cyanoacetamide.11
Repetition of demethylation of 9 with diethylamine
in ethanol at 50-55◦C for 12 h gave 10 in 40% yield
after chromatography.
Encouraged by these results we extended the
reaction to ethyl 2-cyano-3-(3-hydroxy-4-methoxy-
5-nitrophenyl) prop-2-enoate 11 and 2-cyano-3-(3-hy-
droxy-4-methoxy-5-nitrophenyl) prop-2-eneamide 13.
The last two were prepared from 5-nitroisovanillin by
condensing with ethyl cyanoacetate and N, N-diethyl
cyanoacetamide respectively. Upon heating 11 and 13
in ethanol with triethylamine at 70-75◦C for 12 h and the
usual workup, the expected catechol products, 1224 and
entacapone 1 were obtained in yields of 48% and 45%,
respectively (scheme 4) after silica gel chromatography.
In the case of 13, the crude product was obtained
showing very little O-CH3 signal in the 1H-NMR spec-
trum at δ 4.0 ppm; also, significantly the main product
was 1 with only traces of the Z- isomer 2 (LC). Con-
sidering that the starting material 13 was wholely the E
isomer (vide infra), the demethylative process was not
disruptive of the geometry, 1 (scheme 4). The product
mp 162-163◦C was identical with an authentic sam-
ple and was characterized extensively by NMR studies
(vide infra).
Thus our objective of synthesizing 1 from a O-
methylated precursor was realized in moderate yields
in un-optimized experiments under mild conditions
(scheme 4).23
3.3 Scope of demethylation
A limited number of experiments were then carried out
to define the structural features needed for demethylation
under our conditions. 5-Nitroveratraldehyde 14 itself
Scheme 3. Nucleophilic attack of the methyl group on the C-4 methoxyl by triethylamine.
Scheme 4. Synthesis of entacapone 1 and some analogues.

Synthesis of Entacapone and related studies
1983
Figure 1. ORTEP diagram of compound 23.
Figure 2. ORTEP diagram of compound 25.

1984
Attimogae Shivamurthy Harisha et al.
was also demethylated to 5-nitrovanillin 8. 4-Methoxy- group was meta to the nitro was not demethy-
3-nitro derivative 16 underwent demethylation to 17 lated by diethylamine in hot ethanol but gave
in hot ethanolic triethylamine (scheme 4).23 The the salt of 15 as also the corresponding N, N-
nitrovanillin derived isomer 6 of 11, wherein methoxyl diethylcyanoacetamide.11
Figure 3. Single-crystal X-ray structure ORTEP diagram of diethyl amine salt 30 of 1.
Figure 4. Single-crystal X-ray structure ORTEP diagram of piperidene salt 32 of 1.

Synthesis of Entacapone and related studies
1985
A regioisomer 18 was not demethylated but it under-
went a unique reaction to give dihydro and butadiene
products which were observed by us for a large num-
ber of analogues without a nitro group. The results have
been published in our preliminary communication.23
Similar salt formation occurred with 24 derived from
syringaldehyde upon attempted demethylation with
diethylamine and piperidine. Single crystal X-ray of the
diethylamine product 25 again showed that it was a for-
mal salt and not a complex in spite of having additional
electron donating methoxyls at both ortho positions.
The ORTEP is shown in figure 2.
Another regioisomer 19 gave a complex mixture of
products having a 1H-NMR spectrum wherein aromatic
signals were shifted up field indicating reduction of the
NO2 group. This was also observed for the desmethoxy
derivative of 19 obtained from 2-nitrobenzaldehyde. A
similar complex reaction was observed with nitrover-
traldehyde derivative 20.25
Lastly demethylation was attempted on the iso-
vanillin derivatives 2728 and 28 with diethylamine and
triethylamine.
In both cases neither demethylation nor salt for-
mation was observed. Obviously, the OH group is
not acidic enough in the absence of para located
cyanoacrylic groups.
From results obtained thus far, it can be concluded
that O-demethylation with amines in hot solvents
occurs in aromatic systems represented by 29.
Compounds 2126 and 2211 without a NO2 group
derived from vanillin gave only the salts of the amines
used, under demethylation conditions. It has been
shown in recent years that bases and acids can together
form ionic salts or complexes.27
It may also occur with the corresponding aldehydes
although we have studied only one example (3,4-
dimethoxy-5-nitrobenzaldehyde).
In the case of the diethylamine salt 23 from 21, single
crystal X-ray studies showed that it was a formal salt
and not a complex, with the basic nitrogen carrying two
protons, as shown in figure 1.27
Entacapone 1 which has wide spread clinical use has
poor water solubility and water soluble derivatives may
have some clinical usefulness. Our serendipitous obser-
vation of formation of salts in this study cited above
encouraged us to make salts of 1 deliberately. Accord-
ingly, we were able to obtain crystalline colored salts
with diethylamine 30, triethylamine 31, piperidine 32,
diisopropylethylamine 33 and morpholine 34. Among
these, single X-ray structures of the diethylamine and
piperidene salts are available. Their ORTEPS are shown
in figures 3 and 4.
1
This is also evident from a study of the H-NMR
spectrum of 22 and its diethylammonium salt; signals
of H 5 at δ 6.94 and vinyl H at δ 8.22 were shifted to
upfield to δ 6.34 and δ 7.79, respectively.
While there is one reference for the crystal structure
of 1 showing E configuration as shown in figure 5, the

1986
Attimogae Shivamurthy Harisha et al.
Figure 6. Single crystal X-ray structure ORTEP diagram
of 2.
Figure 5. Single crystal X-ray structure ORTEP diagram
of 1.
intermolecular hydrogen bonding, as well as π. . . π
interactions. The packing diagram is shown in figure 7.
Gas phase optimized energies were calculated for
both the isomers (1 and 2) using Gaussian09. The
calculation shows that after optimization 2, shown in
figure 8, is stabler by −3.1 kcal than the optimized 1,
shown in figure 9.
The energy minimization was also carried out for
both isomers using ChemBio3D Ultra 12.0 software
running on MS-Windows 7 using MM2 Force Field
method. Temperature was set 300 K. In total, 727
iterations were set for the process to complete the
energy minimization step. It is interesting to note that
the energy minimized structures of 1 and 2, as shown
figures 10 and 11, respectively show very strong
similarities with the respective crystal structures. We
have calculated the energies to be 6.88 kcal/mole
and −1.45 kcal/mol, respectively for 1 and 2 forms
with 2 again being stabler than 1 by −8.03 kcal. The
implications of these intriguing results need further
study.
same paper has such a study for the Z-isomer 2 only
as the di-benzoate.29 Since 2 is a major metabolite in
humans, it was considered worthwhile to do its crystal
structure. We have been able to prepare good crystals of
2 and solve the crystal structure (table 1). The ORTEP
is given in figure 6.
Z-isomer 2 crystallizes in P 21/c space group.
The crystal structure is stabilized by both intra and
Table 1. Crystal data and structure refinement for 2.
2
Molecular formula
CCDC Number
Formula weight
Crystal system
Space group
a (Å)
C14H15N3O5
1027892
305.29
Monoclinic
P21/c
8.754(2)
8.061(2)
20.695(5)
90
93.491(8)
90
1457.6(6)
4
b (Å)
c (Å)
α (◦)
β (◦)
γ (◦)
3.4 NMR studies on 1 and 2 and other
arylidenecyanoacetic acid derivatives
V (Å3)
Z
μ. (mm−1
T (K)
)
0.11
We also extended our studies elaborating the methods
for determination of E and Z configurations in such
molecules using NMR techniques. A few interesting
observations on the rotational isomerism around the
amide bond were also noted.
295(2)
0.71073
17038
2864
100
0.065
λ (Å)
Reflns. Collected
Unique reflns.
Completeness (%)
Rint
In the 500 MHz 1HNMR spectrum of 1 in CDCl3 the
four CH2 proton of the amide were seen as a broad-
ened singlet at 3.52 ppm (instead of the usual quar-
tet). This broadening has been noticed already in the
400 MHz proton spectra of 1 as its monopivaloate
R1 (F2)
0.04
0.096
wR2(F2)
Goodness-of-fit
Resolution (2θ)
0.98
52

Synthesis of Entacapone and related studies
1987
Figure 7. Intra and intermolecular hydrogen bonding of 2.
Figure 8. (a) Initial geometry (b) Optimized geometry for 2.
Figure 9. (a) Initial geometry (b) Optimized geometry for 1.

1988
Attimogae Shivamurthy Harisha et al.
Figure 10. 2D 1H-1H NOESY spectrum of 1.
Figure 11. 2D 1H-1H NOESY spectrum of 2.
and dibenzoate.29 The corresponding C atom showed
The 1H and 13C NMR spectra of the 2 however gave
two peaks, δ 43.42 and 40.72 (the latter overlaps with two sets of clean quartets for the methylene protons
the carbon signals of DMSO-d6). 13C HSQC showed (δ 3.43 and 3.26) and two well defined triplets for the
that the broad signal in the proton NMR at 3.52 ppm methyl protons (δ 1.10 and 1.01) respectively. In the
was correlated with the above two 13C signals. Like- 13C NMR, the methylene carbon gave two peaks at
wise, the two CH3 protons were seen as two broad sin- δ 43.25 and 39.98 (overlapping with DMSO-d6 car-
glets of 1.32 and 1.28 ppm, respectively, correlating bon signals) and methyl carbons at δ 13.88 and 12.22.
1
in 13C-HSQC with carbon signals at 13.50 and 12.60 13C-HSQC showed correlation between H singlets at
ppm, respectively. The above data indicate that rota- δ 3.43, 3.26, 1.10 and 1.01, respectively with carbon
tion around the amide carbonyl was slower in the 13C signals at δ 39.98, 43.25, 12.22 and 13.88. Results
NMR time scale than in the proton NMR time scale. reported for 2 benzoate are similar.29 From these obser-
1
The H NMR spectra of other enamides 9, 10, 13 and vations it appears that the barrier to free rotation of the
28 reported in this paper as well as the salts of enta- amide bond is less in 1 than in 2.
capone also exhibited broad peaks for the methyl and
In the first attempt at assigning E and Z configura-
methylene protons.
tions for 1 and 2, respectively, we utilized their energy

Synthesis of Entacapone and related studies
Table 3. Proton coupled carbon for 1, 2, 11 and 13.
1989
H
minimized structures 10 and 11. As expected, in the
2D 1H-1H NOESY spectrum of 1 (Figure 10) there was
a strong NOE behave the ene and H-6 protons. In the
similar spectrum for the 2 (Figure 11), there was a
strong NOE between the ene and H-2 protons.
We could achieve even stronger confirmation by
studying the coupling between the C=O and CN car-
bons with the ene proton in the 1 and 2. It was shown
earlier by one of us30 and others38 that the 3J C-H cou-
pling in the ethylenic systems A and B are geometry
dependent with the Jvalues for trans being higher than
the cis.
3J
C
ꢁ
−
H
3J
C
|||
−
O
1
2
11
13
5.78
14.0
10.26
6.94
7.4
9.38
14.03
14.05
X-ray studies published in 2006, which revealed a E-
configuration 37.33 The compound has been shown as
the E-isomer by Woo-Jin Yoo et al.,34 but with no evi-
dence. Likewise, Zhiping Li et al.35 and Chris D Smith
1
et al.,36 have assigned the E-structure by H-NMR. In
the paper of Jose Barluenga et al.,37 the compound
has been obtained as a 70:30 mixture of inseparable
Z/E isomers. The major isomer is said to be Z, but in
the light of our studies and X-ray, it is really E. Sur-
prisingly, all these papers, including the X-ray crystal-
lographic study,33 have ignored the seminal paper of
Kingsbury et al.,38 who have derived the current con-
figuration by using 13C-H splitting for this molecule.
While confirming this assignment by our own study of
13C-H couplings [δ Ph.CO, 195.5 ppm, 3J CH (ene pro-
ton), 10.2 Hz; δCO.OEt, 164.7 ppm, 3J CH (ene pro-
ton), 8.3 Hz], we are providing independent proof from
NOESY studies. Thus, the ene proton shows significant
NOES with CH2 and CH3 protons of the ester which is
possible only in the E-configuration 37.
For this exercise, we studied the 1H coupled 13C spec-
tra of model compounds 3531 and 3632 and obtained the
3J H – C values (table 2.). In the fully proton cou-
pled carbon spectra of 1 and 2, we got values clearly
confirming our previous deductions of H atom and CO
group being cis in 1 and trans in 2 (table 3), thus con-
firming E and Z geometries respectively for 1 and 2.
Data presented in (table 3) for compounds 11 and 13
also establish their geometry as being E. Noting that
the chemical shifts of the ene protons in the aryli-
denecyanoacetates discussed in this paper have chemi-
cal shifts of around δ 8 ppm and the cyanodiethylamides
with the exception of 2 around 7 ppm, they can all be
assigned the E geometry as shown in their respective
structures.
3.5 Biological activities of 1 and analogues
We extended the technique to assign the geometry
of ethyl benzylidenebenzoylacetate 37 (synthesized as
per literature,33 MR 88-89◦C), as literature reports have
been conflicting in the configuration assignment. The
reports have not given definitive proof for the assign-
ments of configurations expect for the single crystal
In a recent article, based upon docking studies with
the InhA,39 entacapone 1 was expected to have anti-TB
activity which was confirmed by in vitro studies. The
MIC was reported to be between 62.5 and 125 μg/ml.
Importantly, it has been noted that it can be useful for
treating MDR and XDR TB infections. In the course of
our research, we had access to 1 and several analogues
(some not reported in this paper) which were exam-
ined for in vitro anti-TB activity against Mtb H37Ra/Rv
were more active with MIC of 12.5μM. These are
being studied further for activity against resistant TB
and cytotoxicity. Likewise, there have been literature
Table 2. 1H coupled 13C spectra of compounds 35 and 36.
3J C - H
trans
cis
35
36
12.2
14.4
7.4
8.5

1990
Attimogae Shivamurthy Harisha et al.
9. Wikberg T, Vuorela A, Ottoila P and Taskinen J 1993
Drug Metab. Dispos 21 81
reports of molecules with an α, β-unsaturated carbonyl
system having in vitro anti-dengue activity.40 Gratify-
ingly, some of our molecules did show this activity.41
We plan to continue our studies to realize the poten-
tials uncovered so far for molecules like 1 in the treat-
ment of resistant TB and dengue for which no therapy is
available as of now and report the results in the future.
10. Pipuri, Honkanen and Haarala 1991 EP Patent
0,426,468
11. Veera Reddy A and Rajendiran C 2005 WO Patent
2005063693A1
12. Cziaky Z 2006 WO Patent 2006064296A1
13. Bartra Sanmarti M, Solsona Rocabert J G, Palomo
Nicolau F E and Molina Ponce A 2008 WO Patent
2008119793
14. Srikanth G, Ray Uttam Kumar, Srinivas Rao D V N,
Badarinath Gupta P, Lavanya P and Islam Aminul 2012
Synth. Commun. 42 1359
4. Conclusions
We have demonstrated the vulnerability to amines
under mild conditions, of an aromatic methoxyl adja-
cent to a nitro group and having strong electron with-
drawing groups in the para position and applied it to
produce entacapone 1, a clinically used adjuvant for the
treatment of Parkinson’s disease. X-ray crystal structure
of the major human metabolite 2 has been established.
Extensive NMR studies have also been carried out to
derive the geometry of 1,2 and similar molecules. Early
15. Veerareddy A and Reddy G S 2014 Synth. Commun. 44
1274
16. Kulkarni P P, Kadam A J, Mane R B, Uday V D and
Wadgaonkar P P 1999 J. Chem. Res. (S) 394
17. Buckle D R and Rockell C 1985 J. Chem. Soc., Perkin
Trans. 1 2443
18. Arcadi A, Cacchi S, Del Rosario M, Fabrizi G and
Marinelli F 1996 J. Org. Chem. 61 9280
19. Nishioka H, Nagasawa M and Yoshida K 2000 Synthesis
2 243
20. Cahn R S 1931 J. Chem. Soc. 1121
indications of anti-TB and antidengue activities for 21. Conreaux D, Belot S, Desbordes P, Monteiro N and
Balme G 2008 J. Org. Chem. 73 8619
some molecules reported in this paper will be pursued.
22. Brescia A, Arents J, Meislich H and Turk A 1973 In
Fundamental of chemistry (3rd ed.) (London: Academic
press) p.413
Supplementary Information
23. Harisha A S, Nayak S P, Nagarajan K, Guru Row T N
and Amar A. Hosamani 2015 Tetrahedron Lett. 56 1427
24. Zhang S J, Zheng X M and Hu W X 2009 Acta Crystal-
logr. Sect. E 65 2351
1H, 13C NMR, HSQC and 2D 1H-1H NOESY spec-
tra for all new compounds are available. available
at www.ias.ac.in/chemsci. Crystallographic data 2
(CCDC1027892), 23 (CCDC1027891), 25 (CCDC10
27893), 30 (CCDC1027894) and 32 (1027895) have
been deposited at the Cambridge Crystallographic
Database Centre and are available on request (http:
www.ccdc.cam.ac.uk).
25. Kametani T, Nyu K and Yamanaka T 1971 Chem.
Pharm. Bull. 19 1321
26. Dai G, Shi D, Zhou L and Huaxue L Y 1995 Chinese J.
Appl. Chem. 3 103
27. Sreenivas Reddy L, Sarah J, Bethune Jeff W. Kampf
and Nair Rodriguez-Hornedo 2009 Cryst. Growth Des.
9 378
28. Mitra A K, Banerjee S K and Sharmistha C 2003 J.
Indian Chem. Soc. 80 921
Acknowledgements
29. Leppanen J, Wegelius E, Nevalainen T, Jarvinen T,
Gynther J and Huuskonen J 2001 J. Mol. Struct. 562 129
30. Ulrich V, Wolfgang V P, Nagarajan K and Nair M D
1978 HeIv. Chim. Acta 61 607
31. Francisco D, Joaquin T, Gerard Z, Miguel L, Rene M
and Jose G 1995 Synth. Commun. 25 753
We thank Alkem laboratories of India for providing
research facilities and OSDD CSIR for in vitro anti TB
results.
32. (a) Saito T, Goto H, Honda K and Fujii T 1992 Tetrahe-
dron Lett. 33 7553; (b) Angeletti E, Canepa C, Martinetti
G and Venturello P 1988 Tetrahedron Lett. 29 2261; (c)
Prajapti D, Lekhok K C, Snadhu J S and Ghosh A C
1996 J. Chem. Soc., Perkin Trans. 1 959; (d) Chalais S,
Laszlo P and Mathy A 1985 Tetrahedron Lett. 26 4453
33. Hema R, Parthasarathi V, Ravikumar K, Sridhar B and
Pandiarajan K 2006 Acta Crystallogr. Sect. E 62 708
34. Yoo W J and Li C J 2006 J. Org. Chem. 71 6266
35. Li Z, Li H, Guo X, Cao L, yu Rango, Li H and Pan S
2008 Org. Lett. 10 803
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