Novel α-amino-acid phenolic ester derivatives with intravenous anaesthetic activity

Article abstract of DOI:10.1016/S0960-894X(03)00025-8

A novel series of α-amino-acid phenolic ester derivatives containing sulphide, sulphoxide, sulphone, ester and amide side chains were prepared and shown to display potent intravenous anaesthetic activity.

Full text of DOI:10.1016/S0960-894X(03)00025-8

Bioorganic & Medicinal ChemistryLetters 13 (2003) 1107–1110
Novel ꢀ-Amino-Acid Phenolic Ester Derivatives with Intravenous
Anaesthetic Activity
Andrew Cooke,* Alison Anderson, Jonathan Bennett, Kirsteen Buchanan,
David Gemmell, Niall Hamilton, Maurice Maidment, Petula McPhail, Donald Stevenson
and HardySundaram
Departments of Medicinal Chemistry and Pharmacology, Organon Laboratories Ltd., Newhouse, Scotland, ML1 5SH, UK
Received 5 November 2002; revised 17 December 2002; accepted 3 January2003
Abstract—A novel series of a-amino-acid phenolic ester derivatives containing sulphide, sulphoxide, sulphone, ester and amide side
chains were prepared and shown to displaypotent intravenous anaesthetic activity.
# 2003 Elsevier Science Ltd. All rights reserved.
Propofol (2,6-diisopropylphenol) is a widely used intra-
venous (iv) anaesthetic. Its mechanism of action involves
the positive allosteric modulation of the neuro-
transmitter g-aminobutyric acid (GABA) at GABAA
receptors.1 The main advantages of propofol are
favourable operating conditions and a rapid recovery
but disadvantages include cardiovascular side effects
and pain on injection.2ꢀ4 We recentlyreported the SAR
and general anaesthetic activityin mice of a series of
a-amino acid phenolic ester derivatives, and Org 25435
2 was tested in volunteers (Fig. 1).5 The lead optimisa-
tion of compound 2 focussed mainlyon structural
modifications to the phenolic and amino moieties. Lim-
ited alkyl side-chain modifications (methyl to butyl)
were explored and led to an increase in anaesthetic
activity. Our aim was to further optimise the anaesthetic
potencyof compound 2 byincorporation of heteroa-
toms into the alkyl side chain.
either 1.0 equiv or 3.2 equiv of sodium nitrite giving the
a-bromo acid derivatives 6 (sulphide side chain) and 7
(sulphone side chain). The acid chlorides were then
prepared using oxalyl chloride in dichloromethane with
catalytic pyridine and coupled with either 2,6-dime-
thoxyphenol or 2,6-dimethoxy-4-methylphenol to give
the intermediates 8 and 9. Sulphide 8 was converted to
sulphoxide 10 using meta-chloroperbenzoic acid in di-
chloromethane. a-Bromophenolic ester intermediates 8,
9 and 10 were converted to final compounds 11–18, 19–
23 and 24–25, respectively, by reaction with selected
amines (NHR02) at room or elevated temperatures.6
The ester and amide analogues 4 were synthesised as
shown in Scheme 2. Maleic anhydride 26 was reacted
with selected amines (NR00 ) in dichloromethane to give
2
substituted maleic acid 27, or reacted with methanol at
50 ꢁC to give substituted maleic acid 28. Reaction of 27
000
and 28 with selected secondaryamines (NR 2) in
In this paper we describe a series of a-amino-acid phe-
nolic ester derivatives containing sulphide, sulphoxide
and sulphone side chains 3, and ester and amide side
chains 4 with intravenous anaesthetic activity( Fig. 2).
methanol (at room or elevated temperatures) followed
bycation exchange with sodium hydroxide gave amino
acid derivatives 29 and 30. Coupling of 29 and 30 with
2,6-dimethoxyphenol or 2,6-dimethoxy-4-methylphenol
using Et3N, DMAP, EDCI in dichloromethane or
chloroform gave final products 31–40 and 41–50
respectively.7
The sulphide, sulphone and sulphoxide analogues 3
were synthesised as shown in Scheme 1. dl-Methionine
5 was diazotised in aqueous hydrobromic acid with
The anaesthetic potencyof compounds was determined
upon their iv administration to mice.8 In each case the
dose required to cause a loss of righting reflex for a
minimum period of 30 s in 50% of treated mice after iv
*Corresponding author. Tel.: +44-1698-736136; fax: +44-1698-
736187; e-mail: a.cooke@organon.co.uk
0960-894X/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0960-894X(03)00025-8

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A. Cooke et al. / Bioorg. Med. Chem. Lett. 13 (2003) 1107–1110
Table 1. Anaesthetic activityof sulphide 11–18, sulphone 19–23 and
sulphoxide 24–25 analogues in mice
Compd
NR02
R
HD50 (mmol.kgꢀ1 a
)
Propofol, 1
Racemic 2
N/Ab
N/Ab
N/Ab
N/Ab
H
Me
H
68.0
21.7
18.2
22.6
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Homopiperidine
Bismethoxyethylamine
Piperidine
Morpholine
Pyrrolidine
Morpholine
Heptamethyleneimine
Pyrrolidine
Tetrahydropyridine
Bismethoxyethylamine
Morpholine
Bismethoxyethylamine
Morpholine
Pyrrolidine
Figure 1. Propofol 1 and Org 25435 2.
<25
H
26.1
31.4
34.8
35
40
39.3
77
Me
Me
H
H
Me
Me
H
99
>106
108
73
H
Me
Me
Me
Figure 2. Sulphides, sulphones, sulphoxides 3 and esters and amides 4.
Morpholine
299
aMale MF-1 mice were used in all studies (nꢂ5).
injection over 10 s was determined byprobit analysis.
This dose is termed the HD50 (hypnotic dose50). Propo-
fol was injected as the commercial veterinaryproduct
(RapinovetTM, Schering-Plough Animal Health, UK) at
10 mg mLꢀ1. Compounds 11–25, 31–40, and 41–50 (free
base or hydrochloride salts) were administered as 5–
10 mg mLꢀ1 solutions in distilled water or suitable
vehicle; all were tested as racemates. The anaesthetic
activityof each series is shown in order of activityin
Tables 1–3. Propofol 1 and the racemate of 2 are given
for comparison.
bN/A=not applicable.
the racemate of 2 (HD50=21.7 mmol kgꢀ1). All sulphide
analogues 11–18 had HD50 <41 mmol kgꢀ1, the most
potent containing a homopiperidine ring 11 (HD50=18.2
mmol kgꢀ1). The more polar sulphoxide and sulphone
analogues displayed weaker activity, with sulphone 19
being the most potent (HD50=39.3 mmol kgꢀ1).
The in vivo anaesthetic potencyof compounds with an
ester containing side chain is shown in Table 2. Com-
pounds 31–33 all showed good potencywhile com-
pounds 34–38 had potencies comparable to propofol.
The in vivo anaesthetic potencyof compounds with a
sulphur containing side-chain are shown in Table 1. Of
these the suphides were the most potent and compared
favourablywith propofol 1 (HD50=68 mmol kgꢀ1) and
The most potent analogue 31 (HD50=22.9 mmol kgꢀ1
)
again contained a homopiperidine ring.
Scheme 1. Reagents and conditions: (i) 1.0 equiv NaNO2, HBr (aq),
0 ꢁC, (ii) 3.2 equiv NaNO2, HBr (aq), 0 ꢁC, (iii) Oxalyl chloride, DCM,
pyridine (cat.), (iv) 2,6-dimethoxyphenol or 2,6-dimethoxy-4-methyl-
phenol, Et3N, DCM, (v) m-CPBA, DCM, 0ꢁ C, (vi) selected amines
(NHR02), Á.
Scheme 2. (i) 1 Equiv selected amine (NR002), DCM, 0ꢁC, (ii) MeOH,
50 ꢁC, (iii) 3–4equiv selected amine (NR0002),MeOH, Á, (iv) 1 equiv NaOH,
MeOH, (v) 1.3 equiv 2,6-dimethoxyphenol or 2,6-dimethoxy-4-methyl-
phenol, CH2Cl2 or CHCl3, 2 equiv Et3N, 1 eq. DMAP, 1.3 equiv EDCI.

A. Cooke et al. / Bioorg. Med. Chem. Lett. 13 (2003) 1107–1110
Table 2. Anaesthetic activityof ester analogues 31–40 in mice
1109
active compounds, propofol 1 and the racemate of 2 are
shown in Table 4. Some of the more potent intravenous
anaesthetics also showed good inhibition of [35S]TBPS
binding to rat whole brain membranes, which suggests
the in vivo anaesthetic activitymaybe mediated in part
bya GABAergic mechanism.
Compd
NR000
R
HD50 (mmol kgꢀ1 a
)
2
31
32
33
34
35
36
37
38
39
40
Homopiperidine
Homopiperidine
Piperidine
H
Me
H
Me
H
Me
Me
Me
H
22.9
26
26.4
46
48
48
48
52
Morpholine
Bismethoxyethylamine
Bismethoxyethylamine
Piperidine
Conclusion
Pyrrolidine
Pyrrolidine
Morpholine
77
79
Manyof the heteroatom side-chain containing com-
pounds described in this paper displaypotent intrave-
nous anaesthetic activity, which may be mediated by a
GABAergic mechanism. The most potent compounds
synthesised belonged to the sulphide 11 (HD50=18.2
mmol kgꢀ1) and ester 31 (HD50=22.9 mmol kgꢀ1) series
and compared favourablywith propofol 1 (HD50=68
mmol kgꢀ1) and the racemate of 2 (HD50=21.7 mmol
kgꢀ1). Further optimisation of this series of a-amino-
acid phenolic ester derivatives is ongoing and will be
reported in the future.
H
aMale MF-1 mice were used in all studies (nꢂ5).
The in vivo anaesthetic potencyof compounds with an
amide containing side-chain is shown in Table 3. Com-
pounds 41–44 all showed good potency(HD 50=34–39
mmol kgꢀ1). The most active analogue 41 (HD50=34
mmol kgꢀ1) is again a homopiperidinyl derivative,
though in this case a change from R=H to R=Me 45
halves the potency(HD 50=67 mmol kgꢀ1). This is unu-
sual, as in most cases the R group does not make a large
difference to the potencyof the compounds.
Acknowledgements
The in vitro effect of selected compounds at GABAA
receptors was also assessed, bydetermination of their
The authors would like to thank the Analytical
Department, Organon, Newhouse.
abilityto
inhibit
[
35S]-tert-butylbicyclophos-
phorothionate ([35S]TBPS) binding to rat whole brain
membranes.8 In each case the concentration of drug
required to inhibit 50% binding of this radioligand was
determined (TBPS IC50). The in vitro results of selected
References and Notes
1. Concas, A.; Santoro, G.; Mascia, M. P.; Serra, M.; Sanna,
E.; Biggio, G. J. Neurochem. 1990, 55, 2135.
2. Bryson, H. M.; Fulton, B. R.; Faulds, D. Drugs 1995, 50,
513.
Table 3. Anaesthetic activityof amide analogues 41–50 in mice
3. Trapani, G.; Altomare, C.; Sanna, E.; Biggio, G.; Liso, G.
Curr. Med. Chem. 2000, 7, 249.
Compd
NR00
NR000
R
HD50 (mmol kgꢀ1 a
)
2
2
4. Tan, C. H.; Onsiong, M. K. Anesthesia 1998, 53, 468.
5. Anderson, A.; Belelli, D.; Bennett, D. J.; Buchanan, K. I.;
Casula, A.; Cooke, A.; Feilden, H.; Gemmell, D. K.; Hamil-
ton, N. M.; Hutchinson, E. J.; Lambert, J. J.; Maidment,
M. S.; McGuire, R.; McPhail, P.; Miller, S.; Muntoni, A.;
Peters, J. A.; Sansbury, F. H.; Stevenson, D.; Sundaram, H. J.
Med. Chem. 2001, 44, 3582.
41
42
43
44
45
46
47
48
49
50
Pyrrolidine
Piperidine
Diethylamine
Piperidine
Pyrrolidine
Pyrrolidine
Pyrrolidine
Diethylamine
Morpholine
Morpholine
Homopiperidine
Piperidine
Morpholine
Piperidine
H
Me
Me
H
34
37.5
38
39
67
81
102
117
>218
>225
Homopiperidine Me
Pyrrolidine
Pyrrolidine
Morpholine
Morpholine
Morpholine
Me
H
H
Me
H
6. Example of sulphide synthesis: butanoic, 2-(1-piperidinyl)-
4-(methylthio), 1-(2,6-dimethoxyphenyl)ester hydrochloride
13. To a stirred solution of dl-methionine (85.4 g) in water
(792 mL) and hydrobromic acid (47% aq, 528 mL) at 0 ꢁC was
added a solution of sodium nitrite (39.5 g) in water (100 mL).
Stirring was continued for 2 h at 0 ꢁC and then allowed to
warm to room temperature and stand for 24 h. The aqueous
phase was extracted with ethyl acetate (500 mL), dried
(sodium sulphate), filtered and concentrated under reduced
pressure to give butanoic acid, 2-bromo-4-(methylthio)-6 (33
g, 27%) as an orange oil. To a solution of 6 (33 g) and pyri-
dine (0.5 mL) in dichloromethane (50 mL) was added a solu-
tion of oxalyl chloride (27 mL) in dichloromethane (50 mL).
The reaction mixture was stirred for 24 h and concentrated
under reduced pressure. To the residue was added dichloro-
methane (100 mL), 2,6-dimethoxyphenol (23.9 g) and the
solution was cooled to 0 ꢁC. A solution of triethylamine (43
mL) in dichloromethane (50 mL) was then added dropwise
and, after complete addition, the reaction mixture was allowed
to warm to room temperature and stir for 2 h. Concentration
under reduced pressure and chromatographyon silica gel gave
butanoic acid, 2-bromo-4-(methylthio), 1-(2,6-dimethoxy-
aMale MF-1 mice were used in all studies (nꢂ5).
Table 4. GABAA receptor modulatoryeffects of selected compounds
Compd
Series
TBPS IC50 mMa
HD50 (mmol kgꢀ1 b
)
Propofol, 1
Racemic 2
N/Ac
N/Ac
18
68.0
21.7
18.2
22.6
29.3
9.7
6.9
4.0
2.7
6.2
4.2
11.7
14.7
11
12
13
15
19
31
33
37
Sulphide
Sulphide
Sulphide
Sulphide
Sulphone
Ester
<25
31.4
39.3
22.9
26.4
48
Ester
Ester
aValues are means of three experiments.
bMale MF-1 mice were used in all studies (nꢂ5).
cN/A=not applicable.

1110
A. Cooke et al. / Bioorg. Med. Chem. Lett. 13 (2003) 1107–1110
phenyl)ester (25.8 g, 48%) as an orange oil. Butanoic acid,
2-bromo-4-(methylthio), 1-(2,6-dimethoxyphenyl)ester (5 g)
and piperidine (5.2 mL) were stirred together for 1 h. Chro-
matographyon silica gel gave butanoic acid, 2-(1-piperidinyl)-
4-(methylthio), 1-(2,6-dimethoxyphenyl)ester (1.3 g, 26%) as a
solid. Conversion to the hydrochloride salt was achieved by
passing hydrogen chloride gas through a solution of the free
amine in diethyl ether. The hydrochoride salt that precipitated
was filtered off to give the title compound 13 as a white solid.
filtered off and washed with ethyl acetate (300 mL) to give
butanedioic acid, 2-(1-piperidinyl)-, 4-methyl ester, sodium
salt (43 g, 79%) as a white solid. To this material (10 g) was
added 2,6-dimethoxyphenol (8.5 g), dichloromethane (100
mL), 4-dimethylamino pyridine (5.2 g), triethylamine (11.7
mL) and 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide
hydrochloride (10.5 g). The resulting suspension was stirred
overnight then chromatographed on silica gel. The resulting
solid was washed with diethyl ether (100 mL) to give butane-
dioic acid, 1-(2,6-dimethoxyphenyl)ester, 2-(1-piperidinyl)-,
4-methyl ester (3.3 g, 22%) as a white solid. Conversion to the
hydrochloride salt was achieved by passing hydrogen chloride
gas through a solution of the free amine in diethyl ether. The
hydrochoride salt that precipitated was filtered to give the title
compound as a white solid 33. Positive ion ESI (M+H)+
353.8. 1H NMR (CDCl3+sodium carbonate); d 2.68–2.83
(3H, m), 2.90–3.01 (3H, m), 3.64–3.78 (7H, m), 3.82 (6H, s),
4.08 (1H, t), 6.62 (2H, d), 7.15 (1H, t). Esters 31–32 and 34–40
and amides 41–49 were synthesised in a similar manner.
8. Anderson, A.; Boyd, A. C.; Byford, A.; Campbell, A. C.;
Gemmell, D. K.; Hamilton, N. M.; Hill, D. R.; Hill-Venning,
C.; Lambert, J. J.; Maidment, M. S.; May, V.; Marshall, R. J.;
Peters, J. A.; Rees, D. C.; Stevenson, D.; Sundaram, H. J.
Med. Chem. 1997, 40, 1668.
1
Positive ion ESI (M+H)+ 353.8. H NMR (CDCl3+sodium
carbonate); d 1.42–1.51 (2H, m), 1.52–1.70 (4H, m), 2.01–2.11
(1H, m), 2.15 (3H, s), 2.17–2.26 (1H, m), 2.60–2.77 (4H, m),
2.80–2.88 (2H, m), 3.61 (1H, t), 3.81 (6H, s), 6.61 (2H, d), 7.12
(1H, t). Sulphides 11–12 and 14–18, sulphones 19–23 and
sulphoxides 24–25 were synthesised in a similar manner.
7. Example of ester synthesis: butanedioic acid, 1-(2,6-dime-
thoxyphenyl)ester, 2-(4-morpholinyl)-, 4-methyl ester hydro-
chloride 33. A solution of maleic anhydride (25.3 g) in
methanol (100 mL) was heated at reflux for 0.5 h with stirring,
then cooled to 0 ꢁC with an ice/acetone bath. Piperidine (67.5
mL) was added and the reaction mixture was heated to 50 ꢁC
for 1 h. Sodium hydroxide (9.2 g) was added and the resulting
suspension was stirred for 1 h. The reaction mixture was con-
centrated under reduced pressure and the resulting solid was